KENYAPOTAMINAE AND SUOIDEA

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 : 347-348

KENYAPOTAMINAE AND SUOIDEA

Our results strongly disagree with a close relationship between Kenyapotamus and any of the considered suoids. The position of Xenohyus within Suidae , postulated as a forerunner of Kenyapotamus (Pickford, 1989, 1993), is again supported ( Ginsburg, 1980; Ginsburg et al., 1988; Made, 1994; Bouvrain & Bonis, 1999; Boisserie et al., 2005b; Orliac et al., 2006; Harris & Liu, 2007). The only feature concretely cited in favour of a Xenohyus – Kenyapotamus close relationship was a similar molar cingula/-ids relative height (Pickford, 1989: 167; Pickford & Morales, 1989: 236), but this feature, beyond its insignificance, is outnumbered by a long list of dental characters that can separate these genera (see Supporting Information).

Any putative close relationship between Doliochoerus , Palaeochoerus , and Taucanamo , on the one hand, and Kenyapotamus , on the other, appears to be unsupported. In a recent informal phylogeny, Pickford & Tsujikawa (2005: fig. 3D) indicated that a sistergroup relationship between palaeochoerids and Hippopotamidae was supported by the ‘posterior groove in upper canines’ and ‘the partial fusion of roots immediately beneath molars forming a substantial base to the crowns’. Unfortunately, an upper canine distal groove is not known in palaeochoerids except in Schizochoerus . First, this genus displays a specialized, bilophodont cheek tooth morphology (see Fig. 1H and Pickford, 1978), making it a particularly unlikely stem for Hippopotamidae . Second, its affinities amongst suoids appear unclear. Although it was initially described as a suid ( Crusafont & Lavocat, 1954), since then it has been considered as an ‘Old World tayassuid’ ( Pickford, 1978) or palaeochoerid ( Made, 1997). A recent cladistic analysis however placed it back within Suidae ( Orliac, 2007) . The position of Shizochoerus within Suoidea was not resolved in our analysis, we thus remain cautious regarding its actual affinities in the superfamily.

The second feature mentioned by Pickford & Tsujikawa (2005), partial root fusion, in support of a clade (palaeochoerids + Hippopotamidae ) does apply differently to hippopotamids and Doliochoerus , Taucanamo , and Palaeochoerus . In these three latter genera, roots display a complete fusion and form only one structure (lingual roots on the upper molars, mesial roots and distal roots on the lower ones), and only labial upper molar roots remain unfused and completely free. Hippopotamids (including Kenyapotamus ) exhibit variable partial fusion with apices always remaining free, and the partial fusion pattern can vary amongst specimens. This condition is also seen in all examined anthracotheriids, except in Merycopotamus and Libycosaurus where the fusion is more similar to that found in palaeochoerids.

In order to further assess hypotheses linking hippopotamids with suoids, we ran some phylogenetically constrained analyses (see Supporting Information). We tested the ‘tayassuid hypothesis’ as formulated by Pickford (1989). Such a scenario was shown to require no fewer than 55 additional steps compared to the unconstrained phylogenetic hypothesis. Similarly, any grouping of hippopotamids with suoids would require 14 additional steps, whereas any clade formed by hippopotamids and palaeochoerids (excluding Schizochoerus ) would require 25 additional steps (or 21 if including Schizochoerus ).

We therefore reject exclusive affinities between Suoidea ( Tayassuidae + Suidae + palaeochoerids) and Hippopotamidae on the basis of dental characters following a careful analysis of the material corner- stone to the ‘tayassuid’ then ‘palaeochoerid’ hypotheses, i.e. that of Kenyapotamus . Close affinities between Suoidea and Hippopotamidae were also previously rejected on the basis of a character sample dominated by craniomandibular characters ( Boisserie et al., 2005a, b). Furthermore, a reanalysis of the most complete previous data matrix ( Boisserie et al., 2005b), without dentally based characters, also led to a clade ( Hippopotamidae + Anthracotheriidae ) that excluded suoids.

To demonstrate that hippopotamids emerged within Suoidea , it would be necessary to: exhaustively discuss the characters and their states presented here, as well as those presented by Boisserie et al. (2005a, b), and demonstrate that they are incorrect or homoplastic; find a significant amount of synapomorphies between Hippopotamidae and any suoid; explain how extant hippopotamids can overall display stronger molecular affinities with extant cetaceans and ruminants than with extant suids and tayassuids (amongst an abundant literature, see notably Irwin & Arnason, 1994; Gatesy et al., 1996; Ursing & Arnason, 1998; Nikaido, Rooney & Okada, 1999; Arnason et al., 2000; Gatesy & O’Leary, 2001; Arnason, Gullberg & Janke, 2004; Price, Bininda-Emonds & Gittleman, 2005; Marcot, 2007).

The purpose of the cladistic analysis was to decipher the relationships of Kenyapotaminae , not to resolve the phylogeny of Suoidea . However, some results on suoid intragroup relationships call for some comments. Regarding the relationships amongst the palaeochoerids (excluding Schizochoerus ), the loss of the ventral vascular groove of the mandible (character 3) seems less likely than a reduction of the P 4 mesiostylid. The former is found in some peccaries as part of a complex adaptation to wide gape and implies significant additional musculoskeletal specializations of the whole skull ( Herring, 1975) – the lack of a vascular groove in Hexaprotodon and Libycosaurus could be linked to a different solution for achieving a wide gape, with a ramus angular process particularly developed and everted. The topology [( Doliochoerus , Taucanamo ), Palaeochoerus ] could therefore appear somewhat more parsimonious, with a parallel evolution of the vascular groove in Taucanamo and Pecari (as well as in some anthracotheriids and hippopotamids). However, a satisfying resolution of palaeochoerid intragroup relationships requires consideration of additional evidence.

Finally, the position of the tayassuid P. tayacu as sister group of palaeochoerids (excluding Schizochoerus ) is interesting, as palaeochoerid affinities with New World tayassuids have been postulated for a long time ( Pearson, 1927). A resolution of this question would require further comparison of fossil tayassuids from northern America with palaeochoerids.

Figures

Figure 1. Occlusal views of upper dentition in Anthracotheriidae, Hippopotaminae, and palaeochoerids. A, Merycopotamus nanus, alveoli of right I1–C, and right P1–M3 (specimen number: Y47189, repository: Geological Survey of Pakistan, Museum of Natural History in Islamabad, Pakistan – GSP). B, Merycopotamus nanus, left* M2–M3 (Y47189, GSP). C, Anthracotherium sp., right P3–M3 (University of Montpellier 2 cast of ACQ 6608, Ecomusée des Phosphatières, Bach, France). D, Hexaprotodon garyam, right M3 (TM059-01-012, Centre National d’Appui à la recherche, Ndjamena, Chad – CNAR). E, Archaepotamus harvardi, left* P2–M3 (KNM-LP 8731, National Museums of Kenya, Nairobi – NMK). F, Ar. harvardi, left* M1–M2 (KNM-LT 23838, NMK). G, Palaeochoerus pusillus, right P4–M3 (Qu 15, Muséum National d’Histoire Naturelle, Paris, France – MNHN). H, Schizochoerus vallesensis, right P3–M3 (TRQ 1018, MNHN). I, Doliochoerus quercyi, left* M1? (Qu 7092, MNHN). * indicates reversed images for comparison purposes. Localities: Y, Potwar plateau, Pakistan; TM, Toros-Menalla, Chad; TRQ, Yassiören, Turkey; LP, Loperot, Kenya; LT, Lothagam, Kenya; Qu and ACQ, Quercy, France. Scale bars = 1 cm.