Changes in vertebrate preservation and bonebed character between lithofacies closely correspond to onshore-offshore changes in depositional setting, indicating that the dominant control of preservation is exerted by physical processes. The strong physical control on marine vertebrate preservation and preservational bias within the Purisima Formation has implications for paleoecologic and paleobiologic studies of marine vertebrates. Evidence of preservational bias among marine vertebrates suggests that careful consideration of taphonomic overprint must be undertaken before meaningful paleoecologic interpretations of shallow marine vertebrates is attempted.

Lithofacies analysis was conducted to place vertebrate fossils within a hydrodynamic and depositional environmental context. Taphonomic data including abrasion, fragmentation, phosphatization, articulation, polish, and biogenic bone modification were recorded for over 1000 vertebrate fossils of sharks, bony fish, birds, pinnipeds, odontocetes, mysticetes, sirenians, and land mammals. These data were used to compare both preservation of multiple taxa within a single lithofacies and preservation of individual taxa across lithofacies to document environmental gradients in preservation. Differential preservation between taxa indicates strong preservational bias within the Purisima Formation. Varying levels of abrasion, fragmentation, phosphatization, and articulation are strongly correlative with physical processes of sediment transport and sedimentation rate. Preservational characteristics were used to delineate four taphofacies corresponding to inner, middle, and outer shelf settings, and bonebeds. Application of sequence stratigraphic methods shows that bonebeds mark major stratigraphic discontinuities, while packages of rock between discontinuities consistently exhibit onshore-offshore changes in taphofacies.

Taphonomic study of marine vertebrate remains has traditionally focused on single skeletons, lagerstätten, or bonebed genesis with few attempts to document environmental gradients in preservation. As such, establishment of a concrete taphonomic model for shallow marine vertebrate assemblages is lacking. The Neogene Purisima Formation of Northern California, a richly fossiliferous unit recording nearshore to offshore depositional settings, offers a unique opportunity to examine preservational trends across these settings.

Funding: Funding sources included a graduate research grant from the Geological Society of America, and a Grant-in-aid of research from Sigma Xi. The lead author is currently supported by a U. Otago doctoral scholarship, which supported the writing of this paper (although the research was carried out beforehand). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

To achieve these goals, numerous types of taphonomic, stratigraphic, and sedimentologic data were recorded for a large sample set ( Table 2 ; Table S1 ) of fossil vertebrates from the Purisima Formation. Comparative taphonomy [8] was utilized to compare preservation of different marine vertebrate taxa within an assemblage, assess problems of bias and differential preservation, and to compare preservation of fossil taxa across inferred depositional settings. Taphofacies analysis [9] was employed to map preservational facies and their lateral and vertical relationships.

The richly fossiliferous late Neogene Purisima Formation of Northern California ( Figs. 1 – 2 ) was deposited in depositional environments ranging from nearshore and estuarine to outer shelf and upper slope settings [34] , [35] . Preservation of fossil vertebrates from different depositional settings in the Purisima Formation allows examination of onshore-offshore gradients in fossil preservation. This study takes advantage of the abundance of vertebrate skeletal material, including baleen whales, toothed whales, pinnipeds, sea cows, marine birds, bony fish, and sharks ( Table 1 ), from numerous depositional environments ( Fig. 3 ) represented by excellent sea cliff exposures of the Purisima Formation at the Santa Cruz section in order to: 1) document onshore-offshore trends in preservation styles and degree; 2) determine the sedimentologic (hydrodynamic) or biogenic processes controlling patterns of marine vertebrate preservation; and 3) construct a preliminary taphonomic framework for understanding marine vertebrate fossil preservation across siliciclastic shelves.

For marine vertebrates, only a few paleontologic studies have compared the trends in preservation along environmental gradients [13] , [14] . A number of actualistic studies have assessed problems of decomposition, disarticulation, sorting, abrasion, scavenging, and bloating in marine vertebrates [15] – [23] . Additionally, several forensic studies focused on decomposition, disarticulation, bloating, hydraulic sorting, bone modification, and marine scavenging, using experiments with pigs or forensic case data, are applicable to marine vertebrate taphonomy [11] , [24] – [28] . All of these studies were conducted in shallow water conditions along shorelines, at the ocean surface, or in the laboratory. Deeper water studies of whale-falls have generated useful taphonomic data (scavenging rates, encrustation, bioerosion) for large cetaceans in outer shelf, bathyal, and abyssal environments [29] – [32] . However, few of these studies have been conducted in the range of water depths characteristic of the continental shelf [33] , and it is unclear how applicable the majority of whale-fall data are to shelf environments. Virtually no actualistic data exist for depositional environments between the shoreline and deep sea. A well-formulated taphofacies model for shallow marine vertebrate assemblages is currently lacking, and no existing actualistic or historical framework is available to place marine vertebrate fossil assemblages within a broader taphonomic context.

Conversely, the taphonomic record of marine vertebrate fossils has received comparatively little study due to the relative rarity of marine vertebrate fossils (compared with invertebrates) and difficulty in conducting actualistic experiments in the marine environment [11] . Taphonomic investigations of processes affecting terrestrial vertebrates are more common because their presence at the land surface make them easier to observe and interpret. Indeed, much paleontologic research has focused on interpreting the genesis of terrestrial vertebrate bonebeds [12] .

The robust shallow marine invertebrate fossil record has been the subject of numerous studies that have broadened the field of taphonomy from a focus on the negative aspects of preservation (taphonomic loss), to one on taphonomic gain and the power of taphonomic data for understanding depositional and biogenic processes. These studies have focused on formation of skeletal concentrations [1] , effect of subsidence rate on skeletal accumulations [2] – [4] , relations between stratigraphic sequence boundaries and preservation [5] , development of new taphonomic field data collection methods [6] , [7] , comparative taphonomy [8] , and recognition of taphonomic facies [9] , [10] .

Norris [34] investigated preservation of invertebrate remains in the Purisima Formation, and found a shift from physical processes (i.e. reworking, transport) dominating shallower marine settings, to ecological processes (i.e. bioturbation, encrustation, in-situ preservation) predominant in deeper marine settings. Norris [34] also observed a decrease in thickness and frequency of invertebrate accumulations with increasing inferred water depth. A study by Boessenecker and Perry [52] identified juvenile fur seal bones with tooth marks attributable to marine mammal teeth from the middle part of the Santa Cruz section.

The age of the Santa Cruz section is well constrained, based on several methods. This section ranges from latest Miocene at the base, to middle-late Pliocene at the top [35] ( Fig. 2 ); the other sections of the Purisima Formation are approximately this age as well. The basal glauconitic sandstone yielded a K/Ar date of 6.9±0.5 Ma [37] . The diatom-bearing lower 90 meters of the Purisima Formation yielded diatom assemblages indicating a similar age, 7–5 Ma [35] . A paleomagnetic study of the Purisima Formation indicates the Santa Cruz section is 6.07 to 2.47 Ma in age, with a depositional hiatus from 4.5 to 3.5 Ma [37] ; this depositional hiatus is marked by one of the bonebeds investigated by this study.

The Santa Cruz section of the Purisima Formation was designated as a supplementary reference section by Powell et al. [35] because it “represents the most continuously exposed and best dated Purisima section.” The type section, exposed in cliffs near the mouth of Purisima Creek near Half Moon Bay, is no longer accessible by foot. Other sections (Point Reyes, San Gregorio, Seal Cove, and Año Nuevo sections) have only received cursory study, and the age of some of these sections remains uncertain [35] , [45] . The Santa Cruz section is 325 meters thick, exposed for 19 km of shoreline along the northern margin of Monterey Bay ( Fig. 1A, D ) [35] . Overall, this section comprises a shoaling-upwards stratigraphic trend with diatomite and mudrock in the lower portion, bioturbated sandstone dominating the middle, and cross-stratified sandstone and coquina in the upper portion [34] , [35] ( Figs. 2 , 4 ). These deposits represent offshore to shoreface, foreshore, and estuarine deposition [34] , [35] . Nine distinct bonebeds occur within the lower and middle parts of the Santa Cruz section, but only six were accessible or satisfactorily exposed for study, here numbered Bonebeds 1–6 ( Fig. 4 ).

The Purisima Formation records an overall change from the earlier biogenic sedimentation of the Middle and Late Miocene (as recorded by the Monterey Formation and Santa Cruz Mudstone), to siliciclastic deposition in Northern California during the latest Miocene and Pliocene [35] . The Tortonian-equivalent (10–12 Ma) Santa Margarita Sandstone (deposited between the Monterey Formation and Santa Cruz Mudstone) can be viewed as the first pulse of coarse siliciclastic marine sedimentation in this region during the Late Neogene. The underlying Santa Cruz Mudstone represents offshore biosiliceous sedimentation [65] , and was already lithified and deformed at the time that Purisima Formation deposition began [66] . The Purisima Formation represents an overall regression that is punctuated by several transgressive-regressive successions [35] .

The Neogene (Miocene-Pliocene) Purisima Formation was named by Haehl and Arnold [36] for fossiliferous marine sedimentary rocks in sea cliffs at the mouth of Purisima Creek in San Mateo County, California ( Fig. 1 ). Ranging in age from 6.9–2.47 Ma [35] , [37] , the Purisima Formation crops out near San Francisco and Santa Cruz, California [38] , [39] , where it is composed of fossiliferous marine conglomerate, sandstone, siltstone, and mudstone, and diatomite. Wrench tectonics associated with strike-slip faulting in Northern California likely controlled basin subsidence [35] , [40] . The four major areas of exposure of the Purisima Formation ( Fig. 1A ), mostly west of the San Andreas fault include: 1) Point Reyes, Marin County, CA, formerly the “Drakes Bay Formation” of Galloway [41] ; 2) Pillar Point, San Mateo County, CA [42] ; 3) Half Moon Bay, San Mateo County, CA [39] ; and 4) Santa Cruz, Santa Cruz County, CA [35] . Although the Purisima Formation crops out in some stream gullies and man-made exposures in the Santa Cruz Mountains [39] , the majority of outcrops are in linear coastal cliffs. Because of local faulting and folding, some Purisima Formation exposures have been mapped but have yet to have detailed stratigraphic sections measured and described. Larger scale faulting (offset along the San Gregorio fault in particular) has caused problems with correlations between different exposures and across faults [43] – [45] . Vertebrate fossils (baleen whales, porpoises, beluga, walruses, fur seals, sea cows, marine birds, fish, sharks, and rays) occur in most Purisima Formation strata [43] , [46] – [59] . Fossil invertebrates and microinvertebrates are abundant in most exposures of the Purisima Formation and include gastropods, bivalves, brachiopods, barnacles, decapods, echinoids, and asteroids [34] , [35] , [42] , [45] , [56] , [60] – [64] .

