Abstract Background Brachylophosaurini is a clade of hadrosaurine dinosaurs currently known from the Campanian (Late Cretaceous) of North America. Its members include: Acristavus gagslarsoni, which lacks a nasal crest; Brachylophosaurus canadensis, which possesses a flat paddle-shaped nasal crest projecting posteriorly over the dorsal skull roof; and Maiasaura peeblesorum, which possesses a dorsally-projecting nasofrontal crest. Acristavus, from the lower Two Medicine Formation of Montana (~81–80 Ma), is hypothesized to be the ancestral member of the clade. Brachylophosaurus specimens are from the middle Oldman Formation of Alberta and equivalent beds in the Judith River Formation of Montana; the upper Oldman Formation is dated 77.8 Ma. Methodology/Principal Findings A new brachylophosaurin hadrosaur, Probrachylophosaurus bergei (gen. et sp. nov.) is described and phylogenetically analyzed based on the skull and postcranium of a large individual from the Judith River Formation of northcentral Montana (79.8–79.5 Ma); the horizon is equivalent to the lower Oldman Formation of Alberta. Cranial morphology of Probrachylophosaurus, most notably the nasal crest, is intermediate between Acristavus and Brachylophosaurus. In Brachylophosaurus, the nasal crest lengthens and flattens ontogenetically, covering the supratemporal fenestrae in large adults. The smaller nasal crest of Probrachylophosaurus is strongly triangular in cross section and only minimally overhangs the supratemporal fenestrae, similar to an ontogenetically earlier stage of Brachylophosaurus. Sutural fusion and tibial osteohistology reveal that the holotype of Probrachylophosaurus was relatively more mature than a similarly large Brachylophosaurus specimen; thus, Probrachylophosaurus is not simply an immature Brachylophosaurus. Conclusions/Significance The small triangular posteriorly oriented nasal crest of Probrachylophosaurus is proposed to represent a transitional nasal morphology between that of a non-crested ancestor such as Acristavus and the large flat posteriorly oriented nasal crest of adult Brachylophosaurus. Because Probrachylophosaurus is stratigraphically and morphologically intermediate between these taxa, Probrachylophosaurus is hypothesized to be an intermediate member of the Acristavus-Brachylophosaurus evolutionary lineage.

Citation: Freedman Fowler EA, Horner JR (2015) A New Brachylophosaurin Hadrosaur (Dinosauria: Ornithischia) with an Intermediate Nasal Crest from the Campanian Judith River Formation of Northcentral Montana. PLoS ONE 10(11): e0141304. https://doi.org/10.1371/journal.pone.0141304 Editor: Matt Friedman, University of Oxford, UNITED KINGDOM Received: May 1, 2015; Accepted: October 7, 2015; Published: November 11, 2015 Copyright: © 2015 Freedman Fowler, Horner. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited Data Availability: All relevant data are within the paper and its Supporting Information files. Funding: Fieldwork and research funding was generously provided to JRH by the Ameya Preserve, Damaris Wagoner, David Sands, and other generous donors to the Museum of the Rockies Student Fund and Horner Fund; and to EAFF by the Tampa Bay Fossil Club and the Montana State University Department of Cell Biology and Neuroscience. Travel funding was generously provided to EAFF by the Doris O. and Samuel P. Welles Research Fund (University of California Museum of Paleontology), the M.A. Fritz Travel Grant for the Advancement of Studies in Palaeontology (Royal Ontario Museum), the Jackson School of Geosciences Student Member Travel Grant (Society of Vertebrate Paleontology), and the College of Letters and Science Student Research Travel Grant (Montana State University). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction Hadrosaurid dinosaurs were prominent members of Late Cretaceous Campanian ecosystems of North America, with an abundance of diverse taxa. The later portion of the Campanian included solid-crested hadrosaurines coexisting with hollow-crested lambeosaurines, but earlier in the Campanian, hadrosaurines were the most abundant hadrosaurids (See Materials and Methods section regarding use of the term “hadrosaurine”. Extensive bonebeds in the Two Medicine and Judith River Formations of Montana preserve numerous individuals of hadrosaurine taxa such as Maiasaura peeblesorum and Brachylophosaurus canadensis [1, 2]. These genera are members of the clade Brachylophosaurini, which includes the recently described basal taxon Acristavus gagslarsoni [3]. Acristavus, named for its lack of nasal crest, is found in the lower Two Medicine Formation of Montana (Fig 1; [3]). Maiasaura and Brachylophosaurus are more typical hadrosaurines in having prominent nasal crests, and are found stratigraphically higher than Acristavus. Maiasaura, with its dorsally projecting nasofrontal crest, is known from the upper half of the Two Medicine Formation [4–6]. Brachylophosaurus canadensis specimens have posteriorly oriented flat paddle-shaped nasal crests, and are likely all from the Comrey Sandstone Zone of the middle Oldman Formation of Alberta and its Judith River Formation equivalent in Montana [7, 8]. A previously published taxon from lower Oldman-equivalent deposits, “Brachylophosaurus goodwini” [9], has been referred to Brachylophosaurus canadensis [1] and is here considered referable only to Brachylophosaurini indet. due to the state of preservation of the holotype and the lack of a preserved nasal crest. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 1. Generalized regional cross section of the Judith River Formation and stratigraphic equivalents, with brachylophosaurin distribution. Members of Brachylophosaurini indicated in blue: Acristavus gagslarsoni, Probrachylophosaurus bergei gen. et sp. nov., Brachylophosaurus canadensis, and Maiasaura peeblesorum. Radiometric dates are indicated in dark gray, and have been recalibrated to the Fish Canyon sanidine standard (28.305 +/- 0.036 Ma) of Renne et al. [10] from the originally published values [11–15]; see text and Table 1 for further recalibration details. https://doi.org/10.1371/journal.pone.0141304.g001 Stratigraphically intermediate deposits between Acristavus and Brachylophosaurus include the lower Judith River Formation of Montana, and the corresponding Foremost and lower Oldman Formations of Alberta. Although several partial skeletons have been excavated from the Foremost and lower Oldman Formations and their Judith River Formation equivalents, no species-level diagnostic hadrosaurid material has previously been collected near the Foremost-Oldman Formation boundary, resulting in a gap in our knowledge of Campanian hadrosaur evolution. This paper describes MOR 2919, a specimen with a relatively complete skull from this stratigraphic interval in Kennedy Coulee, northcentral Montana, which fills in the gap with a transitional morphology between the known taxa Acristavus gagslarsoni and Brachylophosaurus canadensis.

