Growth dynamics, skeletochronology, and histovariability of the theropod dinosaur Berthasaura leopoldinae.

Here we present the bone histology of Berthasaura leopoldinae MN 7821-V, a basal abelisauroid from the Late Cretaceous of Southern Brazil, and discuss the impacts of multi-element sampling in ceratosaurs, including its biological implications. Femur, tibia, and fibula from the holotype were sectioned for this study. Age was estimated using LAG retro-calculation and cyclical growth marks count. The cortex of the femur and tibia consists predominantly of parallel-fibered tissue mixed with regions of fibrolamellar bone, which suggests slow osteogenesis similar to that inferred for abelisauroids rather than ceratosaurids and most theropod dinosaurs. In contrast, the fibula is predominantly composed of a fibrolamellar complex. Whereas the femur is primarily filled by longitudinally oriented vascular canals, with a band of plexiform, the tibia is primarily of longitudinal with a reticular pattern, and the fibula is exclusively of a simple longitudinal network with primary and secondary osteons. The bone cortices are commonly interrupted by lines of arrested growth (LAG) in the femur and tibia, and LAGs plus annuli in the fibula, indicating that the bone growth was periodic. Outer circumferential layers of lamellar bone were not recorded in any of the sampled elements. In addition, the thickness of the growth zones was consistent in the femur and tibia; that, in combination with absent outer circumferential layers, suggests that MN 7821 was a subadult that did not reach sexual maturity before death. This corroborates a previous ontogenetic assessment inferred based on morphology. Inter-elemental variations in bone matrix type and growth record were observed in Berthasaura, showing that such discrepancy would impact metabolic inferences based on single bones. In addition, the particularly narrowing growth zones from the femur of MN-7821-V provided higher values of retro-calculated LAG in comparison to the tibia and fibula. Based on this histovariability observed in the different elements of Berthasaura's skeleton, our work also pointed to an underlying bias concerning LAG retro-calculation based on metric extrapolations from a single bone, which reinforces the importance of working with a multi-elemental sample.