On the origin of high growth rates in archosaurs and their ancient relatives: Complementary histological studies on Triassic archosauriforms and the problem of a “phylogenetic signal” in bone histology

Three possible hypotheses could explain the polarity of the histological features of basal archosauriform and archosauromorph reptiles: either, the fibrolamellar complex is basal; or, the lamellar-zonal complex is basal or finally, the condition varied, and each complex evolved more than once in these early groups. The answer to this question would have broad implications for our understanding of the physiological, ecological, and behavioral features of the first archosaurs. To this end, we sampled the bone histology of various archosauriforms and basal archosaurs from the Triassic and Lower Jurassic: erythrosuchids, proterochampsids, euparkeriids, and basal ornithischian dinosaurs, including forms close to the origin of archosaurs but poorly assessed phylogenetically. The new data suggest that the possibility of reaching and maintaining very high growth rates through ontogeny could have been a basal characteristic of archosauriforms. This was partly retained (at least during early ontogeny) in most lineages of Triassic pseudosuchians, which nevertheless generally relied on lower growth rates to reach large body sizes. This trend to slower growth seems to have been further emphasized among Crocodylomorpha, which may thus have secondarily reverted toward more generalized reptilian growth strategies. Accordingly, their “typical ectothermic reptilian condition” may be a derived condition within archosauriforms, homoplastic to the generalized physiological condition of basal amniotes. On the other hand, ornithosuchians apparently retained and even enhanced the high growth rates of many basal archosauriforms during most of their ontogenetic trajectories. The Triassic may have been a time of “experimentation” in growth strategies for several archosauriform lineages, only one of which (ornithodirans) eventually stayed with the higher investment strategy successfully.