The anatomical position of the ribs and thoracic vertebrae of the Roc de Marsal Neandertal infant.

Recent technical and methodological advances have provided new insights into Neandertal thorax morphology, revealing significant differences compared to modern humans, both in terms of the individual elements (vertebrae, ribs, and sternum), and in the thorax as a whole. However, the thorax morphology of immature Neandertals remains poorly understood, due to the limited and fragmentary fossil record. The Roc de Marsal (RdM) infant is one of the few cases in which the vertebrae and ribs are both relatively well preserved. Previous research has characterised the ribs of the RdM individual as having shafts with low robusticity and short necks. They also have a large radius of curvature (i.e., they are less curved) and a pronounced anterior flare. Theoretically, this would result in a thorax that was more anteroposteriorly projected than that of Homo sapiens, with a maximum width at the 7th rib. Here, we reassess the anatomical position of the thoracic vertebrae and ribs of RdM, evaluating previous observations using a combination of traditional measurements and 3D geometric morphometrics. A morphological assessment of the most complete ribs (7, 9, 10 and 11) reveals marked differences between the ribs of RdM and those of modern human children. These differences include less curvature of the shaft in cranial view, more vertically oriented and straighter (i.e., less sinuous, with less torsion) shafts in external view. At least for ribs 9-11, the shaft portion sternal to the posterior angle is also longer. In some instances, these differences are similar to those present in adult Neandertals, suggesting that the Neandertal thorax was distinct from that of H. sapiens since infancy. This study reinforces the idea that the size and shape of the individual elements (the ribs and vertebrae) provide information about the distinctiveness of the entire thorax.