During this project several consistently recurring patterns of bonebed geometry required development of a new bonebed descriptive scheme to facilitate their interpretation. All Purisima Formation bonebeds contain three intervals ( Fig. 9 ): 1) a lower interval occasionally characterized by an upward increase in bioclast packing (sensu [6] ); 2) a middle interval where bioclast packing is persistently highest; and 3) an upper interval marked by an upward decrease in bioclast packing. For convenience, these intervals were assigned upper case Greek letters for alpha, beta, and gamma ( Fig. 9 ) and termed the α-interval (lowest), β-interval (middle), and γ-interval (uppermost). This scheme specifically uses Greek rather than Latin alphabet characters so as not to be confused with soil horizon descriptive schema; this descriptive scheme may be modified and applied to terrestrial bonebeds, which occasionally coincide with paleosols. Recognition of consistent patterns of bonebed attributes facilitates their description and interpretation. For example, often there may be a sharp erosional surface at the base of the β-interval; the β-interval may also be characterized by multiple erosional surfaces. The α-interval may be barren, or may only have bonebed debris (bonebed bioclastic and clastic material) concentrated within vertical trace fossils and burrows (e.g., Ophiomorpha). The different intervals are often characterized by subtle changes in grain and bioclast size, packing, bioclast mineralogy, as well as changes in vertical thickness and geometry along strike.

The phosphatization scale incorporated two qualitative measures: 1) extent of bone permineralization; and 2) occurrence and relative size of adhering phosphatic matrix or nodules ( Fig. 8 ). Many bones and teeth in the Purisima Formation exhibit varying degrees of phosphate mineral replacement, ranging from heavy, blackened elements, contrasting with lighter shades of gray and brown in non-phosphatized elements. A simple scale was devised to reflect this ( Fig. 8 ): no phosphate replacement (Stage 0), small patches or incomplete phosphate replacement (Stage 1), and complete phosphate replacement (Stage 2). Many of these elements also exhibit varying degrees of adhering phosphatic matrix. To capture this variation, another scale was superimposed on the mineralization scale to indicate the following: no adhering phosphatic matrix (Stage XA), limited adhering phosphatic nodule (Stage XB), and adhering phosphatic nodule covering more than one-third of the element surface area (Stage XC). This resulted in the following possible combinations: Stage 0A, 0B, 0C, 1A, 1B, 1C, 2A, 2B, and 2C. For example, a completely unphosphatized element represents Stage 0A, while 2C represents a blackened element embedded within a phosphatic nodule. All of the other possible stages represent intermediate conditions ( Fig. 8 ). An obvious limitation of this scale is that for bone mineralization (e.g., Stage 0–2X), color change associated with phosphatization must be present and known in an assemblage. Although the color of phosphatized elements in the Purisima Formation is typically black and dark brown, this may vary from formation to formation (or even locality). Effective use of this scale should only be attempted when the color of phosphatized material is established and different from that of non-phosphatized material. Because of the large sample size (n = 1033), petrographic confirmation of phosphate replacement was beyond the scope of this study.

A qualitative scale to assess phosphatization of skeletal elements was devised for this study ( Fig. 8 ). Phosphatization is an early diagenetic process that affects sediment and bioclasts at or below the sediment-water interface during times of phosphogenesis [74] – [76] . Phosphatic rinds may form at the sediment-water interface, but formation of phosphatic nodules occurs below the sediment-water interface [74] – [76] . Vertebrate skeletal elements may be phosphatically permineralized, and may also exhibit adhering phosphatic matrix (usually equivalent to mudrock in terms of grain size) or nodules ( Fig. 6B ), which in most cases exhibit a differing grain size from the surrounding sediment [74] . Early diagenetic permineralization of skeletal tissues and development of adhering phosphatized nodules indicate phosphatization represents a mode of prefossilization [77] . Prefossilization is here defined as early diagenetic permineralization of a bioclast prior to final burial; thus, the presence of prefossilized material within bioclastic concentrations implies that the prefossilized material was exhumed from a temporary deposit (where it underwent early diagenesis).

A modified version of Fiorillo’s [71] abrasion scale was used ( Fig. 5A ). The modified scale includes three stages: unabraded (Stage 0), lightly abraded (Stage 1), and heavily abraded (Stage 2). Although elaborate fragmentation scales have previously been published, only presence/absence of fragmentation was documented ( Fig. 6C ). Articulation and element association was coded on a simple scale ( Fig. 5B ): 1 = articulated skeleton (or articulated elements); 2 = disarticulated skeleton; 3 = cluster of a few associated or articulated elements; 4 = isolated element. Additionally, rare cases of biogenic bone modification such as bite marks and invertebrate bioerosion were noted for individual specimens. Although commonly used in terrestrial taphonomic studies, Behrensmeyer’s [72] bone weathering scale was not employed because analogous weathering attributes have not been recognized for marine vertebrate fossils [29] , [73] . When present, mosaic surface cracking was noted (n = 24; 2.3%) although its significance in marine weathering of bone requires further study. Lastly, polish (light abrasion of element surface resulting in shiny, reflective and often smooth surface) was simply recorded as present or absent ( Fig. 6D , 7 ). These data were imported into a spreadsheet, and calculated percentages were used to generate histograms and pie charts of taphonomic characteristics in relation to lithofacies, taxon, and skeletal element group (bone, calcified cartilage, earbones, and teeth).

To study the taphonomy of each fossil assemblage, the methods outlined by Kidwell et al. [6] and Kidwell and Holland [7] for characterization of bioclast concentration geometry and architecture were applied to all bioclastic (invertebrate or vertebrate rich) units. Detailed field descriptions of the lithology (including clast counts) and sedimentary architecture were recorded for bonebeds, and a large sample of specimens were collected from these bonebeds ( Table 2 ). Bonebeds were recognized as relative concentrations of vertebrate skeletal material [12] and examined along strike to determine their lateral extent and changes in character. Recognition of bonebeds based upon a percentage composition of vertebrate skeletal material (e.g. [68] – [70] ) was avoided because 1) private collection of vertebrate fossils would artificially deflate the size of the vertebrate fraction, perhaps making bonebeds reported herein fall under the minimum threshold value for bonebed recognition; and 2) bulk sampling of all bonebeds was not possible because of safety issues. Taphonomic data (see below) were collected for a large sample (n = 1033; Fig. 3 ; Table 2 ; Table S1 ) of vertebrate fossils. These include specimens collected by R. W. Boessenecker (n = 478) from 2004–2010. Data including taxon (e.g., Chondrichthyes, Osteichthyes, Aves, Pinnipedia, Odontoceti, Mysticeti, Sirenia), abrasion, fragmentation, articulation, phosphatization, and the associated lithofacies were collected for each specimen. This study utilized fossils from museum collections (collected primarily by F.A. Perry) at the Santa Cruz Museum of Natural History (n = 188; SCMNH) and the University of California Museum of Paleontology (n = 295; UCMP) of known stratigraphic provenance (to a distinct bonebed, or distinct stratigraphic position assigned to one of the included lithofacies, based on collector’s field notes). An additional 72 uncurated fossils (also with known provenance) from UCMP and SCMNH collections were recorded. Specimens lacking clear stratigraphic provenance were excluded. Vertebrate fossils were identified to each taxonomic group based on comparisons with previously published Neogene marine vertebrate fossil descriptions and photographs of modern osteological specimens. Vertebrate taxa studied include Chondrichthyes (sharks and rays), Osteichthyes (bony fishes), Aves (birds), Pinnipedia (seals, sea lions, and walruses), Mysticeti (baleen whales), Odontoceti (dolphins and other toothed whales), Sirenia (sea cows), and land mammals ( Table 1 , 2 ). Many bone fragments with typical mammalian histology (i.e. cancellous bone) not confidently assignable (due to taphonomic damage or recent erosion) to any of the aforementioned mammalian groups, and too large to represent birds or bony fish, were identified as indeterminate mammals.

To place the vertebrate fossil assemblages of the Purisima Formation into proper stratigraphic and sedimentologic context, several methods were employed. Three sections representing the lower and middle portions of the Santa Cruz section (sensu [35] ) of the Purisima Formation were measured and described ( Figs. 1 – 2 , 4 ). These sections do not overlap, and represent only part of the lower Santa Cruz section of the Purisima Formation ( Fig. 2 ); measurement and description of a continuous section was not possible due to dangerous outcrop conditions. Data regarding bed thickness and geometry, lithology, sedimentary structures, bedding contacts, ichnofabric index [67] and ichnofossil content were collected for each bed. These data were then utilized to delineate lithofacies ( Table 3 ), with each interpreted relative to hydrodynamic (energy) and substrate conditions. The interpretations are based on inferences of the bedforms and substrate conditions that characterized development of each lithofacies. Commonly co-occurring lithofacies were grouped into lithofacies associations representative of environments characterized by specific related suites of sediment transport processes and substrate characteristics. To the degree possible, bounding surfaces were also noted during section measurement and description; these surfaces were utilized in conjunction with lithofacies associations to develop a sequence stratigraphic framework for interpreting controls on Purisima Formation deposition.

This study focused exclusively on the sea cliff exposures of the Purisima Formation at the Santa Cruz supplementary reference section. To establish paleoenvironmental gradients of preservation within the Purisima Formation, a stratigraphic framework was first established and ‘populated’ with taphonomic data. A combination of sedimentologic, stratigraphic, and taphonomic methods were utilized and are summarized below. The upper portion of the Santa Cruz section was not studied in detail due to its limited exposure and the rarity of vertebrate remains.

The massive mudrock (Mm), laminated diatomite (Mld), and massive pebbly mudrock (Mpm) lithofacies occur together, and more frequently interfinger with each other than with any of the coarser sandstone lithofacies ( Fig. 10 ). This suite of mudrock lithofacies represents deposition entirely below storm weather wave base, and in distal, offshore parts of the continental shelf ( Fig. 12 ). Pervasive bioturbation, in situ phosphate nodules, and (potentially) hiatal bonebeds are all indicative of relatively low sedimentation rates. The fine-grained nature of this lithofacies association is due to suspension fallout of mud and diatom tests offshore at distances far from the reach of fair-weather nearshore (shoreface) sediment transport or storm-generated combined flow delivery of sediment [95] . Additionally, thick sections of diatomite indicate certain areas of the outer shelf were starved of siliciclastic sediment, permitting biogenic sediment to accrue. The laminated nature of some diatomaceous strata indicates anoxic conditions restricted the bioturbating infauna, further suggesting deposition in sediment-starved environments of the outer shelf.

The hummocky cross-stratified (Shc), massive (Sm), and massive pebbly sandstone (Spm) lithofacies commonly occur together, and interfinger more frequently with each other than with finer-grained lithofacies ( Fig. 11 ). This suite of sandstone lithofacies represents deposition ranging from slightly below storm weather wave base to above fair weather wave base on the proximal portion of the continental shelf near the shoreline ( Fig. 12 ). Relative to bottom energy conditions, deposition spanned the middle-lower shoreface to offshore transition zones [91] , [92] . Abundant internal truncations and shell beds indicate frequent, high energy disturbance at the sediment-water interface by storm activity and fair weather wave activity [93] , [94] . The majority of bonebeds within this lithofacies association have internal erosional surfaces, also indicating relatively higher energy than the offshore lithofacies association. Preservation of primary sedimentary structures in some strata indicate higher sedimentation rates and more frequent sediment transport in the shallower nearshore (shoreface) settings than in lithofacies of the more distal offshore shelf environment.

