Origins of slow growth on the crocodilian stem lineage

Crocodilians grow slowly and have low metabolic rates similar to other living reptiles, but palaeohistology indicates that they evolved from an ancestor with higher growth rates.^1,2,3,4,5 It remains unclear when slow growth appeared in the clade due to the sparse data on key divergences among early Mesozoic members of their stem lineage. We present new osteohistological data from a broad sample of early crocodylomorphs, evaluated in a phylogenetic context alongside other pseudosuchians. We find that the transition to slow-growing bone types during mid-late ontogeny occurred around the origin of Crocodylomorpha during the Late Triassic. Earlier-diverging pseudosuchians had high maximum growth rates, as indicated by the presence of woven bone during middle and (sometimes) late ontogeny.^6,7,8,9 Large-bodied pseudosuchians in particular exhibit some of the fastest-growing bone types, giving evidence for prolonged, rapid growth. By contrast, early-branching crocodylomorphs, including a new large-bodied taxon, had slow maximum rates of bone deposition, as evidenced by the presence of predominantly parallel-fibered or lamellar bone tissue during middle-late ontogeny. Late Triassic crocodylomorphs show skeletal anatomy consistent with "active" terrestrial habits,^10,11,12 and their slow growth rates reject hypotheses linking this transition with sedentary, semiaquatic lifestyles or sprawling posture. Faster-growing pseudosuchian lineages go extinct in the Triassic, whereas slow-growing crocodylomorphs do not. This contrasts with the Jurassic radiation of fast-growing dinosaurs on the bird-stem lineage,¹³ suggesting that the End-Triassic mass extinction initiated a divergent distribution of growth strategies that persist in present-day archosaurs.