Institutional Abbreviations CMN/NMC, Canadian Museum of Nature, formerly National Museum of Canada, Ottawa, Ontario, Canada; FMNH, Field Museum of Natural History, Chicago, Illinois, U.S.A.; GPDM, Great Plains Dinosaur Museum, Malta, Montana, U.S.A.; OTM, Old Trail Museum, Choteau, Montana, U.S.A.; MOR, Museum of the Rockies, Bozeman, Montana, U.S.A.; ROM, Royal Ontario Museum, Toronto, Ontario, Canada; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UCMP, University of California Museum of Paleontology, Berkeley, California, U.S.A.; UMNHVP, Natural History Museum of Utah Vertebrate Paleontology, Salt Lake City, Utah, U.S.A.; YPM-PU, Princeton University collection at the Yale Peabody Museum, New Haven, Connecticut, U.S.A.

Materials and Methods MOR 2919 was collected by Museum of the Rockies and University of California Museum of Paleontology crews using standard paleontology field techniques [27] on privately owned land. MOR 2919 was generously donated to the Museum of the Rockies by Nolan and Cheryl Fladstol and John and Claire Wendland. No permits were required for the described study, which complied with all relevant regulations. Terminology in this paper follows traditional conventions and definitions of Hadrosauridae and Hadrosaurinae [28] rather than the proposed alternate names of Saurolophidae and Saurolophinae [29] for reasons of taxonomic stability as detailed in Gates et al. [3]. MOR 2919 was compared morphologically to other hadrosaurine specimens and casts at MOR, ROM, TMP, and UCMP, as well as published descriptions. Measurements were taken directly from the specimens or casts using a tape measure or digital calipers. To determine the phylogenetic relationships of MOR 2919 within Hadrosaurinae, the specimen was coded into the phylogenetic matrices of Gates et al. [3] and Prieto-Márquez [29]. These matrices were selected because Gates et al. [3] is the description of Acristavus, one of the taxa most morphologically similar to MOR 2919, and Prieto-Márquez [29] is a landmark study, the largest, most comprehensive analysis of all Hadrosauridae to that date. The matrices from both studies were used, because although they both have origins in the Horner et al. [28] matrix, the Gates et al. [3] matrix alters few characters, whereas the Prieto-Márquez [29] matrix substantially expands the character list, yielding two extremely different matrices. If both matrices yield the same placement of MOR 2919, then its phylogienetic position can be considered very well supported. To simplify the analyses and focus on relationships within the clade of interest (Brachylophosaurini), as well as to make the taxon lists of these matrices as similar as possible, most lambeosaurine and basal hadrosauroid taxa were removed from Prieto-Márquez’s [29] and Gates et al.’s [3] matrices. Relationships within lambeosaurines and basal hadrosauroids are thoroughly investigated in Prieto-Márquez [29] and need not be repeated here. Iguanodon bernissartensis was retained for use as the outgroup, consistent with Prieto-Márquez [29] and Gates et al. [3]. Bactrosaurus johnsoni was retained as a representative hadrosauroid because it is the most complete hadrosauroid in the matrices. Hadrosaurus foulkii, although not included in the Gates et al. [3] matrix, was retained in the Prieto-Márquez [29] matrix due to the taxon’s variable position in different phylogenetic analyses and its importance in defining the terms Hadrosauridae and Hadrosaurinae. Corythosaurus casuarius was retained as a representative lambeosaurine, consistent with Gates et al. [3]. Several incomplete hadrosaurine taxa (coded with a high percentage of “?”) outside of Brachylophosaurini were removed from both matrices to simplify the analysis and improve resolution of Brachylophosaurini and the other major, well-defined hadrosaurine clades. The removed taxa were: Barsboldia sicinskii, Kerberosaurus manakini, Sabinas OTU, Salitral Moreno OTU (Willinakaqe), Shantungosaurus giganteus, and UTEP OTU. Several character states in the published Prieto-Márquez [29] matrix were recoded, and after initial analysis, some characters were amended or excluded in a second analysis (see Phylogenetic Analyses section). No character states were recoded in the Gates et al. [3] matrix, and no characters were excluded. Thus, phylogenetic analyses were performed on a total of three matrices: 1) the matrix of Prieto-Márquez [29] with all characters included (19 taxa, 370 characters; S1 File); 2) the matrix of Prieto-Márquez [29] with some characters amended or excluded (19 taxa, 367 characters; S2 File); 3) the matrix of Gates et al. [3] with all characters included (13 taxa, 116 characters; S3 File). The matrices were analyzed with parsimony in PAUP 4.0b10 [30] within a heuristic search of 5,000 replicates using ACCTRAN optimization and tree bisection-reconnection swapping to produce the most parsimonious trees, which were then combined into a strict consensus tree, and followed by a bootstrap analysis using a heuristic search of 5,000 replicates. The complete PAUP settings used are provided in the nexus files (S1–S3 Files). For the Gates et al. [3] matrix, the bootstrap replicates were increased to 50,000 due to the support values for certain clades being extremely close to the cutoff value of 50%. Phylogeny figures were time-calibrated by drawing the cladograms using published age ranges (in Ma) for Cretaceous taxa; dates for members of Brachylophosaurini were recalibrated as in Table 1 and Fig 1, using the method of Renne et al. [10]. The left tibia of MOR 2919 was histologically sampled using the techniques of Lamm [31] for large specimens. The mid-diaphyseal segment with minimum circumference was removed, molded and cast, and embedded in resin for histological sectioning. Given the large dimensions of the tibia (anteroposterior mid-diaphysis cross-sectional diameter maximum 13.35 cm, mediolateral diameter minimum 10.60 cm), the transverse cross-section was cut into three parts (anterior, posteromedial, and posterolateral) that were mounted on separate slides and ground to a thickness of 100 μm. The finished slides were imaged at 10x and 40x total magnification on a Nikon Optiphot-Pol polarizing microscope with a Nikon DS-Fi1 digital sight camera utilizing an automated stage to move the slide incrementally. The resulting photomicrographs were compiled with NIS-Elements BR 3.0 software into high-resolution TIFF image files. Adobe Photoshop CS2 was used to combine the images of the three slides and trace the circumference of each line of arrested growth (LAG). These circumferences were then measured with ImageJ 1.46r [32]. Because the left tibia was slightly crushed, several cortical segments were displaced radially inward, resulting in the LAG tracings overestimating the true circumference due to the LAGs on either side of a displacement needing to be connected by a radially oriented line. This was corrected for each LAG individually by subtracting the length of these radial lines at the three areas of greatest displacement (anterolateral, posterolateral, posteromedial) from the total LAG circumference measured in ImageJ. The left tibia of MOR 2919 was originally collected by UCMP in 1981 and curated as UCMP 137272. Limb bones prepared at UCMP at the time sometimes included a metal rebar rod and epoxy inserted into the medullary cavity for rigid support. After the tibia was transferred to MOR collections and sectioned for histology, the rebar was cut out of the tibia segments prior to mounting them on slides to avoid grinding metal on histological equipment. This created the open space seen in the center of the medullary cavity on the finished slides. Nomenclatural Acts The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix "http://zoobank.org/". The LSID for this publication is: urn:lsid:zoobank.org:pub:657DD0A4-799D-443A-BE9F-A2987DFABADD. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: PubMed Central, LOCKSS.