The laminated diatomite (Mld) lithofacies is present in only a single section, representing the last pulse of “Monterey Formation-type” deposition in Northern California [90] , and marking a brief return to the biosiliceous sedimentation that characterized the underlying Santa Cruz Mudstone and Monterey Formation. A combination of high productivity and formation of isolated, sediment starved basins has been implicated in the richly diatomaceous deposits of the Monterey Formation [65] , conditions that likely persisted during deposition of the lowermost Purisima Formation. Absence of trace fossils and invertebrate body fossils from this lithofacies suggests anoxic or dysoxic pore and bottom water. This lithofacies was deposited by a biogenic rain of diatom tests in offshore settings at or near the shelf-slope break well below storm weather wave base [35] ( Fig. 12 ).

The massive mudrock (Mm) lithofacies represents offshore deposition well below storm weather wave base and beyond the limits of sand delivery to the shoreface-offshore transition zone ( Fig. 12 ). Deposition here largely takes place by suspension fallout of silt and clay under low energy conditions. Stacked upward-fining beds with occasional shell lags [63] most likely represent rare distal storm-generated traction transport events. The massive nature of the sediment is again due to pervasive bioturbation. Biogenic activity was relatively unaffected by tractive current disturbance of the substrate, which is also reflected by an abundance of mollusk concentrations preserved in life position [34] . At other Purisima Formation localities, this lithofacies interfingers with turbidites, indicating deposition on the outer shelf near the shelf-slope break [34] that is corroborated by bathyal foraminifera [89] . Abundant trace fossils of the Cruziana ichnofacies suggest deposition in muddy, low energy offshore shelf environments [79] .

The massive pebbly mudrock (Mpm) lithofacies likely formed in a manner similar to the massive pebbly sandstone (Spm) lithofacies. The abundance of phosphatic nodules indicates a substantial decrease in sedimentation rate. Additionally, because phosphate nodules only form below the sediment-water interface [75] , their abundance indicates erosion of the substrate during a long depositional hiatus. Lack of calcareous material may be due to the low pH settings associated with phosphogenesis [75] , [88] , although calcareous macrofossils are generally absent from the diatomaceous portions of the massive mudrock (Mm) lithofacies that brackets (above and below) the only known exposures of the massive pebbly mudrock (Mpm). Finer-grained sediment (massive siltstone and diatomite) in this lithofacies suggests it may record development of distal bonebeds (or distal portions of a bonebed) in offshore environments. Although Ophiomorpha and Thalassinoides of the Skolithos ichnofacies are typical of sandy, high energy environments [79] , their occurrence here is likely due to the high energy associated with bonebed formation. Other observed traces such as Teichichnus and Planolites (Cruziana ichnofacies) are more typical of lower energy, muddy environments [79] . As the massive pebbly sandstone (Spm) likely records bonebed formation within both shoreface and transition zone settings; given the large lateral extent of bonebeds in the Purisima Formation (see 6. Bonebeds ), a single bonebed may extend across the shelf from areas of nearshore massive pebbly sandstone (Spm) deposition to deeper offshore settings where massive pebbly mudrock (Mpm) accumulated ( Fig. 12 ).

The laminated diatomite (Mld) lithofacies occurs only in the lowermost part of the Santa Cruz section. It consists of finely laminated gray-yellow diatomite with a tabular geometry ( Fig. 10D ). Few trace fossils (<1 cm wide) occur in this lithofacies. This facies is sparsely fossiliferous and usually lacks calcareous skeletal material. One horizon in particular (4 meters above the base of the Purisima Formation) exhibits a sharp contact with underlying massive diatomite below, and is mantled by sand, woody debris, and rare vertebrate elements and fragmentary mollusks. This is the same stratigraphic position and locality of a fragmentary ‘whale fall’ assemblage discovered by one of us (F.A. Perry) in 1993. This facies interfingers with the massive mudrock (Mm) lithofacies ( Fig. 11D ).

The massive mudrock (Mm) lithofacies primarily includes siltstone, with lesser amounts of mudstone and diatomaceous lithologies. This facies appears to lack any obvious internal erosional surfaces, and exhibits a tabular geometry ( Fig. 10F ). Planar laminated siltstone occasionally forms couplets with massive siltstone. Some parts of this facies include thin horizons of ripple cross-laminated siltstone. Other parts exhibit stacked beds (∼1 meter thick) of very fine sandstone with occasional shell lags at their base that fine upward into siltstone and mudstone. This lithofacies harbors a variety of trace fossils [63] , including Teichichnus, Planolites, and rare Thalassinoides and Ophiomorpha. The ichnofabric typically consists of cross-cutting traces; small trace fossils and burrows (<1 cm wide) are preserved within this lithofacies. Ophiomorpha is occasionally infilled with sand if close to overlying sandstone. Articulated bivalves (Tresus, Anadara) occur as monotaxic clumps or in isolation; partial colonies of Crepidula are also present. This lithofacies interfingers with the laminated diatomite (Mdl) and massive sandstone (Sm) lithofacies ( Fig. 11B–D ).

The massive pebbly mudrock (Mpm) lithofacies is similar to the massive pebbly sandstone (Spm) lithofacies, and most commonly found in Bonebed 2 and other poorly exposed (unnumbered) bonebeds in Section 1. Interstitial matrix is massive, pervasively bioturbated mud; the coarse fraction consists of very poorly-sorted, matrix-supported pebbles and cobbles with rare terrigenous clasts and phosphatic nodules ( Fig. 10B ). Small zones may be conglomeratic and clast-supported. Phosphatic nodules are internally homogenous and lack mollusk or crustacean skeletal elements. Vertebrate skeletal elements are relatively abundant, with no invertebrate body fossils present. Burrows including Ophiomorpha and Thalassinoides (horizontal branching tube-shaped burrows) are typically infilled with bonebed debris. Other trace fossils include Teichichnus and Planolites (horizontal tube-shaped burrows <2 cm in diameter). Vertebrate skeletal elements are often fragmented and heavily phosphatized (Stage 2A). This lithofacies interfingers with the massive mudrock (Mm) and laminated diatomite lithofacies (Mld) ( Fig. 11D ).

The massive sandstone (Sm) lithofacies represents deposition below storm weather wave base in the shoreface-offshore transition zone ( Fig. 12 ). The massive and monotonous nature of this lithofacies is due to pervasive bioturbation. Because of the greater water depth in this depositional setting, less frequent storm-induced modification of the substrate failed to erase the bioturbatory overprint [34] . Although primary sedimentary structures are lacking, abundance of laterally extensive shell beds and pavements suggest this lithofacies represents storm-deposited beds extensively overprinted and rendered structureless by bioturbation. Sharp scours at the base of rare hummocky cross-stratified beds indicate erosion and reworking of the sediment substrate prior to deposition. The presence of sand below storm weather wave base also suggests sediment introduction by infrequent storm-related event deposition followed by extensive bioturbation during fair-weather periods [87] . Ichnotaxa including Ophiomorpha and Planolites suggest that this lithofacies corresponds to both the Skolithos and Cruziana ichnofacies [79] . Because the Cruziana ichnofacies is typical of slightly deeper water than the Skolithos ichnofacies [79] , presence of both suggests deposition in the shoreface to offshore transition zone. Interfingering of the Sm lithofacies with the hummocky cross-stratified sandstone (Shc) of more proximal high-energy settings and massive mudrock (Mm) of offshore quiet-bottom settings supports this interpretation [34] .

The hummocky-cross stratified sandstone (Shc) lithofacies represents upper and lower shoreface deposition above storm and fair weather wave base ( Fig. 12 ). Hummocky cross-stratification forms under conditions of combined oscillatory and unidirectional flow, with rapid suspension settling of sand [81] , [82] . Combined flow may develop during hyperpycnal flow after heavy runoff produced by sediment-laden river plumes often associated with the effects of intense precipitation during on-shore storms [81] – [83] . Most commonly, as evidenced from modern shallow marine settings, sediment transport of sand and mud involves sediment disturbance by storm-wave resuspension and modification of resulting sediment gravity flows by geostrophic currents [84] – [86] . Storm deposition represents some of the highest energy depositional settings for the Purisima Formation [34] . In addition, rare Ophiomorpha is indicative of the high-energy, sandy substrate conditions of the Skolithos ichnofacies [79] . Frequent reworking of sediment is indicated by truncation and amalgamation of many beds in this lithofacies, and sparse evidence of bioturbation. Laterally extensive hummocky cross-stratified beds with bioturbated tops represent hyperpycnal deposition below fair weather wave base and closer to storm weather wave base, where fewer storms disturb the seafloor and longer periods of inter-storm bioturbation are able to occur [34] . In contrast, non-bioturbated hummocky-cross stratified sandstone beds are interpreted to have been deposited closer to and above fair weather wave base. Abundant well-preserved basal erosional surfaces (mantled with invertebrate bioclasts, mud rip-up clasts, and phosphate nodules) indicate frequent storm-related erosional events. However, the fewer terrigenous clasts and less taphonomically mature invertebrate fossils [34] suggest that although frequency of reworking is much higher than in the massive pebbly sandstone (Spm) lithofacies (i.e. timing between the formation of different bonebeds), the duration of nondeposition is temporally much shorter.

This lithofacies does not interfinger with other lithofacies in the strict sense, but instead truncates underlying strata ( Fig. 12 ). Abundance of phosphatic material and glauconite indicates association with the most extreme periods of non-deposition; truncation of underlying units and wide lateral extent also suggests association with large-scale erosion of the seafloor. The abundance of phosphatized bioclasts and phosphatic nodules also requires significant erosion during genesis of this lithofacies. The presence of Ophiomorpha burrows and Gastrochaenolites and Trypanites borings suggests this lithofacies corresponds with the Skolithos and Trypanites ichnofacies. The Skolithos ichnofacies characterizes non-hardground sandstone within this lithofacies, and is indicative of high-energy, shoreface and transition zone environments with a mobile substrate [79] . Conversely, the Trypanites ichnofacies is limited to hardgrounds (Bonebeds 5 and 6) and rockgrounds (Bonebed 1) and is indicative of high-energy settings with a fully lithified substrate [79] .

The massive pebbly sandstone (Spm) lithofacies (present only in some bonebeds) forms only during bonebed genesis. Abundance of glauconite indicates low to zero-net sedimentation under conditions of sediment starvation [80] . Because phosphatic nodules develop only below the sediment-water interface during periods of low to zero net sedimentation [75] , their presence indicates erosion and exhumation from below the sediment substrate. Some bonebeds exhibit sharp erosional bases (Bonebeds 1, 5, and 6), whereas others (Bonebeds 2, 3, and 4) exhibit gradational contacts. The abundance of phosphatic debris within Bonebeds 2 and 4 suggests that an erosional lower contact was once present but subsequently erased by bioturbators. Clasts of lithified underlying Santa Cruz Mudstone [66] resulted from erosion of a marine rockground during the hiatus prior to Purisima Formation deposition, preserving the sharp lower contact of Bonebed 1; in contrast, formation of a phosphatic hardground preserved the sharp internal contacts of Bonebed 6.

