Early Miocene hippopotamids (Cetartiodactyla) constrain the phylogenetic and spatiotemporal settings of hippopotamid origin

Abstract

The affinities of the Hippopotamidae are at the core of the phylogeny of Cetartiodactyla (even-toed mammals: cetaceans, ruminants, camels, suoids, and hippos). Molecular phylogenies support Cetacea as sister group of the Hippopotamidae, implying a long ghost lineage between the earliest cetaceans (∼53 Ma) and the earliest hippopotamids (∼16 Ma). Morphological studies have proposed two different sister taxa for hippopotamids: suoids (notably palaeochoerids) or anthracotheriids. Evaluating these phylogenetic hypotheses requires substantiating the poorly known early history of the Hippopotamidae. Here, we undertake an original morphological phylogenetic analysis including several “suiform” families and previously unexamined early Miocene taxa to test previous conflicting hypotheses. According to our results, Morotochoerus ugandensis and Kulutherium rusingensis , until now regarded as the sole African palaeochoerid and the sole African bunodont anthracotheriid, respectively, are unambiguously included within the Hippopotamidae. They are the earliest known hippopotamids and set the family fossil record back to the early Miocene (∼21 Ma). The analysis reveals that hippopotamids displayed an unsuspected taxonomic and body size diversity and remained restricted to Africa during most of their history, until the latest Miocene. Our results also confirm the deep nesting of Hippopotamidae within the paraphyletic Anthracotheriidae; this finding allows us to reconstruct the sequence of dental innovations that links advanced selenodont anthracotheriids to hippopotamids, previously a source of major disagreements on hippopotamid origins. The analysis demonstrates a close relationship between Eocene choeropotamids and anthracotheriids, a relationship that potentially fills the evolutionary gap between earliest hippopotamids and cetaceans implied by molecular analyses.

<i>Morotochoerus</i> de Uganda (17.5 Ma) y <i>Kenyapotamus</i> de Kenia (13-11 Ma): implicaciones sobre el origen de los hipopotámidos

“…4, 5). We do not follow the nomenclature proposed by Boisserie et al (2009b) and Orliac et al (2010). It adds little to previously available nomenclature schemes, but it does omit important structures such the anterior, median and posterior accessory cusps and their naming of crista (ids) eliminates one groove on each main cusp, a fundamental feature of suiform teeth.…”

“…It should be noted that the maxilla with DP4/-M1/ of Morotochoerus (MOR II BUMP 350) described by Orliac et al (2010) was originally attributed to a cercopithecid by MacLatchy et al (2003), confusion flowing from the somewhat convergent morphology between the teeth of this suiform and those of monkeys. Re-interpretation of the BUMP 350 maxilla as that of a diminutive hippopotamid by Orliac et al (2010) is equally suspect, the slight morphological resemblances in the cheek teeth to those of hippos being due to convergent evolution rather than to closeness of phylogenetic relationships.…”

“…Confusion due to convergence of characters is on-going (Boisserie et al, 2009a(Boisserie et al, , 2009bOrliac et al, 2010), and failure to take it into account produces weak phylogenetic scenarios, even using the most sophisticated of digital analytical methods.…”

“…Because of this huge gap in the fossil record, some researchers have proposed that anthracotheres (specifically the bothriodontines) represent the missing lineage that links whales and hippos (Boisserie et al, 2005a(Boisserie et al, , 2005b or that the genera Kulutherium and Morotochoerus are primitive hippopotamids which extend the fossil record of the family back to the early Miocene of East Africa (Orliac et al, 2010). The former hypothesis has not found general acceptance, mainly because anthracothere skeletal and dental morphology is widely divergent from that of hippos on the one hand (Pickford, 2008) and that of whales on the other, but also because the group (LibycosaurusMerycopotamus) proposed to fill the morphological gap between whales and hippos occurs appreciably later in time than the earliest known hippos.…”

“…In the particular case of Morotochoerus ugandensis, Orliac et al (2010) had available some partial tooth rows from Moroto I and Moroto II, Uganda, spanning the posterior lower premolar to the m/3 in a mandible and the DP4/-M1/ in a maxilla fragment. It should be noted that the maxilla with DP4/-M1/ of Morotochoerus (MOR II BUMP 350) described by Orliac et al (2010) was originally attributed to a cercopithecid by MacLatchy et al (2003), confusion flowing from the somewhat convergent morphology between the teeth of this suiform and those of monkeys.…”