Origins of Hippopotamidae (Mammalia, Cetartiodactyla): towards resolution

Abstract

The old debate on hippopotamid origins is still unresolved, balancing between two main phylogenetic hypotheses: origins within the Anthracotheriidae vs. origins within the Tayassuidae. The characters used in the literature to support one, the other, or both hypotheses were reexamined in light of a better known primitive hippopotamid morphology. A cladistic analysis was performed on the basis of these characters. On the one hand, although many features similar in hippopotamuses and anthracotheres appear to have evolved in parallel, the family Anthracotheriidae remains the best putative hippopotamus stem group on the basis of the reviewed characters. However, the precise identification of a stem group within this family is still uncertain. On the other hand, the lineage proposed in the tayassuid hypothesis should not be retained, being mostly based on some incorrect observations and without taking into account the derived or primitive nature of the features. The anatomic similarities seen between modern peccaries and Hippopotamus amphibius are the results of convergences between advanced species of both families.

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

“…A second difficulty with this scenario is that the earliest known hippopotamids (Palaeopotamus ternani) occur earlier in time than Libycosaurus species (Pickford, 2007a) the supposed sister group of hippos taken to link hippos to whales. We note that a recently published paper by these authors (Orliac et al, 2010) refutes the hypothesis of Boisserie et al (2005aBoisserie et al ( , 2005b.…”

“…Having abandoned the hypothesis that the sister group of hippopotamids are the bothriodonts Merycopotamus-Libycosaurus, (previously, characters such as a descending plate at the angle of the jaw and the raised orbits, initially considered to represent "apomorphies" perceived to link the two groups, (Boisserie et al, 2005b) are now accepted to be the result of convergent evolution) Boisserie et al (2009b) continue to espouse an origin within Bothriondontinae (sic) on the basis of analyses of teeth, whereas Orliac et al (2009Orliac et al ( , 2010, propose that "Kulutherium and Morotochoerus are sister taxa.... closely related to Miocene hippopotamids". Kulutherium is an anthracothere (Pickford, 2007b), but is not a bothriodont (smooth, thin molar enamel, no signs of enlarged buccal styles, absence of selenodont tendencies in the molars, minute paraconule The upper and lower molars from Ngorora have been flipped over in order to simplify comparisons.…”

“…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.…”

“…If bothriodonts represent the link between hippopotamids on the one hand and Cetacea on the other, as was recently postulated (Boisserie et al, 2005a(Boisserie et al, , 2005b then one needs to address two points which flow from the proposal. Firstly, one would need to hypothesise, as was done by Boisserie et al (2005b) and Orliac et al (2010) that the structure of the dentitions of bothriodonts is so 'plastic' that, in a short period of time during the Late Miocene, the selenodont dentition of bothriodonts could give rise to the brachyodont, bunodont dentition of early hippopotamids, accompanied by major changes in the detailed structure of the radicular system, cusp morphology, relative cusp dimensions, enamel thickness, enamel surface ornamentation and groove system (Fürchenmuster).…”

“…Firstly, one would need to hypothesise, as was done by Boisserie et al (2005b) and Orliac et al (2010) that the structure of the dentitions of bothriodonts is so 'plastic' that, in a short period of time during the Late Miocene, the selenodont dentition of bothriodonts could give rise to the brachyodont, bunodont dentition of early hippopotamids, accompanied by major changes in the detailed structure of the radicular system, cusp morphology, relative cusp dimensions, enamel thickness, enamel surface ornamentation and groove system (Fürchenmuster). This runs counter to evidence that the basic structure of bothriodont cheek teeth, like that of hippopotamids, was stable over extended periods of geological time.…”