Of Fossil Ghosts and Hippos Past

To call hippos “charming” may seem a bit of a stretch, but they are most certainly among the classic charismatic megafauna of the African continent. In the wake of the end-Pleistocene ecological catastrophe – during which waves of extinction denuded the planet of many strange, large-bodied mammals from woolly mammoths to wombats the size of small cars between 20,000 and 10,000 years ago – Africa remained a refuge for some of the last-remaining vestiges of prehistoric megamammal diversity, preserving an oddball assemblage of lineages which had proliferated in the not too distant past. This great pruning back of megamammals, as well as the restriction in range of surviving species, is what allows us to identify elephants, giraffes, and hippos as charismatic herbivores in the first place. The tattered remnants of once more-diverse groups, they are so disparate from other mammals that we can’t help but notice them.

As with many other extant megamammals, the two species of living hippo – Hippopotamus amphibus and the pygmy hippo Choeropsis liberiensis – are the surviving members of a group which was once more diverse and ranged widely through the Old World. We know this from hippo fossils scattered through Africa and Eurasia dating from the past six million years, but understanding where hippos came from in the first place is another matter all together.

Molecular studies have time and again confirmed that hippos are most closely related to whales among living mammals – a finding which fostered the rearrangement of the artiodactyl (even-toed, hoofed mammal) family tree to include whales  – but the fossils which would take us back to the last common ancestor of these two groups have been elusive. The earliest definitive whales, such as the semi-aquatic Pakicetus, date back to about 53 million years ago, but the earliest hippos go back only 16 million years (with hippos of modern aspect only originating about 7.5 million years ago). Given that the two lineages must have diverged before the appearance of the first whales, there is a gap of about 40 million years between the earliest hippos and their last common ancestor with whales.

The fossil gap between hippos and early whales could be filled in by figuring out what creatures the earliest hippos originated from. These missing creatures – predicted to exist but not yet identified – belong to what paleontologists call a “ghost lineage.” True hippos may have originated very recently, but through discerning their closest relatives in the fossil record and working backwards paleontologists would be able to trace their lineage back to the point where the hippo and whale-lines split. Making these determinations has been no easy task. Based upon anatomical studies, different experts have associated early hippos more closely with Old World peccaries (palaeochoerids) or alternatively with a group of hippo-like creatures called anthracotheres.

Despite frustrating scientists for over a century, the study of hippo origins is not exactly a hot topic of evolutionary research. As such there is plenty of material which requires reanalysis and hypotheses which still require testing. Fortunately, scientists Maeva Orliaca, Jean-Renaud Boisserie, Laura MacLatchyd, and Fabrice Lihoreaua have just filled in a few more details about early hippos, publishing a reexamination of two species relevant to hippo origins this past June in the journal PNAS.

The analysis of Orliaca and colleagues focused on two enigmatic African species known from partial jaws – the 20.6 million year old Morotochoerus ugandensis and the ~16 million year old Kulutherium kenyensis. Consensus about what these animals actually were has been hard to achieve. Morotochoerus was initially described as an anthracothere before being identified as an African peccary, while Kulutherium was lumped in with anthracotheres after being proposed as a hippo. Both animals are clearly relevant to the relationships of early hippos, anthracotheres, and perhaps Old World peccaries, but until now they had not been studied in enough detail to place them in their proper evolutionary context.

After running a cladistic analysis of Morotochoerus, Kulutherium, hippos, anthracotheres, suoids (pigs, peccaries, and their close relatives), and other mammals, Orliaca and co-authors found hippos to be most closely related to anthracotheres. (As with many studies of fossil mammals, these results were primarily derived from the comparisons of teeth.) This was expected, but surprisingly Morotochoerus and Kulutherium – along with Kenyapotamus – turned out to be part of an early radiation of hippos, thereby extending the hippo fossil record back to about 20.6 million years and increasing the diversity of archaic hippos. As reconstructed by the paleontologists, hippos primarily remained in Africa until some forms dispersed to Eurasia around 6 million years ago and were subsequently lost as the global climate cooled towards the end of the Pleistocene.

Despite this slight extension of hippos back in time, the details of hippo ancestry still remain uncertain. The most likely scneario is that they originated from a group of anthracotheres called bothriodontines, but whether hippos evolved from an Asian or African branch of this group is unknown. (As a pointed out in a new Nature paper on fossil primates, there is a long history of mammal interchange between the continents.) To trace the history of hippos further, we must identify the ancestors of their ancestors (and we have to get more recursive than that). Resolving this issue will be essential to determining what the last common ancestor of whales and hippos was like, and it would also highlight the high degree of mammalian diversity which has been lost over the past 55 million years. If hippos and whales are each others closest living relatives but last shared a common ancestor at so ancient a time, then entire radiations of strange mammals have flourished and disappeared without leaving any hint of their existence. Without the fossil record, we would have no clue that they were ever here.

Top image: A hippopotamus (left) and a pair of pygmy hippos (right). Images from Wikipedia (here and here, respectively).

References:

BOISSERIE, J., & LIHOREAU, F. (2006). Emergence of Hippopotamidae: new scenarios Comptes Rendus Palevol, 5 (5), 749-756 DOI: 10.1016/j.crpv.2005.11.004

Orliac, M., Boisserie, J., MacLatchy, L., & Lihoreau, F. (2010). Early Miocene hippopotamids (Cetartiodactyla) constrain the phylogenetic and spatiotemporal settings of hippopotamid origin Proceedings of the National Academy of Sciences, 107 (26), 11871-11876 DOI: 10.1073/pnas.1001373107