Osteohistological signal from the smallest known phytosaur femur reveals slow growth and new insights into the evolution of growth in Archosauria.

Fossils of embryonic and hatchling individuals can provide invaluable insight into the evolution of prenatal morphologies, heterochronies, and allometric trajectories within Archosauria but are exceptionally rare in the Triassic fossil record, obscuring a critical aspect of archosaurian biology during their evolutionary origins. Microvertebrate sampling at a single bonebed in the Upper Triassic Chinle Formation within Petrified Forest National Park has yielded diminutive archosauriform femora (PEFO 45274, PEFO 45199) with estimated and measured femoral lengths of ~31 mm and ~ 37 mm, respectively. These new specimens provide the unique opportunity to assess the preservation, body size, and growth dynamics of skeletally immature archosauriforms in North America and compare the growth dynamics of archosauromorphs within an evolutionary and ontogenetic context. We assign PEFO 45199 and PEFO 45274 to Phytosauria (Archosauriformes) based on their strongly sigmoidal shape in lateral view, the presence of proximal anterolateral and posteromedial tubera, the absence of an anteromedial tuber of the proximal end, a teardrop-shaped proximal outline, and a fourth trochanter that is not confluent with the proximal head. Osteohistological analyses of PEFO 45274 reveal a cortex comprising low vascularity, parallel-fibered bone composed of primary osteons that lacks a hatching line and any lines of arrested growth. We interpret PEFO 45274 as a slow-growing, post-hatching individual of less than 1 year of age. Surprisingly, osteohistology of some larger phytosaur femora implies faster growth rates in comparison to PEFO 45274 based on the occasional presence of woven bone and overall higher degrees of vascular density, suggesting the ontogenetic shift from rapid-to-slow growth rates might not occur simply or uniformly as expected in Phytosauria and that non-archosaurian archosauriforms may exhibit size-dependent histological characteristics. This study highlights the importance of including osteohistology from multiple body sizes to investigate non-archosaurian archosauriform ancestral growth rates given the phylogenetic position of phytosaurs near the divergence of Archosauria.