Hexaprotodon, Falconer & Cautley, 1836

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 : 9-12

HEXAPROTODON

Currently, a consensus dominates the discussion on hippo phylogeny: the genus Hexaprotodon depicted in Coryndon (1977) is paraphyletic. Therefore, the taxonomy used for the Hippopotamidae does not reflect the agreed upon phylogeny. Until now, only two questionable solutions have been put forth to resolve this dilemma.

One solution was briefly mentioned previously. Based upon Stuenes’s (1989) discussion of generic characters, Harris (1991) evoked a hippo ‘phenotypic plasticity’ that could alternatively be responsible for a terrestrial morphology (corresponding to Hexaprotodon ) and for an aquatic morphology (corresponding to Hippopotamus ). If this assertion is correct, the current taxonomy could be validated, and thus the two genera employed together, not as evolutionary stages, but as ‘ecological grades’: one terrestrial and one aquatic, respectively. This solution invokes an important ecophenotypism within only one lineage, and which is admittedly somewhat unusual for a large mammal family, and moreover no serious palaeoecological evidence is currently available to support it. In fact, several species of the genus Hexaprotodon cannot really be regarded as more terrestrial and/or more primitive than those of the genus Hippopotamus . For example, the orbit elevation in Hex. karumensis and Hex. palaeindicus compares closely to elevated orbits seen in the most derived specimens of Hippopotamus for this character ( Fig. 8 ). The orbit elevation is recognized as an obvious adaptation to a life near the water surface. Hex. protamphibius is a very close relative of Hex. karumensis, and the same relationship is recognized between Hex. sivalensis and Hex. palaeindicus (see the above parsimony analysis; Hooijer, 1950; Harris, 1991). Hexaprotodon protamphibius and Hex. sivalensis have much lower orbits that would indicate more terrestrial habits. Therefore, a supporter of ‘ecological grades’ should include Hex. karumensis and Hex. palaeindicus in the ‘aquatic grade’ Hippopotamus , while some of their closest relatives would be placed in the ‘terrestrial grade’ Hexaprotodon . This highly confusing situation contradicts Harris’s (1991) remark (p. 57): ‘Whether or not Hippopotamus and Hexaprotodon are merely form genera, there are consistent differences between terrestrially adapted and aquatically adapted hippos that makes it convenient to retain these generic names for purposes of communication’. Such obvious aquatic adaptations are better interpreted as convergences within several distinct lineages.

The other solution is an easy way out: it would consist in merging all such species within one genus, Hippopotamus (following the priority rule). This solution has been proposed on several occasions, but apparently with different motives. Pickford (1983) did not give evidence to justify his use of the name Hippopotamus for the Hexaprotodon species. More recently, Pickford (1993) has employed Hexaprotodon as a subgenus, in the way it was originally put forth, but without further explanation. Stuenes (1989) criticized the characters most frequently used to distinguish Hexaprotodon from Hippopotamus and suggested the irrelevance of the nomen Hexaprotodon . She mainly built her argument around the Madagascan Holocene hippos. One should be cautious to deny the validity of all the observations based on continental material, given the peculiar evolutionary of modes insular faunas ( Sondaar, 1977). The position adopted in Hooijer (1950) has been cited already and is also the most defensible. If Asian hippopotamids constitute an isolated lineage, the attribution of a genus name to this lineage implicates the creation of new genera for the African and European species that show many differences with Hippopotamus . His reluctance to do so was a consequence of the lack of data on these species. Now many are much better known. Above all, a fundamental reason not to merge all hippos in a single genus is that this fusion would conceal taxonomically the important events of the family’s history. It is difficult to approve of such an option, which would moreover contrast strongly with taxonomic recognition for other groups of equivalent diversity, the African Plio-Pleistocene Suidae being a major example (see, among others, Harris & White, 1979; Cooke, 1985). Morphological disparity is strongly marked in the Hippopotamidae , and particularly within the genus Hexaprotodon , which regroups some species as dissimilar as Hex. karumensis, Hex. iberiensis and Hex. sivalensis . Although the employment of several genera, rather than only one, is also subjective, it is more informative to use a taxonomy related to the available phylogenetic data.

