Description and comparisons

Skull

Most of the bones are identifiable, but portions of the maxillae, jugals, nasals, and lacrimals are damaged or missing.

The cranium of CMC VP14128 has diagnostic features of Diplodocidae including: a long prefrontal, paroccipital process with a rounded ventrolateral end, external nares that are retracted and dorsally-facing, a tooth row that does not extend the full length of the maxilla and dentary, low coronoid eminence, and absence of a squamosal-quadratojugal contact. Additionally, the morphology of the basal tubera (robust, triangular, and protruding posteroventrally from the basicranium), the presence of subnarial and maxillary foramina, and the largely peg-like teeth, while not exclusive to Diplodocidae, such morphologies and their combination are commonly observed in diplodocines.

To narrow down the taxonomic identity of CMC VP14128, we compared it with the four Morrison Formation genera of Diplodocinae: Barosaurus, Diplodocus, Galeamopus, and Kaatedocus. Diplodocus is known from eight complete skulls (CM 11255, CM 3452, CM 11161, MOR 7029, SMM P84.15.3, USNM 2672, USNM 2763), Kaatedocus from three skull (AMNH 7530 SMA 0004, SMA D-16/312,17), and Galeamopus is reported from at least three partial skulls (AMNH 969, SMA 0011, and HMNS 17512,27). Fragmentary cranial remains of Barosaurus are reported from the Howe Quarry12 and recently have been recovered from the Aaron Scott Site in the San Raphael Swell of Utah (CMC VP15544). A tooth row not restricted to the anteriormost portion of the skull is present in CMC VP14128, Kaatedocus (SMA 0004), and Galeamopus (SMA 0011). The premaxillae of the newest and smallest specimen, CMC VP14128, express the Massopoda condition of four teeth - seen in Diplodocus and Kaatedocus. In contrast, the reconstructed skull of the Galeamopus pabsti holotype has five27. The exact morphology of the prefrontal in CMC VP14128 is difficult to determine due to taphonomic distortion, but it appears to exhibit the typical diplodocid posterior hook27. The anterior portion of the antorbital fenestra seems to be dorsally situated to the preantorbital fossa as seen in CM 11255, Galeamopus12,27 and possibly in Kaatedocus17, although the damaged margin in CMC VP14128 makes this observation tentative. The posterior margin of the postorbital is gracile and more forked – as in Diplodocus and Kaatedocus, in contrast to less forked in Galeamopus12,27; whereas the dorsomedial process is long and tapered as in Diplodocus and Kaatedocus3,4,17, but not Galeamopus27. The squamosal in CMC VP14128 has a tapered and long anterior process approaching the quadrate, as in Diplodocus and Galeamopus, and not Kaatedocus – an autapomorphy of this genus17. Due to taphonomic damage and distortion, the morphology of the sagittal nuchal crest cannot be accurately discerned in CMC VP14128 (distinct and narrow vs. wide11,12). In contrast, a distinct crest is found in both Kaatedocus and Galeamopus hayi12,27, and CMC VP14129 does exhibit this feature (see Materials and Methods). Therefore the weight of evidence indicates a referral to Diplodocus over the other taxa.

Mandible

The relative proportions of the dentary and surangular are very similar to the lower jaw of the larger, and slightly more mature Diplodocus (CM 112553). Assuming similar proportions of mandibular bones in a mature Diplodocus to CMC VP14128, the skull would have been disproportionally stretched (see Material & Methods). Instead, CMC VP14128 indicates that in early ontogeny, the bones of the lower jaw were not directly proportionate to those of an adult (e.g. dentary to total jaw length). Regarding the dentary, the dorsoventral thickness post symphysis is more uniform as in Diplodocus and Kaatedocus3,12, opposed to strongly tapered as in Galeamopus27. One of the most interesting features of the lower jaw is the tooth row (Fig. 1B,C). As expressed in the upper jaw, the lower tooth row extends more posteriorly than seen in more mature specimens. In the right dentary of CMC VP14128, the tooth row (which is posteriorly obscured and damaged) is located along the anterior most ~1.5 cm. The left tooth row, however, clearly spans ~6.5 cm of the dentary; approximately 46% the length of the dentary compared to 22% in adults. Comparable to the adult condition, CM 11255 was hypothesized3 to have had 10–11 dentary teeth, whereas CMC VP14128 possesses 13. This dental variation may represent intraspecific variation28, considering that the dentary formula of CMC VP 14128 is the same as in immature camarasauromorphs28 and that ontogenetic dental formula reduction is documented in other dinosaurs29,30.

