Browsing by Author "Stefanic, Candice M."

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    The axial skeleton of Poposaurus langstoni (Pseudosuchia: Poposauroidea) and its implications for accessory intervertebral articulation evolution in pseudosuchian archosaurs
    Stefanic, Candice M.; Nesbitt, Sterling J. (PeerJ, 2018-02-14)
    Dinosaurs and their close relatives grew to sizes larger than any other terrestrial animal in the history of life on Earth, and many enormous dinosaurs (e.g., Diplodocus, Spinosaurus, Tyrannosaurus) have accessory intervertebral articulations that have been suggested to support these large body sizes. Some pseudosuchian archosaurs have been reported to have these articulations as well, but few have been characterized in these taxa because of a lower abundance of complete, threedimensional pseudosuchian vertebral material in relation to dinosaurs. We describe the axial column of the large (similar to 4-5 m) poposauroid pseudosuchian Poposaurus langstoni from the Upper Triassic of Texas (TMM Locality 31025 of the Otis Chalk localities; Dockum Group, Howard County, TX, USA). P. langstoni was originally named from pelvic girdle elements and vertebrae; here we describe newly discovered and prepared presacral vertebrae and a presacral rib from the original excavation of the holotype in the 1940s. The well-preserved vertebrae have well-defined vertebral laminae and clear hyposphene-hypantrum intervertebral articulations, character states mentioned in pseudosuchians but rarely described. The new material demonstrates variation present in the hyposphene-hypantrum articulation through the vertebral column. We compared these morphologies to other pseudosuchians with and without the hyposphene-hypantrum articulation. Based on these careful comparisons, we provide an explicit definition for the hyposphene-hypantrum articulation applicable across Archosauria. Within Pseudosuchia, we find the hyposphene-hypantrum appeared independently in the clade at least twice, but we also see the loss of these structures in clades that had them ancestrally. Furthermore, we found the presence of large body sizes (femoral lengths >similar to 300 mm) and the presence of the hyposphene-hypantrum is correlated in most non-crocodylomorph pseudosuchian archosaurs with a few exceptions. This result suggests that the presence of the hyposphene-hypantrum is controlled by the increases and decreases in body size and not strictly inheritance.
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    The evolution and role of the hyposphene-hypantrum articulation in Archosauria: phylogeny, size and/or mechanics?
    Stefanic, Candice M.; Nesbitt, Sterling J. (Royal Society, 2019-08-22)
    Living members of Archosauria, the reptile clade containing Crocodylia and Aves, have a wide range of skeletal morphologies, ecologies and body size. The range of body size greatly increases when extinct archosaurs are included, because extinct Archosauria includes the largest members of any terrestrial vertebrate group (e.g. 70-tonne titanosaurs, 20-tonne theropods). Archosaurs evolved various skeletal adaptations for large body size, but these adaptations varied among clades and did not always appear consistently with body size or ecology. Modification of intervertebral articulations, specifically the presence of a hyposphene-hypantrum articulation between trunk vertebrae, occurs in a variety of extinct archosaurs (e.g. non-avian dinosaurs, pseudosuchians). We surveyed the phylogenetic distribution of the hyposphene-hypantrum to test its relationship with body size. We found convergent evolution among large-bodied clades, except when the clade evolved an alternative mechanism for vertebral bracing. For example, some extinct lineages that lack the hyposphene-hypantrum articulation (e.g. ornithischians) have ossified tendons that braced their vertebral column. Ossified tendons are present even in small taxa and in small-bodied juveniles, but largebodied taxa with ossified tendons reached those body sizes without evolving the hyposphene-hypantrum articulation. The hyposphene-hypantrum was permanently lost in extinct crownward members of both major archosaur lineages (i.e. Crocodylia and Aves) as they underwent phyletic size decrease, changes in vertebral morphology and shifts in ecology.