HIPPOPOTAMOIDEA, sensu Gentry & Hooker, 1988

Boisserie, Jean-Renaud, Lihoreau, Fabrice, Orliac, Maeva, Fisher, Rebecca E., Weston, Eleanor M. & Ducrocq, Stéphane, 2010, Morphology and phylogenetic relationships of the earliest known hippopotamids (Cetartiodactyla, Hippopotamidae, Kenyapotaminae), Zoological Journal of the Linnean Society 158 (2), pp. 325-366 : 351-352

HIPPOPOTAMOIDEA AND RELATED CETARTIODACTYLS

This work has provided additional support for a superfamily Hippopotamoidea sensu Gentry & Hooker (1988) , i.e. including Hippopotamidae and Anthracotheriidae . It is not our intention to describe with any degree of exhaustiveness the extragroup relationships of this clade. However, the taxa involved in the cladistic analysis permit some reappraisal of the possible relationships.

Asian helohyids were suggested to have a sister or stem group relationship with Anthracotheriidae (see, e.g. Coombs & Coombs, 1977b; Ducrocq et al., 1997). These putative relationships were not confirmed here, essentially in relation to paraconid loss (81:1) in all taxa except Gobiohyus and the outgroup. The exclusion of this character led to eight equally parsimonious trees (287 steps) including two with the position previously observed and six with a clade ( Gobiohyus + Cebochoerus + Hippopotamoidea ). In four of these trees, Gobiohyus was found within the sister group of Hippopotamoidea (with Cebochoerus in two trees, alone in the remaining). Independent paraconid loss should not be excluded, as it probably occurred on several occasions, so we cannot reject for now a relatively close relationship between Hippopotamoidea and Asian helohyids.

The results do not agree with Pearson’s (1927) hypothesis of hippopotamid emergence from the archaic Cebochoeridae , but indicate a close relationship between Cebochoerus and the Hippopotamoidea , previously not supported by Boisserie et al. (2005a, b). Interestingly, Theodor & Foss (2005) emphasized the presence of deciduous dental structures (accessory denticles) in cebochoerids found also in deciduous and permanent dentitions of archaic cetaceans (archaeocetes).

These results definitely underscore the need for a better understanding of hippopotamoid affinities amongst Eocene artiodactyls from Eurasia. What is at stake exceeds the simple problem of anthracotheriid origins, and concerns the larger question of relationships amongst major terrestrial, semi-aquatic, and aquatic radiations amongst paraxonians. Eocene archaic artiodactyls, including the earliest known anthracotheriids, are particularly diverse and, most often, not well known. This makes their phylogenetic relationships particularly difficult to resolve. The discovery of complete remains of the raoellid Indohyus brought an apparently radical resolution of cetacean origins ( Thewissen et al., 2007). It is quite possible that equivalent future discoveries will alter this resolution in a similarly dramatic way. It also shows that beyond morphology, the palaeoecology of archaic artiodactyls can be enlightening. That of the earliest anthracotheriids needs further exploration: it would not be surprising if at least some of them were found to be semiaquatic, as for Indohyus ( Thewissen et al., 2007) , given the repeated developments of semiaquatic adaptations in Hippopotamoidea .

Raoellids have previously been related to anthracotheriids and helohyids (for a summary of and contra this hypothesis, see Sahni et al., 1981). Geisler et al. (2007) suggested close relationships amongst cetaceans, hippopotamids, raoellids, helohyids, anthracotheriids, cebochoerids, and Mixtotherium . This view was partially supported by O’Leary & Gatesy (2007), who also found close relationships amongst hippopotamids, some anthracotheriids and helohyids, and raoellids, as well as entelodonts and Andrewsarchus . A better understanding of the exact phylogenetic relationships amongst these taxa and others (including mesonychians and archaic ruminants) should lead to a cetartiodactyl phylogeny congruent with the relationships amongst extant cetaceans, hippopotamids, and ruminants suggested by molecular-based studies.