Abstract
ABSTRACTDinosauromorphs 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.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation 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.
Highlights
The evolution of Dinosauromorpha is characterized by a suite of anatomical features in the appendicular limbs, culminating in the drastically divergent locomotor behaviors of birds, sauropods, and multiple clades of ornithischians (Allen et al, 2009; Hutchinson and Allen, 2009; Sander et al, 2011; Maidment and Barrett, 2012)
In a generalized dinosaurian hip joint, the inner acetabular wall is unossified, resulting in a ring-shaped bony acetabulum (Fig. 3c-i). This “perforated acetabular” (Kuznetsov and Sennikov, 2000; Nesbitt, 2011) morphology is the osteological correlate for the acetabular membrane
The antitrochanter cartilage, which consists of a fibrocartilaginous articular surface and a hyaline cartilage core, occupies the caudal acetabulum
Summary
The evolution of Dinosauromorpha is characterized by a suite of anatomical features in the appendicular limbs, culminating in the drastically divergent locomotor behaviors of birds, sauropods, and multiple clades of ornithischians (Allen et al, 2009; Hutchinson and Allen, 2009; Sander et al, 2011; Maidment and Barrett, 2012). We infer the presence and topology of joint soft tissues using phylogenetically informed osteological correlates (Tsai and Holliday, 2015), identify the polarity and sequence of discrete character transitions using maximum likelihood ancestral state reconstruction (Schluter et al, 1997; Pagel, 1999), and test the homology of osteological characters based on reconstructed soft tissues. We identified 14 characters based on osteological correlates of putatively homologous hip joint cartilages, ligaments, and articular pads in extant diapsids (Table 2; Tsai and Holliday, 2015). These osteological characters serve as proxies for the presence, orientation, thickness, and shapes of articular soft tissues. We reconstructed hip joint articular soft tissues based on the sequence of transitions in osteological correlates for focal taxa along the sauropod and theropod lineages
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