Hippopotamus, LINNAEUS, 1753

Boisserie, Jean-Renaud, 2005, The phylogeny and taxonomy of Hippopotamidae (Mammalia: Artiodactyla): a review based on morphology and cladistic analysis, Zoological Journal of the Linnean Society 143 (1), pp. 1-26 : 19

GENUS HIPPOPOTAMUS LINNAEUS, 1753

Description

Emended diagnosis: Tetraprotodont, and having the following apomorphies: skull with an elongated muzzle; upper canines with a longitudinal and shallow posterior groove, narrow and covered with enamel; lower canines with strong convergent enamel ridges; deep and widely open notch on the orbital anterior border; limbs short and robust with very large quadridigitigrade feet. This genus displays many other features that are derived within the family, but which may be seen in other hippos: antorbital process of the frontal short to absent and a long contact between the maxillary bone and the lachrymal bone; high orbits; short globular braincase, with strong postorbital constriction; mandibular symphysis globular in sagittal section, without incisor alveolar process overhanging frontally; canine processes well developed laterally and frontally; molars high-crowned, compact and relatively long mesiodistally (modified from Gèze, 1980 and Harris, 1991).

Type species: Hippopotamus amphibius Linnaeus, 1758 , extant.

Other material: In Africa, Hip. gorgops Dietrich, 1928 , and Hip. kaisensis Hopwood, 1926); several other species in Eurasia.

Geographical distribution: Africa, Madagascar (see Stuenes, 1989; Faure & Guérin, 1990), continental Europe (including the Caucasus) and the Mediterranean basin (see especially Faure, 1983, 1986; Vekua, 1986; Mazza, 1991); the genus name Phanourios proposed by Boekschoten & Sondaar, 1972) seems to be relevant for the small, peculiar Hippopotamus from Cyprus (see also Houtekamer & Sondaar, 1979).

Temporal distribution: The FAD of the genus Hippopotamus remains unclear, largely because it depends on the identity of some very fragmentary remains. Harris et al. (1988) reported some fossils from the Kataboi member of the Nachukui Formation (West Turkana, Kenya) denoted as Hip. cf. kaisensis. These are the oldest known evidence for this genus at least in the Turkana basin (this member is aged between 4.10 and 3.36 Myr, following Feibel, Brown & McDougall (1989). However, Faure (1994) attributed to Hip. kaisensis some dental and postcranial pieces from the lowest levels of the Nkondo Formation (Western Rift, Uganda), aged about 5.0 Myr.

Discussion

The species belonging to this genus are generally easy to recognize, particularly the morphology of anterior dentition. The monophyly of Hippopotamus has not been recently questioned, and the results of the parsimony analysis confirm this view for the two principal African species ( amphibius and gorgops ). One should note that the number of species and their relationships within the genus are still a matter of debate. For example, all or part of the scarce material attributed to Hip. kaisensis Hopwood, 1926 could correspond to various other species ( Hex. protamphibius, Hex. karumensis or Hip. gorgops , following Gentry, 1999) and/or be a synonym of Hip. amphibius (see Pavlakis, 1990). The discussion about the European forms in Mazza (1995), particularly on Hip. incognitus Faure, 1984 , also illustrates the recent debates on Hippopotamus diversity.