Bone histology and growth in Stenaulorhynchus stockleyi (Archosauromorpha: Rhynchosauria) from the Middle Triassic of the Ruhuhu Basin of Tanzania

The bone histology of non-archosauriform archosauromorphs is understudied but is important for tracing the evolution of growth in archosaurs and their closest relatives. Here, we describe the femoral and tibial histology of a single individual of Stenaulorhynchus stockleyi, a Middle Triassic rhynchosaur from Tanzania. Both elements are composed largely of moderately vascularized parallel-fibered bone tissue, which becomes avascular and more lamellar approaching the periosteum. The spacing between growth marks in S. stockleyi strongly attenuates with age, suggesting determinate growth for this taxon. Whereas the bone tissue suggests slower growth compared to South American rhynchosaurs, which have fibrolamellar bone tissue and exhibit more anastomoses between canals, the determinate growth pattern seems to be shared among hyperodapedontid rhynchosaurs. Although non-archosauriform archosauromorphs vary in the rate of bone deposition and growth in their first year, all taxa examined to date grew relatively slowly compared to most archosauriforms.