Results Systematic Paleontology Dinosauria Owen, 1842 Ornithischia Seeley, 1888 Ornithopoda Marsh, 1881 Hadrosauridae Cope, 1869 Hadrosaurinae Cope, 1869 Brachylophosaurini Gates et al., 2011 Definition. Modified from Gates et al. [3]: Hadrosaurine ornithopods more closely related to Brachylophosaurus, Probrachylophosaurus, Maiasaura, or Acristavus than to Gryposaurus or Saurolophus. Diagnosis. As in Gates et al. [3]. Referred material. UCMP 130139, a partial skull and skeleton originally described as the holotype of Brachylophosaurus goodwini [9], and later assigned to Brachylophosaurus canadensis [1, 23]. Due to the lack of a preserved nasal, and the presence of deep frontal depressions, the specimen cannot be confidently assigned to any current genus of Brachylophosaurini. Horizon and locality. UCMP 130139 was collected from the Judith River Formation of Kennedy Coulee, Hill County, northcentral Montana, in beds equivalent to the lower Oldman Formation, with a published height of approximately 15 m above the Marker A Coal of the Taber Coal Zone of the Foremost Formation [9]. However, a remeasured section shows that the site was actually only a few meters above the Marker A Coal, and lies within the Herronton Sandstone Zone (Mark Goodwin and David Evans personal communication, 2014). Brachylophosaurus canadensis Sternberg, 1953 Holotype. CMN 8893 Referred Material. FMNH PR 862 (partial skull); MOR 720 (braincase); MOR 794 (nearly complete articulated skeleton); MOR 940 (braincase); MOR 1071 (monodominant bonebed); TMP 90.104.01 (complete skull and articulated partial skeleton). Emended diagnosis (revised from Cuthbertson and Holmes [23]). Nasal crest flat and paddle-shaped in adults, covering most or all of the supratemporal fenestrae; prefrontal as in Cuthbertson and Holmes [23]: “prefrontal projecting posteriorly over frontal, and more posteriorly, ventromedially directed to underlie nasal crest and contribute to anterior border of supratemporal fenestra”. These autapomorphies, together with the following traits, form a unique combination of characters: “only the anterior tip of the lacrimal contacting the maxilla; extremely elongated anterior maxillary process” [23]. Remarks. In their rediagnosis of the holotype CMN 8893, Cuthbertson and Holmes [23] reduce the number of autapomorphies listed in Prieto-Márquez [1], and list an additional autapomorphy: “quadratojugal with ‘noncrescentic’ posterior margin variably forming paraquadratic foramen with quadrate” [23]. Because the posterior margin of the quadratojugal and the interpreted presence of a paraquadratic foramen are variable within Brachylophosaurus (see Quadrate and Quadratojugal descriptions and comparisons below), this character is not here considered an autapomorphy, and has been excluded from the emended diagnosis. Horizons and localities. CMN 8893, FMNH PR 862, and TMP 90.104.01 were collected from the Oldman Formation of southeastern Alberta. CMN 8893 and FMNH PR 862 were collected in Dinosaur Provincial Park; TMP 90.104.01 was collected near Onefour and the Milk River. Of these Albertan specimens, the exact stratigraphic position is known only for CMN 8893: the Comrey Sandstone Zone (Unit 2) of the Oldman Formation. MOR 720 was collected from the upper Judith River Formation in badlands surrounding the Missouri River north of Winifred, Fergus County, central Montana. MOR 794, MOR 940, and MOR 1071 were collected from the Judith River Formation of Malta, northern Montana, in beds equivalent to the Comrey Sandstone Zone of the Oldman Formation. Probrachylophosaurus gen. nov. urn:lsid:zoobank.org:act:7B7C87AC-2EFE-4587-9A24-A5D48C908941 Type species. Probrachylophosaurus bergei sp. nov. Etymology. Pro- (Latin) before, -brachylophosaurus (Greek) short-crested lizard, in reference to the new taxon’s stratigraphic position below that of Brachylophosaurus canadensis. Diagnosis. As for type and only species. Probrachylophosaurus bergei sp. nov. urn:lsid:zoobank.org:act:49D503CB-7FA6-4D66-8FC0-0B4E2A3EE106 Holotype. MOR 2919, majority of a skull and skeleton, disarticulated. Cranial material includes a right premaxilla (fragmentary), both maxillae, left jugal, partial right lacrimal, left posterior nasal, partial mid-region of right nasal, articulated braincase (with articulated frontals, parietal, postorbitals, and exoccipitals), both squamosals, both quadrates, predentary, both dentaries, and right surangular (Fig 3). Postcranial material includes atlas fragments and at least 10 other cervical vertebrae, 11 dorsal vertebrae, 29 caudal vertebrae, 19 chevrons, approximately 19 ribs, both ilia, both pubes, both ischia, both tibiae, both fibulae, both astragali, right metatarsal II, and right metatarsal IV. Forelimbs and sacral vertebrae are absent, although an isolated neural spine may belong to a sacral vertebra. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 3. Probrachylophosaurus bergei gen. et sp. nov. skull reconstruction. (A) Preserved skull elements of MOR 2919, left lateral view. Predentary not included due to its poor preservation and diagenetic compression. (B) Outline of skull reconstruction, left lateral view. The outline accounts for diagenetic distortion of the posterior braincase, but otherwise does not correct for distortion of skull elements. Outlined regions where left skull material was not preserved are based on right bones when available. Regions with neither left nor right material preserved are hypothesized reconstructions based on Brachylophosaurus canadensis skulls. (C) Braincase with left nasal crest, dorsal view. (D) Outline of braincase reconstruction with nasal crest, dorsal view. Reconstruction accounts for diagenetic lateral compression and distortion of posterior braincase. Abbreviations: d, dentary; ex, exoccipital; f, frontal; j, jugal; l, lacrimal; m, maxilla; n, nasal; p, parietal; pd, predentary; pf, prefrontal; pm, premaxilla; po, postorbital; q, quadrate; qj, quadratojugal; sa, surangular; sq, squamosal. https://doi.org/10.1371/journal.pone.0141304.g003 Referred specimen. MOR 1097, fragmentary subadult skull material including the right posterior nasal crest, right jugal, left coronoid process of dentary, dentary tooth rows, maxilla tooth rows, partial left prefrontal, and dorsal and ventral condyles of right quadrate. Etymology. Species name bergei in memory of Sam Berge, co-owner of the land where the specimen was discovered, and friend and relative of many members of the Rudyard, Montana community, who have supported paleontologic research for decades. Pronunciation: berg-ee-i Horizon and locality. MOR 2919 was collected from private land north of Rudyard, Montana, just east of the mouth of Kennedy Coulee along the Milk River near the USA-Canada border, in exposures of the Judith River Formation. The site, MOR locality JR-518 (“Superduck”), is within a grey mudstone stratigraphically equivalent to Unit 1 of the Oldman Formation of Alberta. The bone horizon is 17.5 m above the top of the Marker A Coal of the Taber Coal Zone of the Foremost Formation, and 7.0 m above the top of the Herronton Sandstone Zone of the Foremost Formation (Fig 4), with a recalibrated age between 79.5 +/- 0.2 Ma and 79.8 +/- 0.2 Ma [10, 11]. The skeleton was completely disarticulated and there was no preferred orientation of the long bones. Associated microsite material was rare, but dominated by tyrannosaur teeth. MOR 1097 was collected from state-owned land less than 1 km east of MOR 2919, and at a similar stratigraphic height. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 4. Stratigraphic section at Probrachylophosaurus bergei gen. et sp. nov. MOR 2919 quarry, MOR locality JR-518. Datum (0 m) is the top of the Marker A coal of the Taber Coal Zone of the Foremost Formation. Radiometric dates were recalibrated from Goodwin and Deino [11] samples collected southwest of MOR JR-518 in Kennedy Coulee; their stratigraphic heights are indicated on the section. This section is atypical for Kennedy Coulee sections in that it has a relatively thin Herronton Sandstone Zone (HSZ) low in section, and more sandstones in the upper part of the section, whereas most of the coulee has a thick sandstone zone above the Marker A coal, and is dominated by mudstone in the upper regions. The region indicated with “?” below the HSZ may or may not be classified as a continuation of the Taber Coal Zone. https://doi.org/10.1371/journal.pone.0141304.g004 The site was discovered by Kyoko Kishi and partially excavated by a UCMP crew led by Mark Goodwin in 1981 and 1994, which collected some postcranial material (UCMP locality V81232). Additional material became exposed in 2007, so a MOR crew continued excavation in 2007 and 2008, collecting the skull and extensive postcrania. Because all skeletal material at the site belongs to a single individual, UCMP graciously transferred its postcranial material to MOR. Consequently, the specimen numbers have been altered; all bones are now MOR 2919, and the old UCMP numbers are now treated as “field numbers” for purposes of identifying individual bones within MOR 2919. Original UCMP collections numbers were: left tibia UCMP 137272, right fibula UCMP 156955, right metatarsal IV UCMP 399999, right ilium UCMP 172484, left astragalus UCMP 172484, and caudal vertebra UCMP 400000. A jacket containing an unprepared and thus previously uncataloged right tibia was also transferred to MOR. Diagnosis. Probrachylophosaurus bergei is a hadrosaurine hadrosaurid diagnosed by the following features: solid crest consisting entirely of the nasals that overhangs the supratemporal fenestrae by less than 2 cm in adults; nasal crest being extremely dorsoventrally thickened medially, resulting in a strongly triangular frontal plane cross section, with the dorsal angle formed by the paired nasals in posterior view being less than 130 degrees. These autapomorphies, together with the following traits, form a unique combination of characters: posterior lacrimal mediolaterally wide as in Acristavus but not Brachylophosaurus, caudoventral apex of the rostral process of the jugal is posterior to the caudodorsal apex as in Acristavus but not Brachylophosaurus, squamosals contact each other medially as in Acristavus but not Brachylophosaurus, posteriorly-oriented solid nasal crest as in Brachylophosaurus but not Acristavus.