Dentition

The dental formula of CMC VP14128 is 4.8/13; the dental formula of the more mature CM 11255 has a formula of 4.8–9/10–11 dentary teeth, which is comparable to the adult condition of CM 11161 that has a formula of 4.9/11–1231.

The premaxillary teeth of CMC VP14128 exhibit the typical diplodocine condition: long, slightly inclined, pointed, and narrow-crowned – the so-called peg-like condition. However, from the second maxillary tooth posteriorly, the teeth are apicobasally short, with mesiodistally wide and more labial convex crowns. Several teeth have a Camarasaurus-like distal occlusal wear facet (Fig. 3). This relatively basal tooth morphology is consistent with the overall basal-expression form of the cranium.

Figure 3 The dental morphotypes in CMC VP14128. Pre- and maxillary teeth of CMC VP14128 in right and left lateral. Drawing by K. Scannella. Red outlines highlight the zoomed in views on the right. Note the combination of diplodocid peg and camarasaurid spatulate tooth forms. Camarasaurus sp. with the spatulate tooth form (SMA 0002). Diplodocus longus with the peg tooth form (USNM 2672). Camarasaurus and Diplodocus skull modified from McIntosh70. Skulls not to scale. Full size image

Maturational state of CMC VP 14128

CMC VP14128 establishes the immature condition for many features of the skull, jaws, dentition, and anterior cervical vertebrae far beyond what has been previously known. We summarize those changes here, and make note of immature features that correspond to the plesiomorphic character states of Diplodocoidea (for a greater discussion on the possibility of ontogenetic recapitulation, please consult the Supplementary Information).

Size

The estimated skull length of CMC VP14128 is 24.29 cm, which is ~40% the length of the largest adult Diplodocus skull (USNM 2673, ~60 cm). Cranial size differences observed between CMC VP14128 and adult skulls attest to changing body size through ontogeny (Fig. 2). The cranial size difference between CMC VP14128 and CM 11255 – ~5 cm – seems minor, yet the understanding of sauropodomorph paleobiology is dependent on their relative scale. Understanding minor skeletal nuances can have vast ontogenetic repercussions. A mere 25 cm difference in femoral length separates a 6 m 6-year-old from a 27 m 24-year-old Diplodocus (5).

While we do not have complete Diplodocus specimens, we have composites and referable material enabling us to draw some conclusions about adult proportions. Specifically, here we use D. carnegii CM 84 which is a composite, but represents the informal standard for the genus32. Nevertheless, using a ratio from CM 84 assumes isometric growth – contrary to the ontogenetic record of Dinosauria – therefore we should view the resulting estimates as nothing more than generalized proportions.

Using this adult cranial:body length ratio predicts a maximum body length of 9 m for CMC VP14128 and 10.9 m for CM 11255, a difference that would be even greater with a more realistic allometric skull-body length ratio; yet even this isometric trend indicates a minimum difference of nearly 2 m in body length, expressed in ~5 cm of cranial length-difference. While we await more specimens to fill in these crucial ontogenetic intervals, assuming size covaries with age at this locality (see Discussion), we hypothesize that CMC VP14128 was within the recorded MDQ ages of two – six years of age5 and had a body length well under the isometrically calculated 9 m (Fig. 1).

Tooth count

CMC VP14128 has a high dentary tooth count – 13 – in contrast to the lower tooth count – 11 – seen in larger, presumably more mature specimens, such as CM 11161 (see description above). This variation in tooth count, while limited in sample size, may be indicating a trend of dentary tooth count reduction, which is seen in other immature dinosaurs29,30,33. Therefore, the high dentary tooth count of CMC VP14128 indicates its juvenile growth stage. Also, basal eusauropods tend to have a dentary tooth count higher than 1134,35.

Neurocentral synostosis

Three cervical vertebrae with neural arches are preserved with CMC VP14128. In two of the cervical vertebrae (labeled 2 and 3 in Fig. S1) the arches are completely separate from their centra. One vertebra (labeled 1 in Fig. S1) has a fused arch with sutural contacts that are seen on the anterior- and posterior-most margins. The corresponding sutures in a skeletally mature Diplodocus (such as CM 84) are completely closed.

