Palaeobiological inferences for the South American dicraeosaurid Brachytrachelopan mesai (Dinosauria; Sauropoda) based on bone histology of the holotype

Abstract

Brachytrachelopan is a dicraeosaurid sauropod that comes from the Jurassic of Argentine Patagonia. With the purpose to infer different palaeobiological traits (e.g., ontogenetic stage, growth dynamics, temporal correlation between morphological and anatomical parameters of maturation), we perform an osteohistological study on different postcranial elements from the holotype individual, including axial (i.e., cervical hemispinous process, dorsal ribs) and appendicular (i.e., femur) bones. The absence of a distinct external fundamental system indicates that the individual died before full body size achievement (i.e., somatic maturity). Furthermore, although not conclusive, the histological data suggest that despite its body size, which is comparable with fully grown individuals of other dicraeosaurid taxa, the individual also corresponds to a sexually immature specimen. Predominance of highly vascularised fibrolamelar with only a cyclical growth mark (CGM) in the outer cortex of the femur reveals a sustained, accelerated growth, typical feature of neosauropod dinosaurs. The external anatomy (i.e., neurocentral sutures are visible in presacral vertebrae) is consistent with histological data, since both indicate some degree of immaturity. Dorsal ribs are best elements to estimate age. The new evidence reinforces the hypothesis that mechanical forces were higher in the anterior edge of cervical hemispinous processes, although, these findings do not support the presence of horn cores or interspinous ligament system in the neck of Brachytrachelopan.