The massive sandstone (Sm) lithofacies consists of structureless tabular sandstone beds that characterize many Purisima Formation exposures. These massive sandstones are typically fine-medium grained (occasionally very fine grained), moderately-poorly sorted, and contain silty matrix ( Fig. 10E ). In some cases the Sm lithofacies occurs in thick (up to 25 meters thick), monotonous, unfossiliferous sections. Few erosional surfaces are preserved within this lithofacies, and most observed internal changes in lithology (i.e. color, sorting, grain size, ichnofabric) are subtle and gradational. A few thin hummocky cross-stratified sandstone (Shc) beds occur where this lithofacies grades into the hummocky cross-stratified sandstone lithofacies. The massive nature of this lithofacies derives from pervasive bioturbation that has completely homogenized the primary sedimentary fabric. Typically the trace fossil Ophiomorpha is abundant, with the ichnofabric often composed entirely of cross-cutting, overlapping trace fossils. Teichichnus (concave up vertically migrating spreiten), Skolithos (vertical tube-shaped burrows <1 cm wide), and Planolites (small horizontal tube-shaped burrows <2 cm wide) traces are also common [63] . Bioclast-rich portions are rare within this lithofacies, and primarily include thin shell beds, pavements, and stringers. Clumps of articulated bivalves (often Anadara trilineata) in apparent life position also occur [34] . “Articulated” clumps of the colonial gastropod Crepidula are rarely present. This lithofacies often directly overlies laterally extensive bonebeds [34] . Bonebed 3 lacks abundant phosphatic and terrigenous pebbles and is instead composed of this lithofacies (rather than the massive pebbly sandstone (Spm) lithofacies). Vertebrate fossils are rare within this lithofacies; when present, preservation varies from abraded to pristine isolated elements and disarticulated to partially articulated skeletons. This lithofacies interfingers with the hummocky cross-stratified sandstone (Shc), massive mudrock (Mm), and pebbly massive sandstone (Spm) lithofacies ( Fig. 11B ).

The hummocky-cross stratified sandstone (Shc) lithofacies comprises beds of hummocky cross-stratified, very fine-medium grained, well-moderately sorted sandstone ( Fig. 10C ). These beds are typically 20–60 cm thick but range up to 120 cm in thickness [34] . Each bed fines upward, with a discontinuous shell lag present often developed at lower bounding surfaces that are sharp and often wavy. Mollusk shell concentrations typically comprise beds and pavements, with mudrock rip-up clasts (typically 1–3 cm in size, and up to 25 cm), phosphate nodules (typical of thicker shell lags), and rare vertebrate elements also present. Many shells retain adhering mudrock and phosphatic matrix. Terrigenous siliciclastic pebbles occur occasionally along the lower erosional contact, but are much rarer than in the massive pebbly sandstone (Spm) lithofacies. Bioturbation and trace fossils are absent from the lower part of each bed, but burrowing intensity increases towards the top, which is often completely bioturbated and massive. Trace fossils include rare Ophiomorpha. Thinner beds (<40 cm) often lack trace fossils. Shc beds are tabular and can be traced laterally for hundreds of meters [34] . This lithofacies interfingers with the massive sandstone (Sm), massive pebbly sandstone (Spm), and occasionally massive mudrock (Mm) lithofacies ( Fig. 11B ).

Vertical scale = 1 m. (A) exposure of the base of section 1, including Bonebed 1 (at base of cliff) and Bonebed 2 (near top of cliff). (B) exposure of section 2 at Bonebed 3 (near top of photo). (C) Exposure of section 2 at Bonebed 4 (in upper third of cliff). (D) exposure of Bonebed 2 (at base of cliff) in section 1. Solid lines denote sharp contacts, and dashed lines denote gradational contacts. Abbreviations: Mld, laminated diatomite; Mm, massive mudrock; Mpm, massive pebbly mudrock; Shc, hummocky cross-stratified sandstone; Sm, massive sandstone; Spm, massive pebbly sandstone; Scm, Santa Cruz Mudstone.

Spm units are generally tabular with basal erosional surfaces that may be sharp, gradational, or a combination of both. In one case (Bonebed 6), this lithofacies is developed below a complex phosphatic hardground with multiple erosional surfaces preserved within a few tens of centimeters (vertically). The Spm lithofacies is typified by gradational upper contacts (pebbles and bioclasts become less common and smaller up section). This lithofacies interfingers with the hummocky-cross stratified sandstone (Shc) lithofacies, massive sandstone (Sm), and occasionally the massive mudrock (Mm) lithofacies described below ( Fig. 11A–C ). It occurs within laterally extensive (up to several km) bonebeds (Bonebeds 1, 4, 5, and 6), and constitutes the major lithofacies of bonebeds with sand-size matrix (some bonebeds occur within mudrock facies).

Trace fossils are abundant in this lithofacies. Burrows of Ophiomorpha (vertical, 3–5 cm wide, tube-shaped, probable crustacean burrows; [78] ) extending downward as much as 3 meters below this facies are often filled with phosphatic pebbles and bioclastic debris identical to that preserved in overlying bonebeds. Erosional surfaces within this facies may contain similar small flask-shaped clam borings (Gastrochaenolites; 1–4 cm deep, flask-shaped borings from endolithic bivalves; [79] ), and Gastrochaenolites and Trypanites (>1 cm wide subcylindrical borings; [79] ) borings may be present on terrigenous and phosphatic clasts (but not bones).

Phosphatic mollusk steinkerns, phosphatized crustacean remains, and crustacean-bearing nodules comprise a large fraction of the phosphatic nodules. Vertebrate material is abundant and includes fossils of sharks (teeth, calcified cartilage), fish (bones), birds (bones), and marine mammals (bones and teeth). Calcareous mollusk shells are rare, but mollusk steinkerns comprising articulated bivalves or gastropods lacking original shell material are abundant. Less commonly, this lithofacies contains larger phosphatic nodules with a bioclastic framework of disarticulated mollusks, similar in fabric to shelly bioclastic units in underlying strata. Pebble- and cobble-sized clasts of phosphate and reworked porcelanitic pebbles and cobbles of the Santa Cruz Mudstone (Bonebed 1 only) occasionally exhibit bivalve borings up to 3 cm long, and 0.5–1.0 cm in external diameter; boring intensity is highest in extraformational Santa Cruz Mudstone clasts.

The massive pebbly sandstone (Spm) lithofacies consists of thin beds of structureless fine-very coarse grained, poorly sorted sandstone with abundant glauconite sand grains and phosphatic components ( Fig. 10A ). Pebble- and rare cobble-size clasts and bioclasts comprising phosphatic nodules, vertebrate elements, and terrigenous lithic clasts (granules to pebbles) are present. Although typically loosely packed, clasts and bioclasts are occasionally densely packed within the β-interval of bonebeds (e.g., Bonebed 5) and more dispersed within the γ-interval.

Six lithofacies were identified in the Purisima Formation using differences in grain size, sorting, sedimentary structures, and ichnofossil content ( Table 3 ; Fig. 10 ). Three sandstone lithofacies (massive pebbly, massive, and hummocky cross-stratified sandstone), and three mudrock lithofacies (massive pebbly mudrock, massive mudrock, and laminated diatomite) are present. Some of these are similar to those lithofacies identified by Norris [34] .

In summary, the Purisima Formation represents an initial shallowing, after uplift, deformation, and lithification of the underlying Santa Cruz Mudstone, followed by a short transgression (TST). This was followed by deposition of packages of rock showing basinward shifts in facies (parasequences) bounded by discontinuities representing slight shoreward facies offsets (parasequence boundaries/marine flooding surfaces/bonebeds). These parasequences (highstand systems tract) record increasingly shallower facies, eventually grading into terrestrial deposits.

Section 2 includes at least three parasequences, two of which include marine vertebrate concentrations at their basal parasequence boundaries. Parasequence boundaries are also termed Marine Flooding Surfaces (MFS), and represent shoreward offsets in facies [96] . Section 3 preserves the best example of a parasequence, and is capped by Bonebed 6. Overall, the parasequences within the HST represent successively shallower environments. As previously mentioned, the uppermost section of the Purisima Formation represents nearshore, shoreface, and estuarine environments, and may still represent part of the HST ( Fig. 4 ), as terrestrial Pleistocene Aromas Sands appear to conformably overlie the Purisima Formation [35] . Admittedly, this poorly exposed section is not well-studied.

Although only a single vertical section exists, the depositional history of the Santa Cruz section of the Purisima Formation can be explained within a sequence stratigraphic context, as discontinuity bounded units are evident within the Purisima Formation ( Fig. 4 ). The base of section 1 represents a significant shallowing relative to the offshore depositional setting of the Santa Cruz Mudstone [66] . Because of the large basinward offset in depositional setting, this can be interpreted as a ‘forced regression’ [96] . Additionally, tectonic deformation of the Santa Cruz Mudstone prior to Purisima Formation deposition [66] , in concert with the relative change in depositional setting, suggests that a ‘forced regression’ may have been caused by uplift of the basin floor prior to (or during) the depositional hiatus that formed Bonebed 1. Bonebed 1 is identified as a sequence boundary. The next 10 meters of section represents a gradual transition to deeper water sedimentation in the change from the massive sandstone to massive mudrock and eventually laminated diatomite lithofacies ( Fig. 4A ). Because this section represents a gradual transgression overlying a sequence boundary, it is identified as a thin Transgressive Systems Tract (TST). Bonebed 2, present three meters above the base, may represent a distal portion of a transgressive surface of erosion; due to the uncertainty of this feature, the section between Bonebed 1 and 2 is not identified as a Lowstand Systems Tract (LST), and instead assigned to the TST. For example, although the LST in the sequence stratigraphic model of Van Wagoner et al. [96] is bounded below by the sequence boundary and transgressive surface of erosion above, the transgressive surface of erosion may in fact be telescoped with the sequence boundary [2] . Thus, Bonebed 1 may include both the sequence boundary and transgressive surface of erosion, and perhaps the LST is not preserved within the Purisima Formation. The rest of section 1 is difficult to subdivide, but represents gradual shallowing. The unnumbered bonebeds in the upper part of section 1 may represent marine flooding surfaces at the base of parasequences (which typifies sections 2 and 3). Altogether, above the TST, the rest of the Purisima Formation represents a stacked series of shallowing-upward parasequences with bonebeds at their basal marine flooding surfaces, and can all be identified as the Highstand Systems Tract (HST; Fig. 4 ).

Section 3 ( Fig. 4C ) is approximately 30 meters thick, and occurs in the vicinity of Capitola, CA. This section exhibits a basal hummocky cross-stratified sandstone that is truncated by Bonebed 5, which in turn is overlain by massive sandstone grading upward into massive siltstone. This in turn grades back into massive sandstone that becomes increasingly fossiliferous upsection. This massive sandstone grades into hummocky cross-bedded sandstone showing progressive decrease in bioturbation and increase in the thickness of mollusk fossil concentrations at the base. This in turn is capped by Bonebed 6, which is overlain by massive sandstone.