Cervical centrum pneumatization

The cervical centra of CMC VP14128 are excavated by shallow, simple, and weakly divided fossae, typical for young animals, as compared to highly complex fossae and foramina with numerous accessory laminae in adults such as D. carnegii CM 845,13,36,37,38.

Cervical rib histology

In the absence of chronologically-informative bones (such as sauropod dorsal ribs that provide an almost complete growth record39,40,41), a cervical rib of CMC VP14128 was sectioned to obtain an estimate of the relative maturity of the specimen based on patterns of remodeling – in like manner to the Histologic Ontogenetic Stage42. However, we must cautiously note that the origin and development of cervical ribs is still ongoing research (JRH and DCW in prep.). As cervical ribs incorporate a complex developmental relationship of metaplastic and osteogenic processes43,44, at this time we should only compare rates of secondary remodeling (Fig. S2).

Progressing through maturity in diplodocid cervical ribs, there are dramatic changes in tissue composition. In the smallest specimen (SMA 0009) the tissue is composed entirely of highly vascular primary tissue. Progressing to CMC VP14128 the tissue is composed of secondary reconstructions and primary tissue – features indicative of metaplasia45. Finally, within a sub-adult Diplodocus (MOR 592), the tissue consists of regular bony tissues - a core of Haversian bone, periosteally grading from secondary to primary osteons (see greater discussion in the Supplementary Material).

While the ontogenetic development of cervical ribs must be studied in further detail, this analysis supports the hypothesis that they develop via metaplasia from a collagenous to an osseous tissue43,44. Thus, the cervical rib of CMC VP14128, conforming to this developmental pathway, further supports our maturational interferences of immaturity for this specimen.

Systematic Paleontology

Saurischia Seeley 1887

Sauropodomorpha von Huene 1932

Sauropoda Marsh 1878

Diplodocoidea Marsh 1884

Flagellicaudata Harris and Dodson 2004

Diplodocidae Marsh 1884

cf. Diplodocus Marsh 1878.

Comparative description

The Morrison Formation preserves three sauropod clades: Diplodocoidea, Camarasauridae, and Brachiosauridae. The lack of only spatulate teeth, an inclined posterior portion of the premaxilla, projecting external naris, cervical ribs shorter than centrum, and rectangular not rhomboidal cervical vertebrae profiles in CMC VP14128 are more diplodocoid than macronarian morphologies. Although many of the diagnostic characters in the sauropod skull are proportionally or ontogenetically variable46 some of the morphologies within CMC VP14128 are different from those expressed in the adult. While a few traits could be outside of the typical adult expressions, we hypothesize that characters/conditions of CMC VP14128 will at least largely associate with a known genus (as previously demonstrated in Europasaurus47).

CMC VP14128 is referable to Diplodocidae based on the presence of: a long posterior process of the prefrontal, teeth that do not span the length of the maxilla and dentary, a low coronoid eminence on the mandible, lack of crown-to-crown occlusion, cervical rib length that is shorter than the corresponding centrum length, and external nares that are retracted and face dorsally. CMC VP14128 is not referable to Apatosaurus given the presence of a basipterygoid recess, a basipterygoid process that lacks an anteroventral flare, and enlarged cervical ribs that do not project below the centrum. Likewise, CMC VP14128 is not referable to Barosaurus, based on the presence of long postzygapophyses, anteroposteriorly narrow neural spines, and a strongly posteriorly-angled centrum cotyles. CMC VP14128 has its strongest affinities with the slender diplodocines, including Diplodocus, Galeamopus, and Kaatedocus; however, the distribution of shared features is inconsistent, obscuring its lower-level identity. However, based on the number of shared characteristics, CMC VP14128 is most referable to Diplodocus than either Galeamopus or Kaatedocus.

