Comparative histological analysis of vertebrates reveals Triassic climate variability across southern Pangea.

Climate highly influenced the distribution of major animal groups that emerged during the Triassic throughout the supercontinent Pangea. The earliest dinosaurs and associated assemblages (Carnian Stage, Late Triassic, ~230 Ma) are recovered along a paleolatitudinal climate belt spanning southern Pangea. However, developmental responses of different amniotic clades to climate across these environments remain unknown. Characterizing developmental variability, presumably driven in part by climatic variability within this belt, helps constrain the climatic ranges that shaped the earliest dinosaurs prior to their worldwide dispersal, while also providing insights into developmental plasticity of Triassic fauna. We analyzed the bone histology of five vertebrate taxa from the Carnian of Zimbabwe, including sauropodomorph and herrerasaurid dinosaurs, a gomphodontosuchine cynodont, a hyperodapedontine rhynchosaur, and a suchian archosaur (?aetosaur) as a proxy to assess probable climatic influence by comparing various growth strategies/metabolisms of these clades across southern Pangea. The dinosaurs are continuously growing individuals that exhibit well-vascularized woven-parallel and parallel-fibered cortical bone with no apparent growth marks. The cynodont is an immature specimen with a well-vascularized fibrolamellar matrix that has anastomosing vascular canals throughout the cortex and a single growth mark, indicating rapid growth. The rhynchosaurian femur and suchian tibia show moderate to poorly vascularized parallel-fibered and woven-fibered matrixes possessing multiple lines of arrested growth along with an external fundamental system, suggesting these were slow-growing individuals that had reached skeletal maturity at the time of death. Comparing this histology to that of similar assemblages from current-day Brazil, Argentina, and India (which fell along the same paleolatitudinal zone during the Triassic), the mid-continent (i.e., Zimbabwean) dinosaurs exhibit continuous uninterrupted growth in contrast to other histologically sampled dinosaurs from different localities along this climatic belt, whereas the Zimbabwean rhynchosaur and suchian exhibit patterns with more frequent cessation of growth than their South American counterparts. Slower-growing Zimbabwean taxa-presumably, more susceptible to climatic stressors-exhibit characteristics suggestive of a less resource-rich environment (e.g., frequent cessations, more interrupted growth) compared with rhynchosaurs and aetosaurs from South America. Taxa with faster growth rates and higher metabolic regimes (i.e., dinosaurs, cynodont) from this same assemblage apparently grew rapidly-and roughly continuously-throughout the year. This is consistent with an overall more arid but less seasonal climate in Zimbabwe compared to the signals recovered in Brazil, Argentina, and India.