Section 2 (4B) is approximately 30 meters thick, and occurs between Santa Cruz and Capitola, CA. It includes a 10 m-thick monotonous section of massive sandstone at its base, although several outcrops are separated (by incised stream valleys) and it is unclear how many meters of section are missing. This is overlain by massive siltstone grading upward into massive sandstone overlain by hummocky cross-stratified sandstone, and topped with a thin bonebed (Bonebed 3) overlain by massive siltstone. This siltstone is overlain by another bonebed (Bonebed 4). The overlying massive siltstone above the bonebed includes several 0.5–1.0 m-thick fining-upward beds with very fine sand and occasional shell lags at the base, with sand and mollusk concentration increasing upward.

Section 1 (4A) is nearly 50 meters thick, and is located southwest of the city of Santa Cruz, CA. Section 1 includes the basal erosional unconformity of the Purisima Formation, which is mantled by massive glauconitic sandstone and bonebed debris (Bonebed 1). This grades upwards into massive diatomite that includes another bonebed (Bonebed 2), which is in turn overlain by laminated diatomite. Bioturbated diatomite overlies the laminated diatomite, and the rest of section 1 records a gradual increase in grain size from bioturbated diatomite to massive siltstone, and a 25 meters thick, monotonous section of massive sandstone. Two as-yet unstudied bonebeds occur within massively bedded diatomite and sandstone above Bonebed 2 in this section.

The vertical distribution of lithofacies within the Santa Cruz section of the Purisima Formation allows interpretation of its depositional history in the context of successive depositional environments. Four contiguous exposures of the Santa Cruz section of the Purisima Formation exist ( Fig. 1C ). The uppermost section predominantly represents upper shoreface, nearshore, foreshore, and estuarine depositional settings [34] , and due to its lack of vertebrate fossils, was not included in this study. The other three sections are referred to herein as section 1, section 2, and section 3 ( Figs. 2 , 4 ).

Vertebrate skeletal material is abundant within subintervals β 0 , β 2 , and β 3 . Marine mammal bones, bone fragments, and bone pebbles are the most common vertebrate elements. Bones and teeth frequently exhibit abrasion Stage 1–2, commonly occurring as bone pebbles. Many bones are fragmented, and bone shards are common. Most bones in subintervals β 0 and β 3 are completely blackened and phosphatized, occasionally with adhering phosphatic matrix (Stage 2A–C); bones in subinterval β 2 are occasionally phosphatized (Stage 1–2A) and bone surfaces near the upper erosional surface show a phosphatized interval (Stage 1A). Abundant articulated and associated skeletons (Articulation stages 0–3) occur within subinterval β 2 . Vertebrate skeletal elements in subintervals β 0 and β 3 are typically less than 10 cm in greatest dimension (either as nearly complete elements or fragments thereof). They range in size from small (<5 mm) shark, pinniped, and cetacean teeth and Cetorhinus gill rakers to individual bones (mysticete skulls, mandibles) up to 2.5 meters long.

Subinterval β 3 is a 50–70 cm thick massively bedded sandstone with abundant phosphatic material. Calcareous material is absent. The majority of clasts and bioclasts are in the pebble size range. Bonebed debris (phosphate nodules, bone fragments, teeth) mantles the lower surface, and decreases in abundance/packing upwards through the γ-interval. Bonebed 6 can be traced laterally for 1.1 km; blocks of this bonebed occur as boulders in Pleistocene terrace deposits 2.5 km further to the southwest, suggesting nearly 4 km of exposure.

Subinterval β 2 is 10–20 cm thick bioclastic sandstone containing abundant densely-loosely packed mollusk shells. Vertebrate remains are well preserved, and include articulated (and associated) cetacean skeletons. Molluscan shells decrease in abundance towards the top, and mantle the lower erosional surface. Calcareous material can locally be rare or non-existent. Phosphatic nodules are slightly more common than in subinterval β 1 . This unit is also truncated by a sharp erosional surface (the lower contact of subinterval β 3 ), with 20–30 cm of relief. Subinterval β 2 is typically about 10 cm thick, but in some cases large bones protrude more than 10 cm above the base of subinterval β 3 . Abundant endolithic bivalve borings (Gastrochaenolites; 1–3 cm deep) extend down into this surface. This surface is phosphatized as a phosphatic rind or crust.

The α-interval includes hummocky-cross stratified sandstone with bonebed debris-infilled Ophiomorpha burrows at its base that transitions to massively bedded, bioturbated, matrix-supported bonebed conglomerate at the top. Bonebed debris (mollusk shells, phosphate pebbles, and crustacean and vertebrate skeletal elements) increases in abundance in the upper 10–20 cm (which marks the base of the β-interval). Subinterval β 0 is truncated by a generally planar (but wavy on the centimeter scale) erosional surface.

The matrix of Bonebed 6 is a fine to medium grained sandstone with abundant glauconite grains. Phosphatic pebbles are the most abundant coarse clasts, while cobble-size phosphate nodules are less common; the non-bioclastic component of Bonebed 6 corresponds to the massive pebbly sandstone (Spm) lithofacies. Terrigenous clasts are rare. Bones, bone fragments and pebbles, vertebrate teeth, and calcified cartilage are less common than phosphate clasts. While calcareous mollusk shells are abundant in subintervals β 0 –β 1 , calcareous skeletal material occurs only rarely within subinterval β 2 and is not present in subinterval β 3 . In subinterval β 0 , mollusk shells often exhibit phosphatized internal and external molds, whereas in subintervals β 2 and β 3 , many phosphate clasts are steinkerns and external molds. Some nodules include molds of disarticulated, imbricated, and nested bivalve shells. Rare phosphatic nodules exhibit flask-shaped endolithic bivalve borings (Gastrochaenolites) and narrower, subcylindrical borings (Trypanites). Within 50–70 cm of the top of the α-interval, hummocky cross-stratified sandstone gives way to massively bedded sandstone exhibiting extensive burrow mottling. The sandstone within and above Bonebed 6 is massively bedded and pervasively bioturbated.

This bonebed (UCMP locality V99869; Fig. 13F , 14F ) is a phosphate pebble and bone rich cemented hardground located near the top of section 3 ( Fig. 4 ). Bonebed 6 is the most sedimentologically, diagenetically, and taphonomically complex bonebed in the Purisima Formation. It marks a change from hummocky cross-stratified sandstone (Shc) lithofacies below to massive sandstone (Sm) lithofacies above. Bonebed 6 includes three sharp erosional surfaces; the cemented portion of this bonebed was subdivided into four units, Layers A–D, by Friede [88] . Field examination of this bonebed recognized the same units, although while Friede [88] used diagenetic boundaries (i.e. the margins of the cemented zone), this study recognizes four units based on surfaces preserved within. All three surfaces are preserved within the β-interval, and the β-interval includes Friede’s [88] Layer B and Layer C, the top of Layer A, and the bottom of Layer D. Instead, the three surfaces divide the β-interval into four subintervals, termed subintervals β 0 , β 1 , β 2 , and β 3 ( Fig. 14F ) The cemented portion of Bonebed 6 is generally 20–30 cm thick, and localized concretionary tongues may protrude up to 30 cm above or below. These tongues typically form above or below baleen whale skulls or skeletons preserved in subinterval B 2 . Bonebed debris extends for 30–70 cm above the γ-interval, and 50 cm below. Within the α-interval and 1–2 meters below Bonebed 6, abundant Ophiomorpha burrows within hummocky cross-stratified sandstone and other burrows infilled with bonebed debris are present. In total, the bonebed (intervals α-γ) is roughly 2 meters thick.

Vertebrate skeletal elements are most commonly abraded bone pebbles. More complete bones typically include partial cetacean ribs and vertebrae. Teeth in Bonebed 5 typically exhibit Stage 1 abrasion with roots typically more abraded than crowns. Bones typically exhibit abrasion Stage 2–3, although unabraded or lightly abraded (Stages 0–1) bones are less common. Elongate vertebrate elements are frequently fragmented with fragments of larger bones common. The largest vertebrate bioclasts occur within the β-interval. No vertebrate elements are articulated or associated. Bones and teeth are typically heavily phosphatized (Stage 2A, 2C). Vertebrate bioclasts range in size from small elasmobranch teeth and fish bones (<5 mm) to medium sized bones and bone fragments (10–15 cm).

Bonebed 5 (UCMP locality V99866; Fig. 13E , 14E ) is a clast-supported conglomerate located 2 meters above the base of Section 3 ( Fig. 4 ) at a contact between interfingering massive siltstone (Mm) and hummocky cross-stratified sandstone (Shc) below and massive fine-grained sandstone (Sm) above. The underlying siltstone contains flat, tabular concretions 10–20 cm below the contact. The bonebed material mantles a sharp, irregular lower contact with 10–20 cm of local relief; laterally the bonebed instead mantles hummocky cross-stratified sandstone where it has eroded completely through the thin massive siltstone. Abundant wide borings identifiable as Gastrochaenolites (∼3–10 cm wide) extend 20–100 cm below the β-interval and are filled with bonebed debris. Rarely, the bottom of these borings house an in situ pholad clam nearly as wide as the structure; the boring and interior of the pholad clam are filled with bonebed debris. Bonebed matrix includes very poorly sorted fine to very coarse sandstone. Granule, pebble, and (rarely) cobble-size clasts comprise predominantly phosphatic nodules, steinkerns, and external molds of mollusks. Terrigenous clasts (mostly pebbles) are also abundant. Rare large tabular disc-shaped calcareous siltstone cobble-sized nodules (2–4 cm thick, up to 20 cm wide) occur within the β-interval. Some large phosphatic nodules include monospecific clusters of gastropods (Nassarius) and bivalves (Anadara) retaining original calcareous shell material; similar clusters occur in the underlying siltstone. Rare large mollusk-shell bearing calcareous sandstone cobble-size nodules occur as well. Fragments and disarticulated portions of crustacean skeletons (Callianassa and Cancer leg segments and chelae) are abundant, and phosphatic nodules frequently contain partial and articulated crustacean skeletons. Many of these nodules are cylindrical, 2–4 cm wide, up to 10 cm long, and include clusters of lozenge-shaped fecal pellets and occasionally pincers and partial skeletons of Callianassa. Clasts and bioclasts are largest and densely packed (clast supported) within the β-interval directly above the irregular lower surface. The α-interval lacks dispersed vertebrate elements or clasts, and bonebed debris only occurs within the burrows described above. The γ-interval is characterized by gradational upward decreases in clast/bioclast packing (matrix supported, or dispersed) and size (i.e. fining upwards) from that of the β-interval. Pebbles and cobbles occur most frequently within the β-interval, within 10–15 cm of the basal surface or in contact with it. Large pebbles and cobbles are concordantly (and occasionally obliquely) oriented. Some parts of the β-interval are cemented with calcium carbonate. The basal horizon in some places cross-cuts the calcareous siltstone nodule-bearing stratum. Bonebed 5 can only be traced laterally for 50 m.

Cetacean bones and bone fragments are the most common vertebrate skeletal elements. Pinniped bones and teeth, shark teeth, calcified elasmobranch cartilage, fish bones, and bird bones are less common. Bones typically exhibit Stage 0 abrasion with Stage 1–2 less common. Heavily phosphatized bones (Stage 2) are rare, but slightly phosphatized bones abundant (Stage 1) and many bones exhibit adhering phosphatic matrix (Stage 0B–C and Stage 1B–C). The largest vertebrate bones (pinniped and cetacean bones) occur within the β-interval. Vertebrate bones are never articulated, but some partial, disarticulated skeletons (comprising only a few bones; Disarticulation Stage 3) have been found in Bonebed 4. Vertebrate skeletal elements range from small shark and pinniped teeth and Cetorhinus gill rakers under 5 mm, to complete baleen whale bones up to 2 meters in length.

