Taxonomy, habits, and relationships of the subfossil Madagascan hippopotamiHippopotamus lemerleiandH. madagascariensis

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Abstract
The revised taxonomy of the subfossil hippopotami of Madagascar suggests that of the hitherto described four species, only two are valid. Lectotypes of these two species, Hippopotamus lemerlei Grandidier and H. madagascariensis Guldberg, are designated and the species are redescribed. Cranial features suggest differences in functional anatomy and ecology between these two species. The short and deep glenoid fossa together with the lateral wear facets of the incisors in H. lemerlei indicate restricted lateral movements of the mandible, while the long and shallow glenoid fossa and the horizontally worn incisors of H. madagascariensis demonstrate extensive lateral mobility. The long, narrow skull of H. lemerlei, with its elongated facial portion and short postorbital part, points to an amphibious mode of life; while the proportions of the more robustly built skull of H. madagascariensis indicate a mainly terrestrial habit. Tip-to-tip occlusion of the incisors, a posterior groove in the upper canines, and double-rooted first premolars are present in H. madagascariensis, but not in H. lemerlei. Since the two Madagascan species are closely related, these features cannot be used as distinguishing characteristics on the generic level. The ancestral form(s) of the Madagascan hippopotami is not known, but H. amphibius, with its well-developed aquatic adaptations, seems to be a possible ancestor. Like many other fossil hippopotami from isolated islands, the two Madagascan species are dwarfs.