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.

µCT-scanning: All specimens were mounted in foam and scanned using the high-resolution Nikon H225 ST µCT-scanner at Yale University. Scans of each specimen were taken using a 0.1mm copper filter, which facilitates stronger X-ray penetration without oversaturating the image. Scan parameters varied per specimen, but they were set to ensure X-ray penetration and maximize the distribution of greyscale values (see Table 1 in manuscript).

Image processing and segmentation: Raw output from the scans was reconstructed using in-house Nikon software and imported into VGStudio Max v. 3.3 (VGstudio Max 3.3, 2014). In the fossil scans, medullary cavities were clearly distinct from the rest of the bone as darker structures with clear boundaries. The cavities were carefully traced and segmented by hand in each image slice (n=3,143) of the µCT-scan image stack. Boundaries of fused regions in digits III and IV were determined by cracks in the external bone, though they were clearly separated in the proximal end. Each medullary cavity represented a distinct digit and the segmented regions of interest were extracted and viewed in three dimensions to view their internal orientation.  In the extant taxa, using the external ridges as a guide, hypothesized boundaries of the five digits based on where the trabecular bone separated from one another were traced.

Images stacks are 8-bit TIFF files for four species. 

Yohe_giraffe_289_distal: Giraffa camelopardalis (NS 289)
okapi_data: Okapia johnstoni (AMNH 51196)
Yohe_gir2_21930_xy_8bit: YGSP 21930 fossil giraffid from mid-Miocene
Yohe_28199gir_xy_8bit: YGSP 28199 fossil giraffid from Miocene