Patterns and drivers of cranial evolution in rhinos (Rhinocerotoidea; Perissodactyla): allometry, phylogeny and tempo of evolution

Despite their currently scarce taxonomic diversity, rhinocerotoids were among the most evolutionarily successful clades of large herbivorous mammals throughout the Caenozoic. Within an array of morphological adaptations, the cranial horn represents the clades most remarkable and name-giving feature, imposing significant morpho-functional demands on the skull. In this paper, patterns and drivers of the rhinos cranial evolution were investigated for the first time by compiling a three-dimensional geometric morphometric dataset of living and extinct species. Multiple aspects of morphological evolution were explored using phylogenetic comparative methods, including craniofacial evolutionary allometry (CREA - tendency of larger species to have longer faces), phylogenetic constraints, and tempo of evolution. Cranial shape variation linked to horn presence and size evolved under a phylogenetically constrained framework, with a notable transition from hornless to horned species. Rhinos significantly deviated from CREA, likely due to diversity of cranial forms and proportions that evolved independently across the clade in response to varying horn morphologies and dietary habits. Hornless and horned rhinos exhibited similar rates of cranial evolution, and shape variation was obtained through multiple episodes of accelerated evolution. This highlights the role of morphological innovations and Caenozoic global cooling events in the emergence of phenotypic diversity.