Fossil Find May Conclude Controversy Over Hippo Origins

Two pygmy hippos (Choeropsis liberiensis) photographed at the Columbus Zoo by yours truly. The evolutionary origin of such mammals
 has been long unknown, although a recent study in Nature Communications proposes an answer.

A new paper published in the journal Nature Communications has shed crucial light on the ancestry of Africa’s sub-Saharan semiaquatic giant, the hippopotamus. The origins of these animals have long been shrouded in ambiguity but, according to the recent study, can now be definitively placed with the fossil ungulate family Anthracotheriidae. First found in coal deposits, the anthracotheres were aquatic browsers dating back to the late Eocene in Asia and North America.¹ Anthracotheres were among the first animals to colonize Africa, although their range was quite diverse throughout the Oligocene and Miocene epochs.¹ Morphological features such as the flaring snout, wide heavy feet, hippo-like lower jaw, cetacean-like premolars, and prominent tusks of anthracotheres like Elomeryx and Merycopotamus have been cited in support of a link with Hippopotamidae and Whippomorpha (the clade uniting whales and hippos) as a whole.^1,2 The swamp-dwelling tendency of anthracotheres indicated by the presence of their fossil remains in remnant coal seams likely hints at what stimulated differences in morphology and specilization between the otherwise closely related whales and hippos. Stem-whales most probably evolved in coastal environments promoting a carnivorous diet whereas the anthracotherian hippo-progenitors inhabited habitats in which they were restricted to feeding on aquatic plants. As some extant ungulates like pigs occasionally exploit a carnivorous diet, it is not too difficult to imagine stem-whales adopting this trait under restrictive ecological pressures. While fossil stem-cetaceans are numerous and well documented, the ancestry of the 'river horse' has been quite the enigma with ghost lineages remaining between the known anthracothere lineages and the oldest fossil hippopotamus. However, the fossil material described in the Nature Communications publication may help to bridge this paleozoological gap.


Although such a linear evolutionary diagram is an innacurate depiction, anthracotheres such as Elomeryx link extant hippos to the common ancestor of Cetacea and Hippopotamidae (grouped in the clade Whippomorpha) according to the recent Nature Communications study. (Source)

The preceding diagram placed in proper phylogenetic format, and serving to further illustrate the link which anthracotheres form between Hippopotamidae and the common ancestor of Whippomorpha. Direct fossil evidence to substantiate this notion has been lacking, perhaps until now. (Source)

This revelation comes with the discovery of a sheep-sized anthracothere from Kenya, dubbed Epirigenys lokonensis. This animal lived approximately thirty million years ago, older than any previously uncovered hippo ancestor, and has been described as an unambiguous root between Hippopotamidae and the bothriodontine anthracotheres.³ The fossilized molar representative of Epirigenys bears a morphology distinctly similar to that of the two ungulate groups, leading to the conclusion that E. lokonensis acts as a sort of evolutionary transition which is in keeping with modern phylogenetic analyses.³ As a result, it is postulated that the new Kenyan fossil mammal and the modern day hippopotamus are both the descendants of a primitive anthracothere which swam to Africa from Asia around thirty-five million years ago.³ The bothriodontines cited in the Nature Communications paper were, in fact, one of the early anthracotheriid groups which dispersed into Africa. These findings also place the extant hippopotamus as a true native of the Mother Continent, belonging to a lineage populating Africa long before other iconic species such as lions and rhinoceros.³ Overall, these findings help to dispel years of paleozoological controversy and should stimulate further research into this most-interesting evolutionary narrative. 

Diagram illustrating the transitional similarities between the upper molars of an anthracothere (left), the newly-discovered Epirigenys (middle), and a primitive hippo specimen (right). (Source)

Simplified phylogeny detailing the relationship of Epirigenys with the other cetartiodactyls. (Source)

References:

1. Prothero, Donald R., and Robert M. Schoch. Horns, Tusks, and Flippers: The Evolution of Hoofed Mammals. Baltimore: Johns Hopkins UP, 2002. Print.
2. Dixon, Dougal. The World Encyclopedia of Dinosaurs & Prehistoric Creatures: The Ultimate Illustrated Reference Guide to More than 1000 Dinosaurs and Prehistoric Creatures, with 2000 Specially Commissioned Watercolours, Maps and Photographs. London: Lorenz, 2010. Print.
3. Lihoreau, Fabrice, Jean-Renaud Boisserie, Fredrick Kyalo Manthi, and Stéphane Ducrocq. "Hippos Stem from the Longest Sequence of Terrestrial Cetartiodactyl Evolution in Africa." Nature Communications Nat Comms 6 (2015): 6264. Web.