Abstract

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 microstructure of smaller, bipedal Sauropodomorpha and larger, quadrupedal basal Sauropoda allow us to infer how the growth dynamics changed to allow for the evolution of gigantism. Historically, osteohistological studies of Sauropomorpha tended to have focused on either basal taxa (e.g. Plateosaurus & Massospondylus) or on derived taxa (diplodocids & titanosaurs), whereas studies on the growth dynamics of the transitionary groups (i.e. Sauropodiformes & basal Sauropoda) are poorly known. Here, we assess the palaeobiology of two sauropodiformes and a basal 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 was the most common trait among these throughout 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 formation of annuli around fully formed LAGs in Sauropoda indet. as well as the location of LAGs. The number of LAGs in the cortex varied among the taxabut generally the greater accumulation of LAGs were found in the outer regions of the cortex. The growth dynamics of our three sauropodomorph dinosaurs are similar to basal 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 support a mosaic of growth dynamics within transitionary taxa of Sauropodomorpha.