Synchrotron imaging reveals diversity in archosauromorph growth strategies by the end of the early Triassic

Summary Archosauromorphs are a diverse clade of vertebrates that originated in the Early Triassic and persist today as crown birds (ornithodirans) and crocodilians (pseudosuchians). They exhibit markedly different life-history strategies, with ornithodirans characterized by rapid growth, and more crown-ward pseudosuchians with slower growth rates. Understanding how these patterns emerged requires further insight into growth dynamics among early archosauromorphs. We use synchrotron X-ray micro-computed tomography to investigate the bone microstructure in an assemblage of archosauromorphs from the Early Triassic locality Driefontein Farm 11 in the Olenekian-Anisian Langbergia-Garjainia subzone of the Cynognathus zone in the main Karoo Basin of South Africa. Our findings reveal that growth strategies among Early Triassic archosauromorphs already encompass both rapid and slow growth rates. We further deduce that the diversity in growth strategies observed in crown-ward archosaur lineages were not lineage-specific derived characteristics but rather inherited growth traits already present among their Early Triassic members.