Phylogenetic Analyses Analyses based on matrix of Prieto-Márquez [29] Probrachylophosaurus bergei was coded into two versions of the matrix of Prieto-Márquez [29]. The first version included some recodings of previous taxa, with all characters included. The second version included these same recodings, but excluded some characters as detailed below. The first analysis using the Prieto-Márquez [29] matrix included recodings of eight characters for Acristavus based on reexamination of the material. See Prieto-Márquez [29] for complete descriptions of characters and abbreviation codes (e.g., J4 is the fourth jugal character). Character 106 (J4), the relative position of the caudoventral apex of the rostral process of the jugal, was changed from state 1 to 0. Probrachylophosaurus is also coded as 0; Brachylophosaurus and Maiasaura are state 1. The quadrate of the Acristavus holotype (MOR 1155) was not coded into the original matrix, so it has been added here, replacing characters 116–121 (Q1-Q6)?????? with 011211. These are the same codings as the quadrate of Probrachylophosaurus and Brachylophosaurus; Maiasaura has states 001211. Character 259 (PB8), the length/width ratio of the ischial peduncle of the pubis, was recoded from 0 to 1 for Acristavus; Probrachylophosaurus, Brachylophosaurus, and Maiasaura are also state 1. In Brachylophosaurus, Character 138 (F1) was originally coded as state 0, indicating the lack of bifurcation of the anterior frontal margin. However, this is only true for some adult specimens (e.g. GPDM JRF.65); the “slender” adult Brachylophosaurus MOR 1071-7-7-98-86 and subadults MOR 1071-7-13-99-87-I and MOR 1071-C-3-3 have bifurcated anterior frontal margins, so this was recoded to the multistate “01”. When Probrachylophosaurus bergei is added to Prieto-Márquez’s [29] matrix, with extraneous taxa removed and some character states recoded (see above), but all characters included (S1 File), four most parsimonious trees were produced. The strict consensus tree recovers Brachylophosaurus and Maiasaura as sister taxa, with Probrachylophosaurus and Acristavus forming a basal polytomy within Brachylophosaurini (S2 Fig). Characters responsible for the position of Probrachylophosaurus included: 1 (dtth1), number of dentary tooth positions; 15 (mxth1), number of maxillary tooth positions; 36 (dt4), angle of the ventral rostral margin of the dentary; 44 (dt13), lack of caudodorsal point on coronoid process of dentary; 106 (j4), relative positions of caudodorsal and caudoventral apices of rostral process of jugal; 110 (j8), size of caudoventral flange of jugal; 113 (j11), ratio of caudal and rostral constrictions of jugal; and 143 (f6), contribution of the frontal to the orbital margin. The bootstrap 50% majority-rule consensus tree (5,000 replicates) finds 55% support for a Probrachylophosaurus-Brachylophosaurus-Maiasaura clade, with Acristavus as the basal member of Brachylophosaurini, and 64% support for Brachylophosaurus and Maiasaura as sister taxa. Three of Prieto-Márquez’s [29] characters were then excluded from the second analysis. Characters 1 (dtth1) and 15 (mxth1) are based on the number of teeth in the dentary and maxilla, respectively. The number of teeth in hadrosaurid jaws increases ontogenetically [37, 38], so the high tooth counts in MOR 2919 are liable to be due to the specimen’s size and maturity rather than a true phylogenetic signal separating it from Brachylophosaurus and Maiasaura. Character 123 (pf2) describes the shape of the prefrontal contribution to the orbital margin. Many specimens of Brachylophosaurini examined in this study are intermediate between the character states, and so this character was excluded to avoid ambiguous state assignments. Character 143 (f6) in Prieto-Márquez [29] is a modification of character 57 in Horner et al. [28]. The contribution of the frontal to the orbital margin is either present or absent in Horner et al. [28], but Prieto-Márquez [29] divides presence into two states, frontal exposed and forming part of orbital margin, or forming a narrow triangular apex that may or may not reach the orbital margin between the prefrontal and postorbital. This division is problematic because the width of the orbital contribution of the frontal is intraspecifically variable in Brachylophosaurus (MOR 1071 braincases, FMNH PR 862), and potentially may vary similarly in other taxa that currently have smaller sample sizes. Also, the distinction between the two states (frontal forming part of orbital margin or doing so through a narrow triangular apex) is not well defined. Prieto-Márquez [29] codes Acristavus, Brachylophosaurus, and Maiasaura as having the narrow triangular apex, but their contribution to the orbital margin can be more than 1 cm and not triangular. Thus, the frontal contribution to the orbital margin, when present, forms a continuum rather than two discrete states. End members are clear, but most brachylophosaurin specimens fall in between the end points, and so the original version of the character (simply presence or absence of frontal contribution to the orbital margin) in Horner et al. [28] is preferred. Consequently, character 143 (f6) was recoded to merge states 0 and 1, so there are only two states (presence or absence) rather than three. After these exclusions and recodings (S2 File), two most parsimonious trees were produced. The strict consensus tree recovers Acristavus as the basal member of Brachylophosaurini, with Brachylophosaurus and Maiasaura as sister taxa, and Probrachylophosaurus in an intermediate position (Fig 22). Probrachylophosaurus, Brachylophosaurus, and Maiasaura, but not Acristavus, share one apomorphic character state: 134 (sq1) state 0, very short precotyloid process of the squamosal. Probrachylophosaurus differs from Acristavus, Brachylophosaurus, and Maiasaura in its state for three characters: 36 (dt4) state 2, angle of the ventral rostral margin of the dentary; 110 (j8) state 1, size of the caudoventral flange of the jugal; and 113 (j11) state 2, ratio of caudal and rostral constrictions of the jugal; these character states are homoplasies shared with non-brachylophosaurin hadrosaurine taxa. Brachylophosaurus and Maiasaura are united as sister taxa due to two characters: 44 (dt13) state 1, the autapomorphic presence of a caudodorsal point on the coronoid process of the dentary; and 106 (j4) state 1, relative positions of caudodorsal and caudoventral apices of the rostral process of the jugal. Compared to the previous result, this bootstrap 50% majority-rule consensus tree finds increased (66%, was 55%) support for a Probrachylophosaurus-Brachylophosaurus-Maiasaura clade, with Acristavus as the basal member of Brachylophosaurini, and decreased (56%, was 64%) support for Brachylophosaurus and Maiasaura as sister taxa. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 22. Time-calibrated cladogram of hadrosaurines based on Prieto-Márquez [ Time-calibrated cladogram of hadrosaurines based on Prieto-Márquez [ 29 ]. The strict consensus of two most parsimonious trees resulting from adding Probrachylophosaurus bergei gen. et sp. nov. to Prieto-Márquez’s [29] matrix with minor character recodings and exclusions as discussed in text was plotted using the age ranges for each taxon; branch lengths do not reflect the number of character state changes. Values on branches represent bootstrap support; branches without values had less than 50% support. Tree statistics: shortest tree length = 614, Consistency Index = 0.72, Retention Index = 0.68, Rescaled Consistency Index = 0.49. Age ranges of Brachylophosaurini are those recalibrated in Fig 1; age ranges of other taxa are approximate, and are based on unrecalibrated previously published dates [3, 7, 13, 15, 39–48]. Note that the age scale changes before and after 80 Ma. Also note that the age of Bactrosaurus has been variably proposed to be any time from late Turonian to early Maastrichtian in age [49–51]. https://doi.org/10.1371/journal.pone.0141304.g022 Analysis based on matrix of Gates et al. [3] When Probrachylophosaurus bergei is added to Gates et al.’s [3] matrix, with extraneous taxa removed (see Methods), but all characters included (S3 File), two most parsimonious trees were produced. The strict consensus tree recovers Probrachylophosaurus and Brachylophosaurus as sister taxa, with Acristavus as the basalmost member of Brachylophosaurini (Fig 23). Probrachylophosaurus, Brachylophosaurus, and Maiasaura, but not Acristavus, share two apomorphic character states: character 36 state 1, presence of solid nasal crest; and character 37 state 1, circumnarial fossa terminates anterior to nasal crest. Acristavus, Probrachylophosaurus, and Maiasaura, but not Brachylophosaurus, share one apomorphic character state: character 53 state 0, straight to slightly curved ventral margin of the rostral process of the jugal. Probrachylophosaurus and Brachylophosaurus are united as sister taxa due to one apomorphic character state: character 105 state 0, postacetabular process is less than 40 percent the total length of the ilium. Probrachylophosaurus differs from Acristavus, Brachylophosaurus, and Maiasaura in its state for one character: character 72 state 0, low basipterygoid transverse ridge of the basisphenoid. The bootstrap 50% majority-rule consensus tree (50,000 replicates) finds 50% support for Probrachylophosaurus and Brachylophosaurus as sister taxa, with the positions of Acristavus and Maiasaura having less than 50% support. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 23. Time-calibrated cladogram of hadrosaurines based on Gates et al. [ Time-calibrated cladogram of hadrosaurines based on Gates et al. [ 3 ]. The strict consensus of two most parsimonious trees resulting from adding Probrachylophosaurus bergei gen. et sp. nov. to Gates et al.’s [3] matrix was plotted using the age ranges for each taxon; branch lengths do not reflect the number of character state changes. Because species within the same genus had the same codings, only genera are listed in the phylogeny. Values on branches represent bootstrap support; branches without values had less than 50% support. Tree statistics: shortest tree length = 188, Consistency Index = 0.70, Retention Index = 0.65, Rescaled Consistency Index = 0.45. Age ranges are approximate, and are based on those in Gates et al. [3], aside from Brachylophosaurini, which have been recalibrated as in Fig 1. Note that the age scale changes before and after 80 Ma. Also note that the age of Bactrosaurus has been variably proposed to be any time from late Turonian to early Maastrichtian in age [49–51]. https://doi.org/10.1371/journal.pone.0141304.g023 Phylogenetic Relationships Brachylophosaurini is supported as a robust clade in all recent phylogenetic analyses, but the relationships amongst its members vary. Prior to the description of Acristavus gagslarsoni, this clade consisted solely of Maiasaura peeblesorum and Brachylophosaurus canadensis [1, 28, 43]. Prieto-Márquez [29] recovered Maiasaura and Brachylophosaurus as sister taxa, with Acristavus (“Two Medicine OTU”) as the basal member of the clade. However, Gates et al. [3] recovered Acristavus and Maiasaura as sister taxa. When Probrachylophosaurus is added to the Prieto-Márquez [29] and Gates et al. [3] matrices, it is recovered with confidence as a member of Brachylophosaurini (bootstrap support 87% in both analyses). Acristavus is recovered as the basal taxon in all analyses, consistent with its stratigraphic position; thus, Acristavus or a close relative was likely ancestral to the rest of Brachylophosaurini. The inclusion of Probrachylophosaurus in the Prieto-Márquez [29] matrix still results in Maiasaura and Brachylophosaurus as sister taxa, but with much weaker bootstrap support for the clade (56% vs. 69% in [29]). Probrachylophosaurus is recovered as intermediate between Acristavus and the Brachylophosaurus-Maiasaura clade. The inclusion of Probrachylophosaurus in the Gates et al. [3] matrix yields Probrachylophosaurus and Brachylophosaurus as sister taxa, with Maiasaura intermediate between them and Acristavus. The differing positions of Maiasaura relative to Probrachylophosaurus and Brachylophosaurus in each analysis leads to uncertainty regarding the timing of the cladogenic split between the Brachylophosaurus and Maiasaura lineages. The analysis based on the Gates et al. [3] matrix suggests that Maiasaura diverged from the Probrachylophosaurus-Brachylophosaurus lineage sometime between the stratigraphic existence of Acristavus and Probrachylophosaurus (Fig 23). The analysis based on the Prieto-Márquez [29] matrix suggests that the Maiasaura lineage either diverged from the Brachylophosaurus lineage sometime between the stratigraphic existence of Probrachylophosaurus and Brachylophosaurus, or Maiasaura is derived from Brachylophosaurus (Fig 22). In the analysis based on the Prieto-Márquez [29] matrix, Brachylophosaurus and Maiasaura are united by two characters: 44 (dt13) caudodorsal point on the coronoid process of the dentary and 106 (j4) position of the caudoventral apex of the rostral process of the jugal relative to the caudodorsal margin of the rostral process. The caudodorsal point on the coronoid process of the dentary is potentially very useful for taxon identification, but is also problematic. The caudodorsal margins of Brachylophosaurini coronoid processes are very thin, and are broken in many specimens. The caudodorsal margins of the coronoid processes of Acristavus and Probrachylophosaurus are extremely thin and partially broken; they do not preserve points, and the thinness of the bone seems to indicate that there could not have been a point, but this cannot be confirmed with absolute confidence. Also, the caudodorsal point enlarges ontogenetically in Brachylophosaurus and Maiasaura; small individuals lack the point entirely. Thus, the maturity of a specimen lacking the caudodorsal point must be considered before including it in phylogenetic analyses. The Acristavus and Probrachylophosaurus holotypes are from large enough individuals that they should display the adult condition for this character. Character 106 (j4) concerns the relative positions of the caudodorsal and caudoventral points of the rostral process of the jugal. Maiasaura is coded as having the same state as Brachylophosaurus; the caudoventral point is directly ventral to the caudodorsal point, rather than being posterior to the caudodorsal point as it is in Acristavus and Probrachylophosaurus. However, in Maiasaura, the caudoventral point is slightly posterior to the caudodorsal point, and so is actually intermediate between the states of Acristavus-Probrachylophosaurus and Brachylophosaurus. In summary, the phylogenetic analyses concur in placing Acristavus as the basal-most member of the Brachylophosaurini, consistent with its relative stratigraphic position, but the relationships among Probrachylophosaurus, Brachylophosaurus, and Maiasaura vary depending on the cladistic matrix used.

