Hip joint articular soft tissues of non-dinosaurian Dinosauromorpha and early Dinosauria: evolutionary and biomechanical implications for Saurischia

ABSTRACT

Dinosauromorphs evolved a wide diversity of hind limb skeletal morphologies, suggesting highly divergent articular soft tissue anatomies. However, poor preservation of articular soft tissues in fossils has hampered any follow-on functional inferences. We reconstruct the hip joint soft tissue anatomy of non-dinosaurian dinosauromorphs and early dinosaurs using osteological correlates derived from extant sauropsids and infer trends in character transitions along the theropod and sauropodomorph lineagues. Femora and pelves of 107 dinosauromorphs and outgroup taxa were digitized using 3D imaging techniques. Key transitions were estimated using maximum likelihood ancestral state reconstruction. The hips of dinosauromorphs possessed wide a disparity of soft tissue morphologies beyond the types and combinations exhibited by extant archosaurs. Early evolution of the dinosauriform hip joint was characterized by the retention of a prominent femoral hyaline cartilage cone in post-neonatal individuals, with the cartilage cone independently reduced within theropods and sauropodomorphs. The femur of Dinosauriformes possessed a fibrocartilage sleeve on the metaphysis, which surrounded a hyaline core. The acetabulum of Dinosauriformes possessed distinct labrum and antitrochanter structures. In sauropodomorphs, hip congruence was maintained by thick hyaline cartilage on the femoral head, whereas theropods relied on acetabular tissues such as ligaments and articular pads. In particular, the craniolaterally ossified hip capsule of non-Avetheropoda neotheropods permitted mostly parasagittal femoral movements. These data indicate that the dinosauromorph hip underwent mosaic evolution within the saurischian lineage and that sauropodomorphs and theropods underwent both convergence and divergence in articular soft tissues, correlated with transitions in body size, locomotor posture, and joint loading.

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Citation for this article: Tsai, H. P., K. M. Middleton, J. R. Hutchinson, and C. M. Holliday. 2018. Hip joint articular soft tissues of non-dinosaurian Dinosauromorpha and early Dinosauria: evolutionary and biomechanical implications for Saurischia. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1427593.

ACKNOWLEDGMENTS

This study greatly benefited from discussions with A. Hammond, K. Sellers, S. Werning, R. Menegaz, T. Owerkowicz, A. Summers, S. Gatesy, and H. Mallison. The authors are grateful to S. Nesbitt for providing a research quality cast of Asilisaurus kongwe; to H. Mallison for providing computed tomography scan data of Plateosaurus; and to W. Parker for providing laser scan data for Coelophysis for this study. We thank all of the curators and staff for access to fossil specimens and permission for digital data collection. We thank C. Ward and L. Cowgill for reviewing early drafts of this article, which was part of H.P.T.'s University of Missouri doctoral dissertation. We would also like to thank both reviewers for their insightful and helpful comments on this article. H.P.T. was supported by Jurassic Foundation Research Grant, the Doris O. and Samuel P. Welles Research Fund, as well as the University of Missouri Life Science Travel Grant. J.R.H. was supported by Natural Environment Research Council grants NE/G005877/1 and NE/K004751/1. C.M.H. was supported by the University of Missouri Department of Pathology and Anatomical Sciences, the Missouri Research Board, and the University of Missouri Research Council.

Handling editor: Alan Turner.