Variations on a Theme: Morphological Variability and Insular Adaptations in the Pleistocene Hippopotamuses of the Siculo-Maltese Archipelago.

The Pleistocene hippopotamuses of the Siculo-Maltese Archipelago are re-examined through the study of historical collections and newly recovered material. Fossils attributed to Hippopotamus pentlandi and Hippopotamus melitensis were described anatomically and compared morphologically and morphometrically with the continental species Hippopotamus antiquus and Hippopotamus amphibius. Comparison with the continental taxa confirms the mosaic of characters that has long complicated the identification of the ancestral taxon of Hippopotamus pentlandi. At the same time, the Sicilian hippopotamus is shown to possess a distinctive skeletal organization, combining a shortened, more gracile skull with shortened, robust limbs. Most of the Maltese material falls within the range of Hippopotamus pentlandi, although it shows a general trend towards smaller size. The available record remains insufficient to determine the characters of Hippopotamus melitensis as a distinct endemic taxon, while the occurrence of Hippopotamus pentlandi in Malta cannot be excluded. The anatomical pattern documented here provides the morphological basis for future phylogenetic analyses of the Mediterranean insular hippopotamuses.