Comparative Bone Histology in the Ischigualasto Formation (Upper Triassic): Shedding Light on Early Dinosaur Growth Patterns

KRIS CURRY ROGERS; MARTÍNEZ, RICARDO NESTOR; ALCOBER, OSCAR; CARINA COLOMBI

Congreso; Society of Vertebrate Paleontology Meeting; 2017

The Upper Triassic (Carnian-Norian) Ischigualasto Formation preserves basal members of the three dinosaurian subgroups (Ornithischia, Theropoda, and Sauropodomorpha). The formation also yields a suite of non-dinosaurian tetrapods (archosauromorphs, crurotarsans, non-mammalian cynodonts) that overlap with or differ from the earliest dinosaurs in body size, diet, and habitat. The taxonomic and ecological diversity of Ischigualasto vertebrates, combined with the relatively short temporal span of the formation (~ 6 my) and its well-documented paleoenvironmental setting provide an ideal framework for investigating growth dynamics. We employed bone histology to test the following hypotheses: (1) the evolution of elevated growth rates evolved in archosauromorphs before the divergence of distinctive groups (e.g., Crurotarsi); and (2) early dinosaurs exhibited growth strategies distinct from those of contemporaneous non-dinosaurian taxa. We sampled mid-diaphyseal femoral cross-sections from terrestrial Archosauromorpha (Scaphonyx), aquatic Archosauriformes (Proterochampsa, Chanaresuchus), terrestrial Crurotarsi (Sillosuchus, Saurosuchus, Trialestes), terrestrial, potentially omnivorous (Eoraptor) and carnivorous (Herrerasaurus, Sanjuansaurus, Eodromaeus) dinosaurs, and terrestrial, herbivorous Cynodontia (Exaeretodon). Our results suggest that fibrolamellar bone is common to a diversity of non-dinosaur and early dinosaur taxa (this finding is consistent with the reconstructions of other authors). Interestingly, in taxa with more organized primary bone tissue (e.g., Exaeretodon), vascularity is dense and interwoven. Mid-cortical Lines of Arrested Growth are absent in the archosaur sample. Finally, two ?modes? of bone remodeling are developed, and these only rarely co-occur in our sample: some taxa (e.g., Eoraptor, Exaeretodon) exhibit occasional secondary osteons, while others (e.g., Trialestes, Sanjuansaurus) exhibit focused endosteal remodeling. Our results suggest that there may be only a few aspects of early dinosaur growth that are truly distinctive from most of their non-dinosaurian contemporaries.