Bonebed 4 maintains a relatively constant thickness over its lateral extent, but locally shows some thickness variation, and the β-interval pinches and swells from 10–40 cm in thickness. The lower contact is gradational, marking a transition from siltstone to sandstone (and a gradual increase in bioclast packing within the α-interval), and includes many Ophiomorpha burrows infilled with sandstone and bonebed debris, extending 1 meter below the β-interval. The upper contact is also gradational, marking a decrease in grain size and bioclast packing upwards within the γ-interval. Clast and bioclast packing is highest within the β-interval; coarse material is rare within the γ-interval. Clasts and bioclasts are less abundant in the α-interval, and increase in abundance towards the β-interval. The coarse material within the β-interval is mostly matrix supported (loosely packed), and there are localized areas of clast-support (dense packing). Bonebed 4 can be traced laterally for 2 km. To the northeast, Bonebed 4 transitions to a 20–25 cm thick bioclastic bed with horizontally oriented mollusks, and lacking much bioclastic material in the α- and γ-intervals. The β-interval is densely packed and exhibits a sharp planar base, although no sharp sedimentary contact exists.

Bonebed 4 (UCMP locality V6875; Fig. 13D , 14D ), located near the top of Section 2 ( Fig. 4 ), is a tabular unit (10–40 cm thick) containing abundant mollusk shells, large phosphate nodules, and well-preserved vertebrate skeletal material. It is underlain by massive siltstone (Mm) and its matrix comprises very fine to fine grained massive pebbly sandstone (Spm). Gravel-size clasts include abundant mollusk shells (bivalves, gastropods), crustacean skeletal elements, phosphatic nodules, terrigenous pebbles, marine mammal bones, bone fragments, rare bird bones, shark teeth, fish bones, and calcified cartilage. Phosphatic nodules often include steinkerns, external molds, and abundant cylindrical nodules with fecal pellets and partial Callianassa skeletons inside. Many phosphate nodules include original calcareous mollusk skeletal elements; some nodules are up to 15–25 cm wide and contain abundant densely packed mollusk shells.

Cetacean bones are the most common vertebrate elements. Shark teeth, bird bones, and pinniped bones are slightly less common. Vertebrate skeletal elements are sparse, usually isolated, and typically unabraded (Stage 0), or less commonly slightly abraded (Stage 1). No bones exhibit evidence of phosphatization. Vertebrate skeletal elements range in size from small teeth and gill rakers (<5 mm) to complete mysticete bones and skeletons over 1 meter long. A few articulated and associated mysticete skeletons are known from Bonebed 3.

This bonebed (UCMP locality V90042; Fig. 13C , 14C ), located in the uppermost portion of Section 2 ( Fig. 4 ), is a 5–15 cm thick, laterally extensive tabular shell-rich interval with occasional vertebrate skeletal elements. This concentration occurs within massively bedded, burrow mottled sandstone (Sm) lithofacies. The base of a 1 meter thick bed of large-scale hummocky-cross stratified sandstone (Shc) with an erosional scour at its base is present 1–1.5 meters below this stratum. This underlying bed becomes increasingly more bioturbated toward its top, transitioning into massively bedded sandstone. Convoluted bedding and occasional ball-and-pillow structures occur near the top of the non-bioturbated interval (50–70 cm below Bonebed 3). Upper and lower contacts of Bonebed 3 are gradational and demarcated by a gradual decrease in mollusk shells above and below the β-interval. The α- and γ-intervals are less than 10 cm thick. Mollusk shells are the most abundant coarse material within Bonebed 3; terrigenous clasts and phosphatic nodules are rare (always pebble sized), and vertebrate material is slightly less abundant than terrigenous clasts. Mollusk shells are loosely packed, consisting mostly of disarticulated bivalve shells generally oriented concordant and oblique to bedding and rarely nested. Bones, teeth, and pebbles always occur within the β-interval. Large elements (i.e. skeletons, skulls) extend above the β-interval into the γ-interval, but not below into the α-interval. Bonebed 3 extends laterally for 2.4 km.

Cetacean bones and bone fragments constitute the most abundant vertebrate element. Pinniped bones are common, while shark teeth, fish bones, and bird bones less common. Abrasion of these elements ranges from Stage 0–2, but most are unabraded (Stage 0). The majority of bones are preserved as fragments. Most bones appear phosphatized; many of these exhibit phosphatized interstitial matrix and adhering phosphatic nodules. The majority of vertebrate skeletal elements are concentrated within the β-interval, as are the larger elements. Bioclasts and clasts are loosely to densely packed within the β-interval, and increasingly more dispersed in the α- and γ-intervals. No articulated or associated specimens are recorded from this bonebed. Sizes of vertebrate bioclasts range from bone fragments and teeth less than 1 cm to partial cetacean bones up to 40 cm long.

Bonebed 2 (UCMP locality V99877) is located 3 meters above the base of the Purisima Formation ( Fig. 4 , 13B , 14B ) within a massively bedded, pervasively bioturbated and burrow-mottled diatomite. The matrix of this bonebed corresponds to the massive pebbly mudrock (Mpm) lithofacies. No visible change in lithology occurs within the bonebed or within a meter above or below. The bonebed is tabular with gradational upper and lower contacts. The majority of clasts and bioclasts are concentrated in the β-interval, and bioclast packing decreases above (γ-interval) and below (α-interval). Most large pebble- and cobble-sized clasts and large bioclasts occur in the β-interval; clast/bioclast size decreases upwards and downwards from the β-interval. The β-interval pinches and swells, and is generally patchy; clasts and bioclasts are typically floating. (loosely packed, but occasionally densely packed). The α- and γ-intervals are similar in their architecture and contain dispersed clasts and bioclasts that often occur as localized clumps or pods (including pebble-size clasts/bioclasts) oriented vertically to oblique (sensu [6] ). Occasionally, these pods (sensu [6] ) of bonebed debris fill Ophiomorpha burrows. These clast-bioclast pods are often densely packed and clast-supported; some pods occur up to 2.5 meters below the bonebed. One meter below Bonebed 2, there is a sharp, irregular contact between massive glauconitic sandstone below and massive diatomite above. This contact in some exposures is mantled by debris similar to that of Bonebed 2, in some places appearing as a thinner, discontinuous bonebed. Where exposed in plan view, bonebed debris at this horizon appears to be confined to horizontal connected burrows forming a polygonal pattern. Clasts are primarily phosphatic pebbles and cobbles in the 1–5 cm size range with rare terrigenous pebbles. Most phosphatic clasts are black, well-rounded nodules. Bonebed 2 can be traced laterally for 0.5 km.

Postcranial bones (complete and fragmented) of cetaceans, sirenians, and pinnipeds are common, with cetacean and sirenian ribs the most frequently encountered elements. Shark teeth and fish bones are less common, and mammal teeth and bird bones are rare. Most bones exhibit Stage 1 abrasion, and some bones exhibit Stage 2–3; few bones are unabraded (Stage 0). Most bones exhibit some fragmentation or fracturing. No bones or teeth exhibit any phosphatization. Most vertebrate skeletal elements are within 10–15 cm of the lower contact (within the β-interval), and most large bones are in contact with or in close proximity to the basal surface. Within the β-interval, no articulated remains occur. Associated remains are extremely rare in this interval (one pair of associated walrus tusks were found from this lower zone). Articulated and associated skeletons occasionally occur 30–50 cm above the base within the γ-interval, along with well-preserved (unabraded, unfragmented) isolated vertebrate skeletal elements ranging in size from small teeth and bone fragments (<1 cm wide) to complete mysticete ribs up to 1 meter long.

Bonebed 1 (UCMP locality V99875) is located above the unconformable contact between the Santa Cruz Mudstone and overlying Purisima Formation ( Fig. 4 , 13A , 14A ). The Santa Cruz Mudstone below the contact consists of interbedded couplets of unconsolidated siltstone and silicified porcelanite. The lower contact of this bonebed is highly irregular, with 20 cm of relief; many burrows (Ophiomorpha) extend up to 2.5 meters below the contact and are infilled with glauconitic sandstone and coarse bonebed debris. The matrix lithology of the bonebed is primarily massive (and burrow-mottled) medium grained, glauconite-rich sandstone (Spm) with occasional granules. Coarse clasts are extraformational porcelanite pebbles and cobbles from the Santa Cruz Mudstone and extrabasinal terrigenous pebbles (and rare cobbles). Cobbles of the Santa Cruz Mudstone often exhibit flask-shaped clam borings identified as Gastrochaenolites (circular aperture with flask-shaped cross section, 1–3 cm long) on all sides, in addition to conchoidal fracturing of many surfaces. Bonebed 1 is an approximately 50 cm thick matrix-supported conglomerate. Coarse clasts and bioclasts are most densely concentrated (matrix supported or loosely packed) in the basal 20 cm thick β-interval, and are increasingly more dispersed within the overlying γ-interval. Cobbles and large bioclasts are almost always in the lower 20 cm and occasionally in contact with the truncated Santa Cruz Mudstone. Lithic gravel and bioclast size decreases upwards. The thickness of this generally tabular bonebed is maintained laterally, and it can be traced laterally along the shoreline for 0.7 km.

Six bonebeds from the Santa Cruz section of the Purisima Formation were studied in detail ( Table 4 ). Several other bonebeds were observed – two in the middle of section 1, and a third within section 3, several meters above Bonebed 5. Bonebeds 1, 4, 5, and 6 are exposures of the massive pebbly sandstone (Spm) lithofacies, whereas Bonebed 2 is an exposure of the massive pebbly mudrock (Mpm) lithofacies, and Bonebed 3 an exposure of the massive sandstone (Sm) lithofacies. These bonebeds occur in sections 1, 2, and 3 ( Fig. 4 ).

When the relative abundance of each tissue type is plotted by lithofacies, several trends are apparent ( Fig. 24 ). Bones are the most abundant type of element in all lithofacies. Teeth are most abundant in bonebeds and in proximal settings. Calcified cartilage occurs only in the massive pebbly sandstone (Spm) and massive mudrock (Mm) lithofacies, while earbones are most common in the massive pebbly sandstone (Spm) and less abundant in the massive sandstone (Sm) and massive mudrock (Mm) lithofacies. This suggests that while bones are the most commonly fossilized elements, teeth, earbones, and calcified cartilage are best represented within bonebed lithofacies and massively bedded offshore sediments. Teeth in particular are only well-represented in proximal settings linked with frequent reworking and disturbance (Shc), slow sedimentation and intermittent reworking (Sm), and periods of widespread reworking of shelf sediments during bonebed formation (massive pebbly sandstone and mudrock; Spm and Mpm, respectively).

