Meeting Abstract
39.2 Wednesday, Jan. 5 Reconstructing growth of the basal archosauromorph Trilophosaurus WERNING, S*; IRMIS, RB; Univ of California, Berkeley; Univ of Utah; Utah Museum of Natural History swerning@berkeley.edu
Despite extensive research on the evolution of bird-line archosaur growth strategies, the basal condition for archosaurian growth rates, bone histology, and ontogenetic changes in surface morphology remain undescribed. The basal archosauromorph Trilophosaurus buettneri is well-known from hundreds of individual elements from a monodominant Late Triassic (~220Ma) bonebed near Otis Chalk, Texas. This large sample preserves multiple ontogenetic stages and presents an unusual opportunity to examine growth in a proximal outgroup to Archosauria. We measured over 300 humeri, ulnae, femora, and tibiae across all sizes of Trilophosaurus, constructed allometric growth curves for each element, and compared allometric changes among elements. We then histologically sampled ontogenetic series of femora and tibiae to obtain skeletochronological age estimates and constructed age-size growth curves. We also described the osteohistology of a representative adult humerus and ulna. Growth lines occur in the smallest specimens we sampled, and features indicating cessation of skeletal growth occur in the largest. Throughout ontogeny, Trilophosaurus shows a lamellar tissue organization, low-to-moderate levels of vascularity, radially-organized simple longitudinal canals with few anastomoses, and highly organized osteocytes. Finally, we examined changes in bone surface morphology with size and age. Increases in the robusticity of muscle attachments and epiphyses generally do not occur until at least 2/3 total adult length, several years after hatching. All methods suggest very slow growth throughout ontogeny for Trilophosaurus, and that the high growth and bone deposition rates achieved by more derived archosaurs are not characteristic of all members of Archosauromorpha. Our results help characterize the basal condition for growth, bone histology, and limb ontogenetic morphology for Archosauria.
