On the cranial anatomy of the smallest insular hippopotamus (Cyprus, Pleistocene)

The Pleistocene Cyprus dwarf hippopotamus (c. 132 kg) is the smallest insular hippopotamus known thus far. This species was part of an extremely depauperate fauna, in which the only other large herbivore was represented by a dwarf elephant, in agreement with a high degree of geographic isolation. Here we describe for the first time its cranial anatomy and reconstruct a virtual model of its skull. Unique characters of this taxon are the lack of upper fourth premolar in adult specimens and lophodont teeth. All other cranial phylogenetically relevant characters firmly place the Cyprus dwarf hippo within the genus Hippopotamus. The increased frontation of its orbits and the elevated neutral position of its head are derived features in respect to its hypothetical ancestor, H. antiquus. These features are here explained as correlated to a higher degree of terrestriality and deviation from a grazing lifestyle. The peculiarities of its dentition indicate a wider dietary niche than in other hippopotamids, probably containing a higher percentage of browse intake compared to mainland Hippopotamus. Wear facets on the incisors point to an increased lateral component during chewing, which results in more efficient chewing, probably to cope with potential (seasonal) resource limitations on the island. The mandible is either anteriorly broad with large canines, or narrower with small canines, probably due to sexual dimorphism which was retained from its ancestor. The combination of extreme body size shift, a higher degree of terrestriality, and a change in dietary niche is here hypothesised as an evolutionary adaptation to this island environment, facilitated by ecological release from competition and predation that allowed the species to survive for hundreds of thousands of years.