Earbones were least affected by phosphatization (57.7% Stage 0A), while calcified cartilage was most frequently phosphatized (28.5% Stage 0A; Fig. 23 ). The greatest variation in phosphatization stage occurs in bones and earbones, while phosphatized cartilaginous elements are typically Stage 2A (57.1%), and phosphatized teeth are typically Stage 1A (20.9%) or 2A (31.4%). Teeth rarely have adhering phosphatic matrix (Stage XB–C), while bones often do; calcified cartilage and earbones exhibit adhering nodules less often. Intermediate mineralization (Stage 1) is rare in calcified cartilage, suggesting that this tissue type becomes phosphatized rapidly.

To address possible bias between different types of biomineralized tissues, skeletal elements from this study were grouped into four broad groups: bones, earbones (restricted to cetaceans), teeth (absent in mysticetes and birds), and calcified cartilage (restricted to chondrichthyes), and compared in terms of abrasion, fragmentation, phosphatization, and polish ( Fig. 22 ). Because these elements vary widely in density and hardness, it is reasonable to hypothesize that they may be subject to preservational bias. Frequency of abrasion in these groups is strongly correlated with the hardness of biomineralized tissue. Calcified cartilage and bones are most frequently abraded (71.5% and 70.7%, respectively; Stage 1–2), and teeth were the least commonly abraded (26.6%), with earbones displaying intermediate abrasion (47.9%). Little difference in fragmentation was evident between bones (52.6% fragmented) and earbones (47.9% fragmented), while cartilage was the most fragmented (61.9%), and teeth were the least affected (33.4%).

Differential phosphatization among these different taxa has numerous implications. While phosphatized elements may be more susceptible to fragmentation due to their increased brittleness, they are probably less sensitive to abrasion than ‘fresh’ elements. Furthermore, phosphatization often increases the density of the element, decreasing the likelihood (or slowing) of transport as bedload, and possibly exaggerating hydrodynamic sorting. Phosphatization may be a mechanism for increasing the preservation potential of an element (prefossilization; [100] , [101] ) as phosphatized bones appear more resistant to abrasion, with the addition of phosphatic nodules on skeletal elements further inhibiting their taphonomic destruction. Preferential phosphatization of certain taxa (odontocetes and bony fish) may result in a taxonomically skewed assemblage biased towards these taxa by exaggerating hydraulic sorting and increasing the durability of their phosphatized remains.

Phosphatization affects the skeletal elements of certain taxa differently ( Fig. 21 ). Bony fish elements are most frequently phosphatized (78%, non stage 0A), while sharks, pinnipeds and odontocetes also share relatively high frequencies of phosphatization (54–59%); bird, mysticete, and sirenian elements have lower frequencies (25–41%). This indicates bias towards the phosphatization of fish, sharks, and small marine mammals. The phosphatized sample of most groups (with the exception of mysticetes) contains a large proportion of blackened, mineralized elements lacking nodules (Stage 2A). Trends regarding the occurrence of nodules are also apparent: birds, sharks, sirenians, indeterminate mammal bones, and mysticetes rarely exhibit adhering phosphatic nodules (Stage XB or XC), while bony fish and odontocetes include a large number of Stage 2C specimens with large overgrowths. Phosphatization only operates within a thin zone below the sediment-water interface, and thus is biased towards small skeletal elements [99] , potentially explaining the limited effect on mysticete bones. Pinnipeds exhibit an intermediate amount of nodule-bearing specimens.

Fragmentation follows a slightly different pattern amongst taxa ( Fig. 18B ). Fragmentation is most frequent in indeterminate mammal bones (83%), sirenians (62%), bony fish (63%), and mysticetes (45%). Skeletal elements of odontocetes, pinnipeds, and sharks all exhibit lower rates of fragmentation (27–33%); most intriguingly, only 22% of bird bones are fragmented. Sirenians and bony fish have extremely dense postcranial bones (pachyosteosclerotic and avascular bone, respectively), and accordingly may fragment due to their higher brittleness; likewise, the fragmented nature of indeterminate mammal bones is the reason why they are unidentifiable. A lower incidence of fragmentation for bird bones may be related to their lower mass and density relative to other vertebrate skeletal elements. Perhaps a bone with lower mass is less likely to suffer an impact with sufficient force to incur fracturing. Similarly, bird bones are only rarely abraded or polished, and in general show little physical taphonomic modification. It is unclear whether this reflects a predominance of “fresh” elements and relatively quick taphonomic degredation, or a genuine resistance to damage due to the lower mass and density of bird bones. Actualistic tumbling experiments using bird bones may address this problem.

With the exception of indeterminate mammal bones, cetacean skeletal remains possess the highest degrees of abrasion (68.5% Stage 1–2, Mysticeti and Odontoceti combined). Abraded mammal bone pebbles are abundant (93.4% Stage 1–2), and the majority of these, although too incomplete to identify, are probably cetacean in origin based upon size and histology. Elasmobranch and bird elements are the least affected by abrasion (33.2 and 33.8% respectively, Stage 1–2), whereas pinniped and bony fish elements exhibit an intermediate (53.6% and 57.2% respectively, Stage 1–2) frequency of abrasion ( Fig. 18A ). This difference is probably due to the relatively robust nature of shark teeth, while cetacean bones are osteoporotic. In the case of diagnostic cetacean cranial elements, these may be abraded into bone pebbles past the point of identification, and smaller odontocetes are predicted to be more susceptible to taphonomic destruction than large bodied odontocetes with sturdier bones. Although possibly due to bioerosion, the roots of shark teeth in Bonebed 1 are almost always missing (or incomplete) and superficially appear abraded. The enameloid crowns of these teeth are usually pristine and intact (although occasionally fragmented). Sometimes the majority of the osteodentine ‘core’ (except for a residue of osteodentine remnants) is missing, suggesting that this process only affected the osteodentine and not the enameloid; it is possible that the roots are bioeroded by microborings (see [98] ). This has not been observed in any other taxa or localities, and represents a strong bias against the preservation of shark teeth within a single stratum.

Phosphatization shows a slightly different trend than other taphonomic modifications and is rare in fossils from the massive sandstone (Sm) and massive mudrock (Mm) lithofacies, and absent in the laminated diatomite (Mld) lithofacies. Phosphatized elements are common in the massive pebbly sandstone (Spm), massive pebbly mudrock (Mpm), and hummocky cross-stratified sandstone (Shc; Fig. 17 ). Furthermore, stage 2 phosphatization is less common in the massive pebbly sandstone (Spm) than in the hummocky cross-stratified sandstone (Shc) or massive pebbly mudrock (Mpm). In contrast, phosphatized vertebrate fossils of the massive pebbly mudrock lithofacies (Mpm) are nearly all stage 2A and 2B. The proportion of stage 1 and 2 phosphatization is similar in the massive sandstone (Sm) and mudrock (Mm) lithofacies. Large phosphatic (Stage XC) nodules are absent from the massive pebbly mudrock (Mpm), although present in both the hummocky cross-stratified sandstone (Shc) and massive pebbly sandstone (Spm) lithofacies ( Fig. 17 ). Phosphatization thus appears to be correlated with lithofacies where erosion is implicit in its mode of formation.

Polish is most abundant within the massive pebbly mudrock (Mpm; 42.8%), absent within the laminated diatomite (Mdl; 0%), and low within the massive sandstone (Sm; 5.9%) and mudrock (Mm; 4%) lithofacies ( Fig. 15C ). The hummocky cross-stratified sandstone (Shc) and massive pebbly sandstone (Spm) exhibit intermediate percentages (17.8% and 25.7%, respectively) of polished elements. Polish parallels abrasion and fragmentation in terms of abundance by lithofacies ( Fig. 15C ).

Fragmentation follows a pattern similar to abrasion ( Fig. 15B ). The most abundantly fragmented vertebrate samples occur in the massive pebbly sandstone (Spm) and mudrock (Mpm) lithofacies (48.2% and 48.2%, respectively). Vertebrates are not fragmented within the laminated diatomite (Mdl), while the massive mudrock (Mm) exhibits a low percentage of fragmented remains (18%); slightly higher percentages of fragmented elements characterize the hummocky cross-stratified sandstone (Shc) and massive sandstone (Sm; 32.1% and 30.9%, respectively). Generally, the degree of fragmentation decreases offshore ( Fig. 15B ).

Articulated and associated remains are generally rare in all lithofacies ( Fig. 16 ). Articulation is virtually absent in the massive pebbly sandstone (Spm; 98.6% Stage 4), massive pebbly mudrock (Mpm; 98.2% Stage 4), hummocky cross-stratified sandstone (Shc; 96.4% Stage 4), and laminated diatomite (Mdl; 100% Stage 4). However, in the massive sandstone (Sm) and massive mudrock (Mm) lithofacies, articulated and associated elements (articulation stage 1–3) are more common (14.3 and 18.0%, Sm and Mm respectively; stages 1–3). Lack of articulation in laminated diatomite may reflect a sampling artifact, given the low sample size from this lithofacies (n = 6; Table 2 ); this lithofacies is broadly similar to laminated mudrocks that have produced Mesozoic marine vertebrate lagerstätten (e.g., [97] ), and would be predicted to exhibit the highest frequency of articulation. Future fossil discoveries are necessary to test this prediction.

Abrasion is most extensively developed within the massive pebbly sandstone (Spm; 58.2%, Stage 1–2) and mudrock (Mpm; 71.4%, Stage 1–2) lithofacies; the massive mudrock (Mm; 34.0%, Stage 1–2) and laminated diatomite (Mld; 16.6%, Stage 1) exhibit the least abraded skeletal elements, while the hummocky cross-stratified sandstone (Shc; 42.8%, Stage 1–2) and massive sandstone (Sm; 47.6%, Stage 1–2) samples are intermediate between the two extremes ( Fig. 15A ). This correlates well with bonebed-related lithofacies forming under conditions of extensive erosion during depositional hiatus, and documents an offshore decrease in abrasion amongst the non-bonebed lithofacies.

Bonebeds form from submarine erosion, depositional hiatus, or a combination of the two ( Fig. 11 ). The abundance of phosphatic debris and phosphatized skeletal elements indicates periods of low net sedimentation promoting conditions conducive for phosphogenesis, corroborated by the presence of glauconite. Erosion by fair-weather and storm-generated waves resulted in exhumation and redeposition of ‘prefossilized’ vertebrate skeletal elements in the bonebed assemblage. More poorly phosphatized skeletal elements likely represent specimens that experienced lower duration of phosphogenesis. Fragmentation is more abundant within this taphofacies; perhaps weaknesses form during early diagenesis in buried bones and subsequently result in fragmentation during exhumation and transport. The Bonebed taphofacies does not interfinger with other taphofacies, but cross-cuts all other taphofacies.

Taphonomically modified vertebrate skeletal concentrations occur in laterally extensive bonebeds that mark vertical lithofacies offsets. Vertebrate skeletal material from the Bonebed taphofacies exhibits the highest degree of taphonomic modification (abrasion, fragmentation, phosphatization). In rare cases, articulated and associated skeletons occur within bonebeds. A cluster of associated mysticete bones occurs in Bonebed 4, and dozens of articulated skeletons are known from subinterval β 2 of Bonebed 6. Additionally, a single cluster of odontocete vertebrae was observed in subinterval β 3 of Bonebed 6, and a cluster of sirenian bones representing a disarticulated skeleton within Bonebed 1. Most cases of polished skeletal elements occur within this taphofacies.