Discussion Ontogeny is intimately tied to evolution, as in a very general sense, ontogeny recapitulates phylogeny [58, 59]. This is often expressed in juvenile dinosaurs, which may bear morphologic traits of their adult ancestors (peramorphy) that are not expressed in the adults of the later taxon, e.g. juvenile Bactrosaurus dentition resembling that of iguanodonts [60]. Because juveniles may retain ancestral characteristics, without proper stratigraphic data they may be mistaken as older, more basal taxa. Indeed, when juvenile specimens are coded into a phylogenetic matrix, they are placed more basally on the resulting cladogram than are adults of that taxon [61, 62]. Within hadrosaurine and lambeosaurine skulls, the most drastic ontogenetic changes occur in the morphology of the premaxillae, nasals, and supraoccipitals [63]. As the individual grows, its premaxillae and nasals generally enlarge and change shape to form the crest [63], with the crest typically moving posteriorly during ontogeny [38, 64]. Bones far from the incipient crest (quadrates, dentaries, and maxillae) change the least [63]. The number of tooth rows increases [37, 38], articular surfaces become more rugose, and the relative size of the orbit decreases [37]. These general trends are observed in both Probrachylophosaurus and Brachylophosaurus. The nasal crest of Brachylophosaurus canadensis enlarges late in ontogeny. Prieto-Márquez [1] separated Brachylophosaurus specimens into “slender” and “robust” morphotypes, and stated that these individuals were approximately the same size. Although specimens of the two morphotypes have roughly the same interorbital and postorbital widths [1, 23], these widths have been affected by diagenetic compression. The robust specimen MOR 794 is laterally compressed, decreasing its cranial width; the slender specimens MOR 1071-7-7-98-86 and MOR 1071-7-16-98-248 are dorsoventrally compressed, increasing their cranial widths. Thus, although their apparent sizes are similar, after accounting for differential compression slender Brachylophosaurus specimens are slightly smaller than robust specimens. Slender and robust Brachylophosaurus specimens are both found in nearly the same horizons at Malta, Montana, and are therefore most parsimoniously ontogenetic stages of the same species. Cuthbertson and Holmes [23] noted that the slender and robust morphotypes are not discrete; Brachylophosaurus specimens such as FMNH PR 862 and TMP 1990.104.001 possess crests of intermediate sizes. The presence of intermediate crest morphologies supports individual or ontogenetic variation. Also, individual variation in body size could account for the presence of specimens with different crest sizes at similar body sizes. Like many dinosaur taxa, hadrosaurids reached nearly their full adult size relatively quickly, and then grew increasingly slowly for the rest of their lives [65]. In modern animals such as cassowaries, the cranial crest grows rapidly late in ontogeny, often after the rate of skeletal growth has greatly slowed and the individual is nearly at full adult size [66]. A near-adult cassowary with a skull length of 165 mm possesses a small incipient crest, and after a mere 5 mm (3%) of additional growth in skull length, acquires a full adult casque that covers most of the dorsal skull. Pachycephalosaurus and Stegoceras juveniles have flat skulls, gaining their distinctive frontoparietal domes later in ontogeny [67, 68]. Centrosaurine ceratopsids also developed their pronounced cranial ornamentation late in ontogeny [69]. The nasal crests of slender and robust Brachylophosaurus follow this pattern of rapidly increasing crest size with a very small increase in overall skull size. In Brachylophosaurus, the nasal crest elongates and flattens posteriorly ontogenetically, and the posterior margin of the nasofrontal suture migrates posteriorly as well. In MOR 2919, the nasofrontal suture is anteriorly placed as in a subadult Brachylophosaurus, and the nasal crest is shorter than that of any adult Brachylophosaurus. Yet, MOR 2919 is not a subadult; in addition to its large size, the braincase elements and vertebral neural arches are completely fused externally, with most sutures obliterated, and histology confirms that MOR 2919 was nearing skeletal maturity. In slender adult Brachylophosaurus (MOR 1071), cranial sutures are still clearly defined, and sutures are still visible in the robust morphology of adult Brachylophosaurus (MOR 794). Although all sutures of braincase elements of MOR 2919 are fused and nearly obliterated, the nasofrontal suture and internasal suture are disarticulated and show no indications of fusion. The nasofrontal suture in Brachylophosaurus becomes more rugose ontogenetically, stabilizing the suture as the nasal crest grows. The lack of nasofrontal fusion in MOR 2919 suggests that the nasal crest was still enlarging at time of death. The holotype of Brachylophosaurus goodwini (UCMP 130139) and MOR 2919 were both collected from the Kennedy Coulee area, but the Brachylophosaurus goodwini holotype was located stratigraphically lower in the coulee. Unfortunately, UCMP 130139 does not preserve the posterior nasals, which are a critical character in defining Probrachylophosaurus bergei. UCMP 130139 is adult-sized, and its frontals exhibit an anteriorly-located nasofrontal suture that does not cover the entire frontals, similar to that of MOR 2919 and subadult Brachylophosaurus canadensis. This supports UCMP 130139 and MOR 2919 being more closely related than either is to B. canadensis. However, UCMP 130139 exhibits anomalously deep frontal depressions that preclude it from being referred to P. bergei. If the deep frontal depressions are not pathologic, taphonomic, diagenetic, or individual variation, then they may potentially be used as an autapomorphy to resurrect the species “B.” goodwini, although it will need a different genus name. Given the poor preservational condition of the frontals of UCMP 130139, the specimen should be referred to Brachylophosaurini indet. pending additional specimens with similar frontal depressions. The morphologies of elements of Probrachylophosaurus are consistently more similar to members of Brachylophosaurini than to other hadrosaurids. Within Brachylophosaurini, elements of Probrachylophosaurus are either most similar to Brachylophosaurus or Acristavus, or intermediate between these taxa, as predicted by their relative stratigraphic positions. This supports the hypothesis of Probrachylophosaurus bergei representing an intermediate taxon within the Acristavus-Brachylophosaurus lineage (Fig 29). PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 29. Ontogenetic and anagenetic hypothesis of brachylophosaurin evolution. Probrachylophosaurus bergei gen. et sp. nov. is proposed as an intermediate member of