However, it must be stated that specimens previously assigned to Diplodocus, and how we phylogenetically recognize and identify this genus are being reexamined12,32. Some historically recognized Diplodocus specimens are now being referred to other genera – such as USNM 2673 possibly representing Galeamopus12 and even CM 11255 to Barosaurus48. Additionally, some Morrison Formation taxa have little to any known or described cranial material (Barosaurus, Dystrophaeus, Haplocanthosaurus, Supersaurus, Suuwassea), therefore there are several taxa we cannot adequately compare CMC VP14128 to or assess. Additionally, while Whitlock11 identified three cranial autapomorphies for Diplodocus (preantorbital fenestra with well-defined fossa, pterygoid medial to ectopterygoid on transverse palatal hook, teeth inclined anteriorly relative to axis of jaw) since no skulls to date are unquestionably associated with post-crania, Tschopp et al.12 questioned these characters. While the latest phylogenetic analysis of Diplodocidae would advocated that no unambiguous diplodocinae cranial synapomorphies are recognized12, the exact taxonomic assignment of CMC VP14128 within Diplodocinae remains uncertain. With the current lack of no known diplodocinae synapomorphies12, one could taxonomically identify CMC VP14128 simply as diplodocinae indeterminate. However, given the predominance of similar morphologies between CMC VP14128 and Diplodocus sp. in comparison to the other Morrison Formation diplodocids, we tentatively opt to refer CMC VP14128 to cf. Diplodocus. While both identifications (diplodocinae indeterminate and cf. Diplodocus) are testable hypotheses, we currently believe that it is more fruitful and more constructive for future works to support/refute our identification of CMC VP14128 as cf. Diplodocus versus reanalyzing starting from a subfamily level identification.

Phylogenetic analyses

Separate data sets

The cranial + postcranial parsimony and Bayesian analyses of the Whitlock11 data set recovered CMC VP14128 as the basalmost member of Dicraeosauridae, while the cranial only analysis recovered CMC VP14128 as the sister species of Dicraeosauridae, and Dicraeosaurus + Amargasaurus, respectively. The cranial + postcranial parsimony and Bayesian analyses of the Tschopp et al.12 data set recovered CMC VP14128, as a diplodocine more derived than Diplodocus, while the cranial only analysis recovered CMC VP14128 in a polytomy within Flagellicaudata.

Combined data set

In the combined cranial + postcranial parsimony and Bayesian typology, there is a degree of basal uncertainty (no united Macronaria), yet there is a structured and organized Diplodocoidea (Fig. 4). In the parsimony typology, CMC VP14128 is recovered as the sister species to Diplodocus, while in the Bayesian analysis, it is recovered as a derived member of Flagellicaudata and basal to Diplodocidae. For the combined cranial only analyses, the typologies are similar to that of the cranial + postcranial analyses. There are distinct Rebbachisauridae, Dicraeosauridae, and Diplodocidae branches, and CMC VP14128 is recovered as the sister taxon to Diplodocus. Apart from minor changes in posterior probability, the Bayesian topology is very similar to the earlier analysis12. It likewise recovers CMC VP14128 in a flagellicaudatan polytomy in which Dicraeosauridae forms one branch. Yet we would caution that while these two analyses superficially produce similar results, the low support for groupings indicates that these relations are not definitive.

Figure 4 Dendrograms of parsimony (left column) and Bayesian (right column) phylogenetic analyses. (A–C) Consist of cranial and postcranial characters, while (D,E) consist of only cranial characters. (A and D) CMC VP14128 coded into the matrix of Whitlock11. (B and E) CMC VP14128 coded into the matrix of Tschopp et al.12. (C and F) CMC VP14128 coded into a combined matrix. Full size image

The minutia between the parsimony and Bayesian analyses do vary, but in general the encompassing skeletal analyses do little to elucidate the taxonomic identify of CMC VP14128. In the analysis of Whitlock11, CMC VP14128 is recovered in the same dicraeosaurid position, while in the analysis of Tschopp et al.12 taxonomy is slightly more refined as CMC VP14128 is recovered as a derived diplodocine versus a flagellicaudatan polytomy, and finally in the combined analysis, CMC VP14128 is still recovered as a diplodocoid. Certainly the encompassing analyses do reveal much more overall taxonomic resolution, but said resolution is largely irrespective to CMC VP14128. A cranial only approach could appear redundant or unnecessary, yet given the vastly differing phylogenetic placements between the matrices used herein, we would suggest that an elemental or regional styled analyses can be used to further check or verify specimens that are recovered in seemingly unusual or suspect positions.