The five digits of the giraffe metatarsal

Evolution has shaped the limbs of hoofed animals in specific ways. In artiodactyls, it is the common assumption that the metatarsal is composed of the fusion of digits III and IV, while the other three digits have been lost or are highly reduced. However, evidence from the fossil record and internal morphology of the metatarsal challenges these assumptions. Further, only a few taxonomic groups have been analyzed. In giraffes, we discovered that all five digits are present in the adult metatarsal and are highly fused and modified rather than lost. We used high resolution µCT-scans of the metatarsals of two mid and late-Miocene giraffid fossils and the extant giraffe and okapi. In all the Giraffidae analyzed, we find a combination of four morphologies: (1) four articular facets, (2) four, and in most cases, five separate medullary cavities internally, (3) a clear, small digit I, and (4) in the two fossil taxa of unknown genus the presence of external elongated grooves where the fusions of II and V have taken place. Giraffa and Okapia, the extant Giraffidae, show a difference from all the extinct taxa in having more flattened digits tightly packed together, suggesting convergent highly fused digits despite divergent ecologies and locomotions. These discoveries provide evidence for new understandings as to how bones fuse and question current hypotheses of digit loss.