the lineage leading from Acristavus gagslarsoni to Brachylophosaurus canadensis. Shaded blue areas indicate known elements of Probrachylophosaurus. Skull outlines of Acristavus and Brachylophosaurus are used courtesy of Terry A. Gates. The reconstruction of the MOR 1071 Brachylophosaurus skull is a composite of an articulated skull roof with a scaled-down copy of the MOR 794 skull outline. All skulls are scaled to the same 10 cm scale bar. The horizontal axis is not to scale; the MOR 1071 reconstruction is much closer to MOR 794 in size and hypothesized maturity than MOR 1097 is to MOR 2919. Radiometric ages have been recalibrated to the Fish Canyon sanidine standard (28.305 +/- 0.036 Ma) of Renne et al. [10] from the originally published values [3, 11–13]; see text and Table 1 for further recalibration details. The age of the Acristavus holotype was precisely estimated by Gates et al. [3]. The age of the Comrey Sandstone Zone of the Oldman Formation is not tightly constrained, leading to uncertainty in the exact age of Brachylophosaurus. https://doi.org/10.1371/journal.pone.0141304.g029 In some morphologic characters within Brachylophosaurini, the subadult morphology of the stratigraphically younger taxon matches the adult condition of the stratigraphically older taxon, illustrating a peramorphic trend of immature animals of a descendant taxon resembling adults of the ancestral taxon [60]. The palatine process of the jugal in subadult Probrachylophosaurus resembles the condition of adult Acristavus. The nasal crest in Brachylophosaurus increases in size late in ontogeny: subadults lack any crest, similar to the adult form of their proposed ancestor Acristavus; small adults have a small crest reminiscent of their proposed ancestor Probrachylophosaurus; and large adults have a large broad paddle-shaped crest. The nasofrontal suture of small subadult Brachylophosaurus is similar to that of adult Probrachylophosaurus, although Probrachylophosaurus has the deep rugosities of a more mature suture. The prequadratic process of the squamosal of small subadult Brachylophosaurus is also more similar to that of adult Probrachylophosaurus than to adult Brachylophosaurus. Probrachylophosaurus bergei shares many morphologic similarities with Brachylophosaurus canadensis, but in other characters is more similar to Acristavus gagslarsoni. Thus, placing P. bergei as merely a new species of either genus would be problematic. Depending on the cladistic matrix used, P. bergei may be the sister taxon to B. canadensis and could be classified as a member of the same genus (Fig 23), or Maiasaura peeblesorum may be the sister taxon to B. canadensis (Fig 22), requiring P. bergei to be classified as a separate genus. Given this phylogenetic uncertainty, Probrachylophosaurus bergei should be considered a unique genus. Future discoveries of additional intermediate specimens will hopefully clarify evolutionary relationships within the Brachylophosaurini clade.

Conclusions In the early years of dinosaur paleontology, specimens were collected as isolated points, each so morphologically unique that their evolutionary relationships were difficult to determine. As more fossil specimens are collected, the gaps in morphology that previously separated species are being filled. With larger sample sizes resulting in more continuous series of fossils with good stratigraphic resolution, variations in morphology can be analyzed in a more complete context, and attributed to ontogeny, evolution, taphonomic alteration, biogeography, or individual variation [70]. A better understanding of stratigraphy and advancements in radiometric dating enable precise temporal correlation of geographically separated localities. Taxa can then be placed in temporal sequence, allowing tests of evolutionary hypotheses. A precise stratigraphic framework is critical for determining whether morphological variations of adult specimens are due to evolution or are variations within a roughly contemporaneous population. If closely related taxa do not overlap stratigraphically, the pattern is more parsimonious with anagenesis than cladogenesis. Recent research has greatly increased the sample size and stratigraphic resolution of specimens from the Judith River and Hell Creek Formations of Montana and their Canadian equivalents, revealing several potential anagenetic lineages in Campanian and Maastrichtian ornithischians [7, 45, 71–73]. Because Probrachylophosaurus bergei is stratigraphically older than all Brachylophosaurus canadensis specimens, it is hypothesized to represent a basal brachylophosaur morphology, early in the evolution of this lineage from a non-crested ancestor. Thus, the small crest of Probrachylophosaurus would represent a transitional nasal morphology between a non-crested ancestor such as Acristavus and the larger crests of adult Brachylophosaurus. The fourth member of Brachylophosaurini, Maiasaura, would represent a cladogenic event, diverging from the lineage that led to Brachylophosaurus at a currently unknown point.

Acknowledgments Many thanks to Mark Goodwin, Dave Varricchio, Don Brinkman, Matt Lavin, Denver Fowler, Ray Rogers, Dave Evans, Michael Ryan, Terry Gates, and Nic Campione for morphologic, stratigraphic, and cladistic advice and discussions. For specimen collection, thanks to the 2007–2008 MOR field crews, especially Brian Baziak, Holly Woodward, and Cary Woodruff, and 1981 and 1994 UCMP field crews, especially Mark Goodwin and Kyoko Kishi. For fieldwork support, hospitality, and land access, great thanks as always to Dan and Lila Redding. For land access and specimen donation, and for suggesting the species name, grateful thanks to Nolan and Cheryl Fladstol and John and Claire Wendland. Thanks to all members of the Rudyard, Montana community, whose continuing support of local paleontology makes this work possible. Excellent preparation was done by Carrie Ancell, Jamie Jette, Aleen Kienholz, Patsy Hookey, and volunteers at the MOR Bowman Prep Lab. For histological assistance, thanks to Ellen Lamm, Holly Woodward, Brian Baziak, and Christian Heck. For specimen transport, thanks to Robert and Sarah Boessenecker and Daniel Traub. Thanks to Kevin Seymour and David Evans (ROM); Don Brinkman, Brandon Strillsky, and Graeme Housego (TMP); Pat Holroyd, Mark Goodwin, and Kevin Padian (UCMP); Sue Frary (GPDM); and Sharon Emond (Phillips County Museum) for comparative specimen access. Dave Varricchio, Don Brinkman, and Matt Lavin provided helpful comments on drafts of this paper. Terry Gates and an anonymous reviewer offered additional comments that improved the quality of this paper.

Author Contributions Conceived and designed the experiments: EAFF JRH. Performed the experiments: EAFF. Analyzed the data: EAFF. Contributed reagents/materials/analysis tools: JRH. Wrote the paper: EAFF.