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. 2018 Oct 11;8(1):14341.
doi: 10.1038/s41598-018-32620-x.

The Smallest Diplodocid Skull Reveals Cranial Ontogeny and Growth-Related Dietary Changes in the Largest Dinosaurs

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Free PMC article

The Smallest Diplodocid Skull Reveals Cranial Ontogeny and Growth-Related Dietary Changes in the Largest Dinosaurs

D Cary Woodruff et al. Sci Rep. .
Free PMC article

Abstract

Sauropod dinosaurs were the largest terrestrial vertebrates; yet despite a robust global fossil record, the paucity of cranial remains complicates attempts to understand their paleobiology. An assemblage of small diplodocid sauropods from the Upper Jurassic Morrison Formation of Montana, USA, has produced the smallest diplodocid skull yet discovered. The ~24 cm long skull is referred to cf. Diplodocus based on the presence of several cranial and vertebral characters. This specimen enhances known features of early diplodocid ontogeny including a short snout with narrow-crowned teeth limited to the anterior portion of the jaws and more spatulate teeth posteriorly. The combination of size plus basal and derived character expression seen here further emphasizes caution when naming new taxa displaying the same, as these may be indicative of immaturity. This young diplodocid reveals that cranial modifications occurred throughout growth, providing evidence for ontogenetic dietary partitioning and recapitulation of ancestral morphologies.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Skeletal reconstruction of CMC VP14128 to scale with a mature D. carnegii (dark grey). Grey bones are missing, while those in ivory are those present in CMC VP14128. Skeletal reconstruction based on the Diplodocus by S. Hartman. Silhouettes by S. Hartman and PhyloPic (Creative Commons Attribution-ShareAlike 3.0 Unported; http://phylopic.org/image/3cb1d5bf-7db5-4db2-82a6-4c39f6a4441b/; https://creativecommons.org/licenses/by-sa/3.0/), modifications made. Skeletal reconstruction of CMC VP14128 redrawn from D. carnegii skeletal by S. Hartman (http://www.skeletaldrawing.com/sauropods-and-kin/diplodocus). Human scale is Andrew Carnegie at his natural height of 1.6 m. Skeletal and silhouettes to scale. (B) CMC VP14128 in right lateral view with accompanying schematic. (C) CMC VP14128 in left lateral view with accompanying schematic. Schematics by DCW. The four portions of the skull numbered on accompanying schematics. Lateral views and schematics to scale. a: angular, al: alisphenoid, aof: antorbital fenestra, d: dentary, f: frontal, h: hyoid, l: lacrimal, m: maxilla, n: nasal, oc: occipital condyle, os: orbitosphenoid, p: parietal, paof: preantorbital fenestra, pf: prefrontal, pm: premaxilla, po: postorbital, pro: prootic, q: quadrate, sa: surangular, sq: squamosal. L and r before bone denotes if it is left or right.
Figure 2
Figure 2
Cranial ontogeny in Diplodocus sp. (A) CMC VP14128; (B) CM 11255 (redrawn from Whitlock et al.); (C) CM 11161 (redrawn from Wilson and Sereno). Skull drawings by K. Scannella. Skulls to scale. (D) Silhouettes of CMC VP14128 and a mature D. carnegii to illustrate body length differences between skulls of A and C size. Diplodocus silhouette by S. Hartman and PhyloPic (Creative Commons Attribution-ShareAlike 3.0 Unported; http://phylopic.org/image/3cb1d5bf-7db5-4db2-82a6-4c39f6a4441b/; https://creativecommons.org/licenses/by-sa/3.0/), modifications made. Skeletal reconstruction of CMC VP14128 redrawn from D. carnegii skeletal by S. Hartman (http://www.skeletaldrawing.com/sauropods-and-kin/diplodocus). (E) Transformation grid highlighting the ontogenetic cranial changes. Adult skull is the same in part C (Wilson and Sereno).
Figure 3
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 McIntosh. Skulls not to scale.
Figure 4
Figure 4
Dendrograms of parsimony (left column) and Bayesian (right column) phylogenetic analyses. (AC) Consist of cranial and postcranial characters, while (D,E) consist of only cranial characters. (A and D) CMC VP14128 coded into the matrix of Whitlock. (B and E) CMC VP14128 coded into the matrix of Tschopp et al.. (C and F) CMC VP14128 coded into a combined matrix.
Figure 5
Figure 5
Life reconstruction of CMC VP14128. Note the cranial morphologies interpreted to denote differing feeding strategies: in CMC VP14128 the narrow snout with posteriorly elongated and morphologically varied tooth row for bulk feeding vs. the widened snout with anteriorly restricted peg-shaped teeth for ground-level browsing in adults. Also note the camouflaged ontogenetic color change suggesting young diplodocids may have sought forested refuge. Reconstruction by A. Atuchin.

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