Choeropsis, LEIDY, 1853

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 : 15-16

GENUS CHOEROPSIS LEIDY, 1853

Description

Emended diagnosis: Small-sized genus, distinct from all the other known Hippopotamidae by its downwardly bent nasal anterior apex, which clearly passes the premaxillae-nasal contact anteriorly; orbits clearly below the cranial roof; strong posterior nasal spine of the palatine; large and elongated tympanic bulla, which is apically rounded and without marked muscular process; presence of a lateral notch on the basilar part of the basi-occipital, immediately posterior to the muscular tubercles; down-turned sagittal crest. Choeropsis shares many characters with the most primitive Hippopotamidae : weak extension of the canine processes (both lower and upper); facial crest regularly convex in ventral view, gradually sloping from the zygomatic arch toward the maxilla; slen- der zygomatic arch in ventral view; lachrymal separated from the nasal by a long maxillary process of the frontal; orbit anterior to the level of contact between M/2-M/3 seen in lateral view; weak supraorbital apophyses; braincase elongated and transversally rounded. Choeropsis also presents some derived features convergent on several other Hippopotamidae : diprotodont mandibular symphysis, short and upright; gonial angle laterally everted; P4s generally without accessory cusps.

Type species: Choeropsis liberiensis ( Morton, 1844) , extant, same diagnosis as for the genus.

Geographical distribution: Coastal valleys and plains of the Guinean gulf ( Sierra Leone, Liberia, Ivory Coast, Nigeria, Guinea-Bissau?), Western Africa (from Eltringham, 1993, 1999). Corbet (1969) created a subspecies, C. liberiensis heslopi , for the delta of Niger populations.

Temporal distribution: Genus exclusively known in the present, with only one species classified vulnerable by the IUCN ( Eltringham, 1993, 1999); the oriental subspecies may be already extinct.

Discussion

The extant Liberian hippo shows a mosaic of characters. First, almost all of its cranial character states appear plesiomorphic in the parsimony analysis, except for characters 8 and 12 (large orbit size, state 1; sagittal crest slope, state 3). Second, the mandible and the dentition display some very derived features, also found in some other hippopotamids (convergences). The symphysis is very inclined (character 18, state 1), the gonial angles are divergent (character 25, state 1), the reduced number of incisors (characters 26 and 31, respectively, states 1 and 2), the P4s are simple (characters 30 and 36, states 1). The overall morphology of the mandible is, similarly, both derived and primitive, the symphysis being short but without strong canine processes. Finally, this species has some autapomorphies (noted ‘A’ on Figs 2 and 3 ): the anterior morphology of the nasal (A3), the presence of a notch on the basilar part of the basi-occipital (A2), the developed posterior nasal spine of the palatine (A1). These noninformative characters have not been included in the cladistic analysis. A fourth autapomorphy, the downturned sagittal crest (character 12, state 3) is relatively frequent in the Anthracotheriidae and is generally regarded as a plesiomorphy, reinforcing the ‘primitive aspect’ of this species.

All these plesiomorphies, autapomorphies and convergences show that this species probably issued from a lineage distinct from all the other hippopotamids a considerably long time ago. If the Pliocene Chadian hippos are the closest relatives, their respective histo- ries diverged for more than 5.0 Myr. The evolution of this species, having resulted in an overall unique morphology within this family, justifies its distinctness at the generic level, as recommended in Harrison (1997). Hence, I propose to maintain this species within the genus initially created for it: Choeropsis .

Remarks: The position of the Liberian hippo with Saotherium within the sister group of all other hippopotamids, except Kenyapotamus , calls for some comments on ecology. This hippo is frequently mentioned as more terrestrial than the other modern species, Hip. amphibius . However, it presents several physiological characters that are related to a semiaquatic way of life: peculiar skin without sweat glands and retaining few hairs, strong muscular valves to obstruct the ears and nostrils underwater. These characters are shared with the common hippo. Hence, they could have been present in the common ancestor of those animals, which, according to the phylogenetic hypothesis proposed here, is the ancestor of all hippos except Kenyapotamus . This would slightly reinforce the idea that aquatic behaviour in whales and hippos could be inherited from a common ancestor. This would also preclude characterizing the semiaquatic habits of fossil species only on the basis of elevated orbits.

Figures

Figure 2. Cranial character states. A, ventral view of a Hippopotamus amphibius skull. B, ventral view of a Hexaprotodon liberiensis skull. C, Schematic view of Hex. harvardi tympanic bulla area. D, Schematic view of A. ulnifer glenoid articular area. E, Three dorsal views of different bone contacts in the lachrymal area (from bottom to top: in Hex. harvardi, in Hex. protamphibius, in Hip. amphibius). A1 and A2 are Hex. liberiensis autapomorphies (see text).

Figure 3. Cranial character states. A, lateral view of a Hexaprotodon liberiensis skull. B, Lateral view of a Hippopotamus amphibius skull. C, lateral view of a Hex. mingoz skull. D, three dorsal views of the braincase (from bottom to top: in Hex. harvardi, in Hex. mingoz, in Hip. amphibius). E, four schematic anterior views of the left orbit (from right to left: in Hex. protamphibius, in Hex. harvardi, in Hip. gorgops, in Hex. sivalensis).