The Bear Otter

Folks have been all a-twitter about the giant fossil rabbit found on the Spanish isle of Minorca during the past week, but there was another oversized mammal announced in the same issue of the Journal of Vertebrate Paleontology that has so far gone unnoticed. About the same age as the behemoth bunny, this other giant lived alongside some of our own close relatives in northeastern Africa. If the suggestions of the scientists who discovered it are correct, this creature might best be described as the “bear otter.”

The bones of the enormous otter were found in the 3.2 – 3.4 million year old deposits of Dikika in Ethiopia’s Afar Valley. Among paleontologists, this area is best known for discoveries of early humans – such as the juvenile Australopithecus afarensis named “Selam”  – but these rocks have also yielded remains representing other parts of the local fauna. One, a raccoon dog named Nyctereutes lockwoodi, was just described last year, and the otter was another previously-unknown component of the ancient ecosystem.

The partial skull of the "bear otter", as see from the front (left) and the right side (right). From Geraads et al., 2011.

Described by Denis Geraads, Zeresenay Alemseged, René Bobe, and Denné Reed, the otter has been given the name Enhydriodon dikikae. A partial skull, a portion of the lower jaw, a humerus, and a few femur fragments are all that have been found so far. That is not very much to go on, but the authors of the new paper argue that this otter was quite unusual.

“The most striking feature of the [partial skull] is its very large size,” Geraads and colleagues state, “more suggestive of a bear than of a modern otter.” If Enhydriodon dikikae had a skull proportioned like its living relatives, the whole head would have been nearly ten inches long. This is about three inches longer than the skull of the largest living otter – the appropriately-named giant otter of South America – and may indicate that the bear otter was well over seven feet long.

Exactly how big the whole animal was, though, is difficult to say. No total length estimate was provided, and, based upon comparison of molar size with other otters, the scientists gave a wide weight estimate of about 200 to 400 pounds. More precise estimates of this large otter’s size will have to await further material.

How this otter made its living is also unclear. The authors of the new study suggest that the bear otter was more terrestrial than modern otter species, but their reasoning suffers from two significant flaws. First, citing a 2008 study of fossil otter femurs by paleontologist Margaret Lewis, Geraads and co-authors state that the femur fragments of their new species are anatomically similar to the femur of a large fossil otter found in Hadar, Ethiopia and designated AL-166-10. The femurs from the two species are quite similar, but it actually argues against the bear otter spending more time on land. Lewis found that AL-166-10 possessed features associated with a semi-aquatic lifestyle, perhaps similar to that of living sea otters.

A feature of the bear otter’s humerus was also cited as evidence for a life spent mostly on land. Down near the elbow joint, there is a prominent facet of bone called the medial epicondyle which was an important attachment site for muscles, ligaments, and tendons in the lower arm. In semi-aquatic species like the sea otter and giant otter, this knob of bone is quite large and has a squared-off outline, but the medial epicondyle is relatively reduced in size in the bear otter. Since another fossil otter found in Chad called Sivaonyx beyi also had a relatively small medial epicondyle and was interpreted as spending much of its time on land, Geraads and co-authors infer that the bear otter was “mostly terrestrial.”

The argument that Enhydriodon dikikae was a landlubber is relatively weak. Only one characteristic of the humerus is cited as supporting this conclusion, and the anatomy of the otter’s femur may directly contradict the hypothesis. The details of the otter’s prehistoric environment do not do much to help. Fossils of fish, crocodiles, and hippos are found in the same layers of strata, as well as fully-terrestrial animals. Future fossil discoveries and in-depth study of the otter’s remains may provide a greater degree of resolution about this species and other fossil otters. For now, though, our understanding of otter evolution is frustratingly incomplete. Many prehistoric species are known only from teeth and maybe a few skeletal scraps. As ever, we need more bones.

Top Image: A giant otter (Pteronura brasiliensis). Image from Flickr user VIDYO.

References:

Geraads, D., Alemseged, Z., Bobe, R., & Reed, D. (2011). Enhydriodon dikikae, sp. nov. (Carnivora: Mammalia), a gigantic otter from the Pliocene of Dikika, Lower Awash, Ethiopia Journal of Vertebrate Paleontology, 31 (2), 447-453 DOI: 10.1080/02724634.2011.550356

LEWIS, M. (2008). The femur of extinct bunodont otters in Africa (Carnivora, Mustelidae, Lutrinae) Comptes Rendus Palevol, 7 (8), 607-627 DOI: 10.1016/j.crpv.2008.09.010

PEIGNÉ, S., DE BONIS, L., LIKIUS, A., MACKAYE, H., VIGNAUD, P., & BRUNET, M. (2008). Late Miocene Carnivora from Chad: Lutrinae (Mustelidae) Zoological Journal of the Linnean Society, 152 (4), 793-846 DOI: 10.1111/j.1096-3642.2008.00377.x

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