TESTING TRIASSIC CLIMATE VARIABILITY AND VERTEBRATE DISTRIBUTION ACROSS SOUTHERN PANGEA WITH COMPARATIVE HISTOLOGICAL ANALYSIS

Climate strongly influenced the distribution of major animal groups that emerged during the Triassic period throughout the supercontinent Pangea. The earliest dinosaurs and their associated faunas (Carnian Stage, Late Triassic, ~230 Ma) are recovered along the same paleolatitude across southern Pangea. Understanding climatic variability within this single climatic belt will help constrain the climate ranges of the first dinosaurs, before they spread worldwide. We describe the bone histology of 5 vertebrate taxa from the Carnian of Zimbabwe, including sauropodomorph and herrerasaurid dinosaurs, a gomphodontosuchine cynodont, a hyperodapedontine rhynchosaur, and an aetosaur. The dinosaurs exhibit well-vascularized cortical bone and no apparent growth marks, suggestive of continuously growing individuals. The cynodont femur also has a well-vascularized fibrolamellar matrix with anastomosing vascular canals throughout the cortex and a single growth mark, indicating it was experiencing rapid growth. In contrast, the rhynchosaur femur and aetosaur tibia show moderate to poorly vascular bone matrix, possessing multiple lines of arrested growth (LAG) and an external fundamental system (EFS), suggesting that these specimens are slow-growing mature individuals. Comparing this histology to that of similar assemblages from Brazil, Argentina, and India, the central Pangean (i.e., Zimbabwean) dinosaurs exhibit more continuous growth than other dinosaurs along this climatic belt, whereas the Zimbabwean rhynchosaur and aetosaur exhibit slower growth patterns with more frequent cessation of growth than their South American counterparts. The slower-growing Zimbabwean taxa exhibit characteristics suggestive of a harsher environment (e.g., slower growth, frequent cessation), however, the probable endotherms (i.e., dinosaurs, cynodont) from this same assemblage apparently grew rapidly and continuously throughout the year. This is consistent with an overall harsher but less seasonal climate in Zimbabwe compared to Brazil, Argentina, and India. This hypothesis can be further tested via paleoclimate proxies and will contribute to a better view of Triassic climate across Pangea, providing clarity regarding the ancestral environmental tolerances of early dinosaurs.