The above analysis fully confirmed the paraphyly of the genus Hexaprotodon . The genus Hippopotamus seems to be based firmly on synapomorphies. Moreover, the analysis effectuated here shows that several clades can be recognized on morphological grounds within the family Hippopotamidae . The revision of hippo phylogeny is linked with the requisite adoption of phylogenetic systematics, that supports a splitting of the genus Hexaprotodon and names for these clades. This reformation can be carried out with the results of the parsimony analysis presented above. However, this approach is not wholly satisfying. Some remarks of Harris (1991) in regard to the importance of aquatic adaptations cannot be neglected. Thus, Weston (2000) concluded as follows: ‘The parallelism that results from what appears to be successive invasion of aquatic and terrestrial niches needs to be taken into consideration when unravelling the history of this extraordinary group of mammals’. This parallelism frequently affects cranial structures (see Fig. 8 ), which of course play a major role in the definition of the basal nodes of the phylogenetic trees. Several strong intraspecific selective pressures may also induce the parallel modifications seen in the skull regions committed into fights, i.e. mainly the rostrum.

In order to circumvent this difficulty, at least partially, further attention must be given to the morphology of the mandibular symphysis. Indeed, the symphyseal area serves to effectively discriminate different Hippopotamidae species and, for some, indicates clear evolutionary trends during the Mio- Pliocene ( Weston, 1997, 2000; Boisserie et al., 2003; Boisserie & White, 2004). Hence, its phylogenetic significance is undoubtedly important. However, this structure can only be described accurately through a set of continuous variables. Given the difficulties attendant on coding these conditions, only a few such symphysis characters have been included in the present data matrix (see above, the character list). Other mandibular data were, however, examined and compared. Principally, they are: relative proportions of the cross section of the symphysis (height/length); relative robustness of this cross section (thick or thin); length of the symphysis relative to the width. A different feature, also examined, was the length of the premolar row relative to the length of the molar row. Those parameters were compared between taxa included in the cladistic analysis. Morphologies, especially those of the sagittal cross-section of the symphysis, appear to be similar between taxa within the following groups: (1) Hex. mingoz and Hex. cf. mingoz ( Boisserie et al., 2003) ; (2) Hex. sivalensis , Hex. palaeindicus and Hex. bruneti ( Boisserie & White, 2004) ; (3) Hex. karumensis, Hex. protamphibius, Hex. aethiopicus; (4) Hip. amphibius , Hip. gorgops . The other species showed mandibular proportions and shape that distinguished them from all the other taxa. These data are summarized graphically in Figure 9 . In fact, the comparison of mandibular morphologies lead to visually discriminate groups similar to the clades of smallest rank obtained in the parsimony analysis. On this basis, it was more difficult to conclude on relationships between those different clades. Nevertheless, this confirms that mandibular shape and proportions allow us to recognize when species are closely linked. Therefore, when it was possible, these mandibular features were used to define the relationships of species that were not included in the cladistic analysis.

As a consequence, the new classification proposed below for the Hippopotamidae is based on the foregoing parsimony analysis, but also on those complementary observations (which are further discussed in the following section). Here two new genera are proposed, the genus Choeropsis is validated and the genus Hexaprotodon is redefined and reconstituted.

Figures

Figure 8. An example of convergence in the Hippopotamidae: orbit elevation. From bottom to top, right lateral views of the neuro-crania: KNM-WT 19633, Hippopotamus gorgops from the Nachukui Formation, West Turkana, Kenya, housed at the NMK, Nairobi; 36824, Hexaprotodon palaeindicus from the Narbada beds, Central India, housed at the NHM, London; KNM-ER 798, holotype of Hex. karumensis, from the Koobi Fora Formation, East Turkana, Kenya, housed at the NMK, Nairobi. The elevated orbit is related to an aquatic way of life (Mazin & Buffrénil, 2001), indicating a preferential position at the air/water interface. These three species belong to three different lineages and evolved from forms with much lower orbits.

Figure 9. Mandibular anatomy within the Hippopotamidae. This figure shows the new taxonomic divisions of the family Hippopotamidae and, for each discussed taxon, some of the mandibular characters that provided additional support to the clades identified in the parsimony analysis (boxes in this figure). These features include: the general shape of the mandible, with expansion of the canine processes and relative length of the symphysis (seen in the dorsal outlines); the shape of the symphysis sagittal cross section; the length of the premolar row relative to the length of the molar row. The figure shows the following features for the taxa listed under each genus name: Saotherium, very inclined symphysis with thin cross-section and poorly developed canine processes; Archaeopotamus, relatively long and shallow symphysis with poorly developed canine processes and longer premolar rows than in any other clade; Hexaprotodon, wide symphysis but with poorly differentiated canine processes, very robust symphysis in cross section; Choeropsis, very short symphysis globular in cross section and poorly developed canine processes; Hippopotamus and aff. Hippopotamus, short symphysis globular in crosssection (lacking a projected incisor alveolar process) and strong extension of the canine processes – the latter feature being not salient in the Afar species (aff. Hip. coryndoni, aff. Hip. afarensis) and aff. Hip. cf. protamphibius from Kanapoi.