The complete skeleton of a straight-tusked elephant calf from Cova del Rinoceront (Late Pleistocene, NE Iberian Peninsula): New insights into ontogenetic growth in Palaeoloxodon antiquus

Abstract

A nearly complete skeleton of an elephant calf was excavated between 2012 and 2014 from breccia deposits in a vertical karstic cavity named Cova del Rinoceront, exposed during limestone quarrying. The skeleton was found in the 120 cm-thick breccia layer III (Unit 1) of the 11 m-long sedimentary sequence filling the cavity. IRSL (128–129 ka) and AAR (126–142 ka) dating indicate that the layer was probably deposited during MIS 5e (∼115–135 ka). The diagnostic morphological traits of the occlusal surface of the molariform teeth, their position in the tooth progression and wear indicate that the skeleton belongs to a Palaeoloxodon antiquus calf about 5 years old, with a putative height at the shoulder of about 178–187 cm and a weight of about 1450–1500 kg. The sex cannot be confidently determined because the tusks were poorly preserved and the young age reduces the diagnostic value of the pelvis girdle.The rarity of calf skeletons in the fossil record of continental straight-tusked elephants renders the Cova del Rinoceront individual of great interest for better understanding the ontogenetic processes in continental straight-tusked elephants.The results obtained evidence, on the one hand, the difficulties entailed in properly assessing the ontogenetic growth process, given the low number of fully informative specimens of straight-tusked elephants known to date. On the other hand, they indicate an early beginning of distal epiphyseal fusion in humerus and tibia and of the proximal epiphysis in radius. Moreover, the nearly fused lateral epicondylus and condylus and the unfused and separate diaphysis and epiphysis on the medial side of the right femur suggest not only that fusion patterns may show high levels of intraspecific variation, but also that the process of epiphyseal fusion may vary in the homologous bones of a single individual.Notwithstanding the limitations resulting from the preservation status of the calf bones, our analysis yielded sufficient information to infer the ontogenetic age of the Cova del Rinoceront elephant, estimate its body mass and evaluate allometric growth in P. antiquus long bones. We obtained slightly different results when analysing samples that did or did not include the Cova del Rinoceront calf. In the former sample, confident isometry was limited to a few predictors but could not be excluded because for most of the independent variables, isometry and negative correlation were roughly balanced. Most of the dependent and independent variables showed the same or a similar scale ratio, but fewer responded almost similarly to bending stresses imposed by loading under their own weight. When the calf was excluded from analysis, negative or possibly negative correlations and variables responding almost similarly to bending stresses imposed by loading under their own weight prevailed. However, the hypothesis that some changes in limb bone proportions occurred during ontogenetic growth requires further support from an analysis of large samples of fully informative mature and immature P. antiquus individuals.