Cranial Appendages in Ruminants: Diversity, Evolution, Development, and Molecular Basis of Horns, Pronghorns,...

Ruminants are unique among living mammals in possessing paired bony cranial appendages in the form of horns, pronghorns, antlers, or ossicones. This review summarizes current knowledge on the structure, development, function, and evolution of these appendages and integrates recent findings from genomics, transcriptomics, and single-cell analyses. Two competing hypotheses have been proposed to explain their origin. According to the independent-origin hypothesis, horns, pronghorns, antlers, and ossicones evolved separately in different pecoran lineages, possibly through repeated recruitment of similar developmental capacities of the frontal region of the skull. According to the common-origin hypothesis, these appendages derive from a single ancestral osseous cranial structure that was subsequently modified in different lineages. Comparative anatomy and developmental data reveal major differences among the four types of appendages, including dermally ossifying horn cores in bovids, annually regenerated antlers arising from frontal bone pedicles in cervids, deciduous keratin sheaths in pronghorns, and skin-covered ossicones in giraffids. The fossil record does not currently resolve these hypotheses, because the main appendage types are first documented within a relatively short early Miocene interval and transitional forms remain unknown. In contrast, molecular and cellular studies identify shared developmental pathways, cranial neural crest-derived progenitors, and conserved regulatory genes, including RXFP2, ALX1, SOX9, and components of Wnt signaling, which are consistent with a shared developmental module. We conclude that the homology of pecoran cranial appendages remains unresolved and that further paleontological, developmental, and comparative molecular studies are required to determine whether these structures share a common evolutionary origin or instead represent convergent recruitment of similar developmental programs.