W. Scott Persons IV &Â Philip J. Currie (2019)
The anatomical and functional evolution of the femoral fourth trochanter in ornithischian dinosaurs.
The Anatomical Record (advance online publication)Â
The femoral fourth trochanter is the attachment site of the caudofemoralis musculature, which is the primary hindlimb retractor in most nonâavian dinosaurs. Early ornithischian dinosaurs are uniquely characterized by a fourth trochanter with a prominent pendant process. Throughout the evolution of ornithischians, the fourth trochanter repeatedly converged on two major morphological changes: (1) the distal migration of the trochanter down the femoral shaft and (2) the loss of the pendant process. Both changes, as well as the original evolution of the pendant form, relate to a single major functional shift emphasising caudofemoral leverage. Direct evidence of muscle scarring across the surface of the pendant process affirms that it served to extend the attachment of the primary caudofemoralis brevis tendon distally. A proximally located fourth trochanter is the basal condition in dinosaurs and other archosaurs, and the development of a pendant process lengthened the functional lever arm with regard to the insertion of the caudofemoralis. This adaptation afforded improved mechanical advantage, perhaps beneficial in the context of the newly assumed herbivorous diets of basal ornithischians. As some derived ornithischians increased in body size, a highâleverage system with a more distal caudofemoralis attachment evolved. In some groups, the fourth trochanter as a whole descended down the femur, eventually reaching a point where the pendant process was unnecessary. Sauropodomorphs, the other great lineage of dinosaur herbivores, converged on the same highâleverage distal fourth trochanter arrangement, but without first transitioning through a prominent pendant form.
Brandon P. Hedrick, Erika Goldsmith, Hector RiveraâSylva, Anthony R. Fiorillo, Allison R. TumarkinâDeratzian & Peter Dodson (2019)
Filling in gaps in the ceratopsid histologic database: Histology of two basal centrosaurines and an assessment of the utility of rib histology in the Ceratopsidae.
The Anatomical Record (advance online publication)Â
Bone histology grants substantial insight into the growth and biology of fossil vertebrates. Many of the major nonâavian dinosaurian clades have been extensively sampled for bone histologic data allowing reconstruction of their growth as well as the assessment of the evolution of growth changes along phylogenies. However, horned ceratopsians are poorly represented in paleohistologic studies. Further, the ceratopsian taxa that have been examined are unevenly sampled phylogenetically with very basal forms and highly derived forms making up the majority of studied taxa. In order to rectify this, we have histologically sampled Avaceratops from Montana and Yehuecauhceratops from northern Mexico to assess how midâsized basal centrosaurines grew relative to more basal and derived forms. Based on results from these taxa, basal centrosaurines present a mosaic of growth characters intermediate between those seen in basal ceratopsians and more derived centrosaurines. Further, Yehuecauhceratops has many lines of arrested growth preserved, suggesting that the large number of lines of arrested growth found in a highâlatitude Pachyrhinosaurus specimen may be a result of phylogeny rather than geography. Since lines of arrested growth are not preserved in long bones of many ceratopsians, especially chasmosaurines, we also histologically sampled ribs of Avaceratops and Pachyrhinosaurus. However, the largest ribs were highly remodeled obscuring lines of arrested growth, making it unlikely that rib histology will clarify growth trends in ceratopsians. These centrosaurines add to the growing ceratopsian histological database and demonstrate that basal centrosaurines grew in a manner intermediate between nonâceratopsid taxa and derived centrosaurines.