Palaeobiogeography and evolution of insular bovids: ecogeographic patterns of body mass variation and morphological changes

The study of the plants and animals that live on island has been a frequent key to understanding the processes of evolution that determine biotic diversity globally. Insular endemic mammals represent a special ground for the study of evolutionary mechanisms and speciation processes in response to the special characteristics of island environments. Endemic bovids are intriguing elements of insular faunas and offer an extensive and diversified sample for exploring patterns of insular evolution of large-sized mammals. This thesis provides the first comprehensive study on the evolution of extinct and living insular bovids, exploring the selective biotic and abiotic factors that influenced ecogeographic patterns of body mass variation and peculiar morphological changes (e.g., hypsodonty and low-gear locomotion) of these taxa. I show that the majority of island bovids, as large mammals, do follow the main prediction of the island rule, showing a body size reduction, and that ecological release and resource limitation are the main factors influencing body size evolution of these taxa. My results indicate that hypsodonty (increase in molar crown height), a quite common morphological trend in insular artiodactyls, is shared by the majority of insular bovids independently from the abrasiveness of the current diet. The evolution of this feature appear to result from an expansion of the dietary niche under resource limitation, although alternative, but complementary, hypotheses (e.g., increase in reproductive lifespan/longevity and changes in eating methods) cannot be excluded. The acquisition of a low-gear locomotion or the maintaining of a cursorial aptitude in insular bovids can be explained in the light of habitat selection (niche availability) and predatory pressure (ecological release) operating on each island. Finally, my results confirm the crucial role of time in isolation, with each of the above evolutionary phenomena becoming more developed for bovid populations with longer residence times on the islands. Another factor that needs to be mentioned is original bauplan, working as a constraint in the evolution of all the species in the study. By integrating research into the evolution of body mass variation and peculiar morphological changes exhibited by both extinct and extant bovids, this thesis documents patterns that have often been only hinted at previously, and identifies some that appear to be entirely new, providing new insights into the phenomena of the island syndrome and bovid evolution.