The position of Hippopotamidae within Cetartiodactyla

Abstract

The origin of late Neogene Hippopotamidae (Artiodactyla) involves one of the most serious conflicts between comparative anatomy and molecular biology: is Artiodactyla paraphyletic? Molecular comparisons indicate that Cetacea should be the modern sister group of hippos. This finding implies the existence of a fossil lineage linking cetaceans (first known in the early Eocene) to hippos (first known in the middle Miocene). The relationships of hippos within Artiodactyla are challenging, and the immediate affinities of Hippopotamidae have been studied by biologists for almost two centuries without resolution. Here, we compare opposing hypotheses implicating several ''suiform'' families. This morphological analysis of a comprehensive set of taxa and characters offers a robust solution to the origins of Hippopotamidae. This family appears to be deeply nested within the otherwise extinct artiodactyl family Anthracotheriidae, most precisely within the most advanced selenodont forms. The proposed sister group of hippos is the middle to late Miocene African semiaquatic Libycosaurus. Any close relationships of hippos with suoids, particularly with Tayassuidae, are rejected. Furthermore, the clade (Hippopotamidae, Anthracotheriidae) is proposed as the sister group of the Cetacea, offering broad morphological support for a molecular phylogeny, such support being also consistent with the fossil record. Corroboration of this relationship requires an exploration of anthracothere affinities with other Paleogene artiodactyls. Among those, the position of Ruminantia is a central question, still to be solved. Further progress in this debate is likely to come from morphological studies of paleontological data, whether known or still to be discovered.hippo origin ͉ phylogeny ͉ Anthracotheriidae ͉ suoids ͉ archaeocetes

Stable isotope evidence for an amphibious phase in early proboscidean evolution

“…Finally, the anthracothere results are of interest given that recent phylogenetic studies have placed these artiodactyls as stem hippopotamids (45). All but one of the data points for the L-41 anthracothere are clustered close together, with one seemingly anomalous data point falling far away from the other samples (indeed, far away from all of the other L-41 samples) (Fig.…”

Understanding Evolutionary Trees

“…As with "fishes," the category of "reptiles" is phylogenetically inconsistent is an ancestor of the other nor even that the common ancestor looked anything like either of the two groups. For example, the hypothesis that whales and hippopotamuses are sister groups (e.g., Boisserie et al 2005) does not imply that the ancestor of whales was a hippo nor that it would even have been thought of as being similar to a hippo were it encountered when it was alive. Not surprisingly, the fossil record of whales, which is becoming increasingly extensive, shows that the early ancestors of whales (e.g., Pakicetus, Ambulocetus) bore no substantial resemblance to modern hippos at all (Thewissen and Bajpai 2001;Thewissen and Williams 2002).…”

<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.…”

“…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).…”