This taphofacies represents attritional accumulation of vertebrate skeletal material in distal, mud-rich environments. Vertebrate skeletons and elements occur in mudrock and diatomite deposited in low-energy offshore settings by suspension settling of sediment; no evidence of higher-energy traction transport of sediment or skeletal material is present. Vertebrate skeletal material likely remained at the original site of deposition, with many isolated elements shed from floating carcasses. Shark teeth may have accumulated after being shed during feeding. This taphofacies reflects offshore or outer shelf preservation.

Vertebrate skeletal elements are extremely rare, distributed randomly throughout the sediment, and rarely mantle surfaces within taphofacies 3. Vertebrate bones and teeth lack taphonomic modification or polish, are unabraded, complete, and unphosphatized. Bones and teeth are typically isolated. Two partially articulated skeletons, including a mysticete skeleton with preserved baleen and a chemosynthetic mollusk assemblage typical of whale falls occur within this taphofacies (F.A. Perry, unpublished data). Aside from the aforementioned molluscan assemblage, mollusks and crustaceans are absent from this taphofacies. This taphofacies corresponds to the laminated diatomite (Mdl) and massive mudrock lithofacies (Mm), and interfingers with Taphofacies 2.

Taphofacies 2 represents a combination of attritional accumulation of vertebrate hardparts and occasional concentration of skeletal material along storm-generated erosional surfaces. This taphofacies was deposited near and below storm weather wave base, within the shoreface-offshore transition zone. Vertebrate elements shed from drifting carcasses are likely responsible for the majority of isolated elements preserved ‘floating’ in sediment, as low energy environments below storm weather wave base lack sediment transport processes capable of transporting and dissociating bones. As a result, taphofacies 2 exhibits the largest sample of articulated and associated skeletons that remained relatively complete after arrival at the sediment-water interface. Scavengers and bioturbators may cause some of the disarticulation seen in some specimens. Minor phosphatization shows that at least some elements were exhumed by storm erosion and redeposited by hyperpycnal flow, along with ‘fresh’ skeletal elements from the sediment-water interface. During deposition of the massive sandstone (Sm) lithofacies, occasional storm currents concentrated some vertebrate skeletal material into laterally extensive shell beds and pavements. This taphofacies reflects transition zone or middle shelf preservation.

Vertebrate skeletal elements in Taphofacies 2 are rarely modified. In general, evidence of fragmentation and abrasion is sparse, affecting only a minority of specimens. Phosphatization and polish of bones is rare. Taphofacies 2 has the highest frequency of articulated and associated skeletons relative to other taphofacies, and isolated bones and teeth are abundant. In some cases, bones and teeth are concentrated within these mollusk shell pavements and thin shell beds; vertebrate skeletal material exhibiting rare evidence of abrasion or fragmentation is confined to these thin mollusk shell concentrations. Vertebrate skeletal elements occur more frequently within these shell concentrations than in ‘background’ sediment. This taphofacies corresponds to the massive mudrock (Mm) and massive sandstone (Sm) lithofacies, and interfingers with Taphofacies 1 and 3.

The higher degree of taphonomic modification in this taphofacies (relative to Taphofacies 2 and 3) is interpreted as being produced by higher energy conditions related to shoreface settings. The abundance of hummocky cross-stratified sandstone and thick shell beds suggests that this taphofacies represents vertebrate skeletal material preserved within shoreface deposits, above storm weather wave base and in some cases above fair weather wave base. Frequent storm reworking is probably responsible for disarticulating and dissociating skeletons [21] , and exhuming some vertebrate skeletal material from underlying strata (evidenced by occasional phosphatized elements). This taphofacies reflects shoreface or inner shelf preservation.

The distribution and taphonomic condition of fossil vertebrate elements preserved indicates a higher energy environment than that of Taphofacies 2 and 3. Isolated vertebrate elements are most often concentrated in shell beds associated with erosional (or hiatal) surfaces. The abundance of vertebrate skeletal elements mantling erosional surfaces and occasional phosphatization (and adhering phosphatic matrix) indicates many of these bones have been exhumed from the underlying substrate. This is corroborated by the abundance of phosphatic nodules and invertebrates with adhering phosphatic matrix in these beds. Bones and teeth devoid of taphonomic modification may represent skeletal input during minor hiatuses or material yet to experience enough transport and burial/exhumation cycles to produce modification.

Vertebrate skeletal elements in Taphofacies 1 exhibit a range of taphonomic modifications. Vertebrate material is mostly isolated, and associated specimens are rare. Specimens are occasionally fragmented and often abraded. Vertebrate skeletal elements within this taphofacies display a wider range of phosphatization than in Taphofacies 2 and 3, and slightly more skeletal elements display phosphatization. Roughly one-third of these elements are phosphatized or exhibit adhering phosphatic matrix. Phosphatization is not as common as in the Bonebed taphofacies. Specimens preserved within this taphofacies are rarely polished. Vertebrate skeletal elements occur more often within shell concentrations than in the bioclast-poor “background” sediment layers between bioclastic accumulations. This taphofacies occurs within the hummocky cross-stratified sandstone (Shc), and interfingers with Taphofacies 2.

To elucidate taphonomic gradients within vertebrate assemblages four vertebrate taphofacies [9] , [10] were delineated ( Table 5 ). A taphofacies is a body of sedimentary rock “which is distinguished from other vertically and laterally related bodies of rock on the basis of its particular suite of taphonomic properties” ( [8] :227). For the Purisima Formation, taphofacies analysis utilized variation in preservation of vertebrate skeletal elements only. No single taphonomic characteristic (e.g., abrasion) was found to define any single taphofacies ( Table 5 ); thus, combinations of preservational features of vertebrate fossils were used. With the exception of the Bonebed Taphofacies, other taphofacies are designated Taphofacies 1, 2, and 3. The lack of discrete boundaries for any given taphonomic characteristic highlights the gradational nature of marine vertebrate preservation in shelf environments.

Discussion

Bioturbation and Bonebed Architecture This study and previous research have illuminated the utility of bonebed (or shellbed) cross-sectional architecture for interpreting its mode of formation [1], [6], [105]. Purisima Formation bonebeds vary in terms of thickness, presence or absence of debris-filled trace fossils, bioclast packing, and expression of erosional surfaces. Some bonebeds lack distinct erosional surfaces (Bonebeds 2–4) and instead have gradational upper and lower contacts, while others preserve a distinct basal scour (Bonebeds 1, 5) or multiple scours (Bonebed 6; Fig. 13, 25). Despite lacking an erosive base, Bonebeds 2 and 4 exhibit loosely packed bioclasts/clasts, and Ophiomorpha burrows that extend up to 2 meters below the β-interval that are infilled with densely packed bonebed debris. These bonebeds also exhibit clear trace fossils within the bonebed. These data indicate that certain bonebeds have been bioturbated and biologically mixed, directly modifying their internal architecture; such biologically mixed concentrations are difficult to interpret (e.g., [73]). The presence of coarse bonebed debris infilling burrows below bonebeds indicates bioturbating invertebrates were able to transpose clasts and bioclasts up to 5 cm in length, and up to 2–3 meters below bonebeds (e.g., Fig. 26). This indicates that the architecture of a bioclastic accumulation (when bioturbated) may be misleading when applying the bioclastic concentration model of Kidwell [1]. Bonebeds 2 and 4 both contain a large amount of phosphatic nodules and phosphatized bioclasts, indicating that seafloor erosion was a factor in its formation, although the lack of a clearly preserved scour means its architecture would be interpreted as a hiatal concentration in Kidwell’s [1] scheme (Fig. 25). This has implications for the interpretation of other marine vertebrate bonebeds; for example, Pyenson et al. [105] interpreted the middle Miocene Sharktooth Hill Bonebed as a hiatal concentration rather than a lag concentration due to the lack of evidence of erosion. However, it is possible that an erosional scour was present at some stage, and subsequently erased by bioturbators (Fig. 27); this possibility is borne out by the abundance of fragmented and otherwise taphonomically damaged skeletal elements reported from the Sharktooth Hill Bonebed [105]. These new observations from the Purisima Formation indicate that bonebed architecture – just like primary sedimentary structures – can be biologically modified after deposition, with the potential to drastically affect interpretations of bonebed genesis. Furthermore, this style of information loss means that it may not be possible to determine a mode of formation from bonebed architecture (such as in [1], and [105]) in the case of bioturbated bonebeds. PPT PowerPoint slide

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larger image TIFF original image Download: Figure 27. The effects of bioturbation on cross-sectional bonebed geometry. (A) Erosion winnows coarse, dense material and bioclasts onto an erosional surface. (B) A return to positive sedimentation results in burial of the lag concentration. (C) Subsequent bioturbation by burrowing organisms and other infauna erase the erosional surface and transpose bioclasts. (D) The resulting concentration may be misinterpreted as a hiatal bonebed. https://doi.org/10.1371/journal.pone.0091419.g027 The α-interval of most bonebeds includes abundant burrows (Ophiomorpha, Gastrochaenolites ) extending 1–3 meters below the β-interval that contain bonebed clasts and bioclasts (e.g., Fig. 26). Material infilling such traces only occur immediately below bonebeds, and is best interpreted as bonebed debris infilling open burrows at some point during bonebed formation. This demonstrates a surprising potential for bioturbators to rework small (<10 cm) bioclasts down section into older strata; similar down section reworking by bioturbators has been reported by Martill [104]. This has implications for surprisingly old taxa in condensed strata; for example, Koretsky and Sanders [112] reported on surprisingly old true seal (Phocidae) femora from the late Oligocene Chandler Bridge Formation of South Carolina. The formation is less than one meter thick and unconformably overlain by fossiliferous Pleistocene strata; in theory, it is possible that Plio-Pleistocene fossils could be reworked by contemporary bioturbators into older strata. This applies equally to other cases of unexpected fossils appearing in stratigraphically condensed sections.

Relationship between Phosphatization and other Taphonomic Characteristics The majority of polished elements (73%) also exhibit Stage 2 phosphatization, with the frequency of polish and phosphatization stage appearing to be positively correlated (Fig. 28). This suggests that polish primarily occurs after prefossilization, as suggested by Rogers and Kidwell [101], and that phosphatization is a common mode of prefossilization in the shallow marine fossil record. Abrasion and phosphatization show a similar trend. Specimens with adhering nodules and stage 1–2 permineralization are more frequently abraded. Stage 2 specimens are the most abraded. A similar relationship also exists between fragmentation and phosphatization (Fig. 28B); specimens with adhering nodules and stage 1 and 2 phosphatization are more frequently fragmented than stage 0 specimens and those without nodules. Specimens with stage 1A and 2A phosphatization are roughly similar, while 1B, 1C, 2B, and 2C specimens are more fragmented (but roughly similar to each other), suggesting that style rather than the degree of phosphatization has more of an effect on fragmentation. PPT PowerPoint slide

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larger image TIFF original image Download: Figure 28. Histograms showing relationship between phosphatization stage and other bone modifications. Bone modifications include (A) abrasion, (B) fragmentation, and (C) polish; values displayed as percentage of elements showing each phosphatization stage and b