Palaeobiology and osteohistology of South African sauropodomorph dinosaurs

Several sauropodomorph dinosaurs have been excavated from the Elliot Formation (EF) of Southern Africa which include important taxa such as Massospondylus, Melanorosaurus and Antetonitrus. The study of the bone microstructure of smaller, bipedal Sauropodomorpha and larger, quadrupedal Sauropoda allow us to infer how the growth dynamics changed during the evolution of gigantism. Historically, osteohistological studies of Sauropodomorpha tended to have focused on either early diverging taxa (e.g. Plateosaurus & Massospondylus) or on derived taxa (diplodocids & titanosaurs), whereas studies on the growth dynamics of the transitionary groups (i.e. Sauropodiformes & early Sauropoda) are poorly known. Here, we assess the palaeobiology of two sauropodiformes and an early sauropod by analysing their bone histology. Thin sections of the long bones of two indeterminate sauropodiformes NMQR 3314 and NMQR 1551, and an indeterminate sauropod SAM-PK-K382 were prepared. The general histology of the long bones of all three dinosaurs were similar. Rapid growth through the deposition of fibrolamellar bone tissue characterised their respective ontogenies. Lines of arrested growth (LAGs) were commonly located in the mid and outer cortex signalling the onset of uninterrupted growth. Differences in the histology of these dinosaurs were principally related to the pathological bone tissue evident in the femur of the sauropodiform NMQR 1551 and to the formation of annuli around LAGs in Sauropoda indet., as well as in the location of LAGs in the compacta. The number of LAGs in the cortex varied among the taxa but generally the outer regions of the cortex showed an accumulation of LAGs. The growth dynamics of our three sauropodomorph dinosaurs are similar to early sauropods such as Antetonitrus. It appears that the abundance of fibrolamellar bone tissue and uninterrupted growth at later ontogenetic stages are likely key traits in the early evolution of gigantism in Sauropoda, which supports the occurrence of a mosaic of growth dynamic patterns among transitionary Sauropodomorpha.