Prehistoric Hippo Potholes are the First Fossil Mammal Swim Traces

When Charles Darwin was composing On the Origin of Species, he had to face an extremely frustrating problem. The fossil record, despite initially inspiring some of his evolutionary notions, was largely a mystery. Part of the problem stemmed from the gaps and missing chapters in life’s history. But there was also a more hopeful aspect of the argument that Darwin hasted to point out as he wrote new editions of his most famous book. Despite the fossil record’s unavoidable incompleteness, paleontologists of Darwin’s day had only just begun to discover the vestiges of prehistoric life that the world’s rocks contained. There was far more to find than had been turned up. And that’s still true. Case in point – paleontologists have only just now uncovered the first known fossilized traces of a mammal as it swam.

Paleontologists found the traces at a site best-known for different fossils. Koobi Fora, on the east shore of Kenya’s Lake Turkana, has yielded multiple fossils of early humans that lived in the region between ~4.3 million years ago and 600,000 years ago. The footfalls of prehistoric people are known from the site, too, but the more numerous tracks of other creatures fill out what the environment was like when Homo erectus was ambling around. Excavations conducted over three decades ago found footprints of hippos and a wading bird that slogged through the shallows 1.4 million years ago, for example, and upon revisiting the track surface in 2008, researchers found additional, crude divots in the ancient stone. These newly-found tracks – described by paleontologists Matthew Bennett, Sarita Morse, and Peter Falkingham – were made by swimming hippos.

Fantasia to the contrary, hippos are not very graceful on land. They can swing their legs in a quick run, true, but most of the time they have to keep three legs on the ground at any point in the step cycle because of their girth. But a submerged hippo is an elegant beast. They punt off the bottom with their four-toed feet and glide through the water, quite at home under the surface. And it’s that kind of “bottom-walking” behavior that created the newly-described Koobi Fora tracks.

The 240 traces Bennett and coauthors describe roughly fit into five different types, ranging from tracks with well-defined toes and nails to “oval- or tear-shaped prod-like impressions” and messy tracks on top of tracks. These could have been made by adults and juveniles of the large Hippopotamus gorgops, or the small tracks could be attributed to a smaller species that lived in the area named Hippopotamus aethiopicus.

Unfortunately the tracks don’t offer the clues to tell the difference between the species, not to mention that the fore- and hindfeet of hippos are so similar that distinguishing which foot made which track isn’t easy. But despite these complications, the preserved anatomy shows that the traces are distinctively hippo and record the underwater excursions of several individuals. The fact that there are many tracks but there don’t seem to be any clear trackways indicates that the water horses where floating along beneath the surface and occasionally giving themselves a push off the lakebottom. Of course, the water had to be fairly deep to allow hippos to swim, so this hints that the Homo erectus tracks found in the same level may be a sign of the people wading into the lake.

And this discovery is more than a fossil first. Bennett and colleagues tie the discovery to long-running debates over swimming sauropod dinosaurs.

Despite the swampbound “Brontosaurus“swampbound “Brontosaurus“swampbound “Brontosaurus“ I saw in books and movies as a kid, there is no good evidence for swimming sauropods. The few trackways that seemed to show dinosaurs like Brachiosaurus walking themselves along the bottom, their hind legs and tails buoyed up in the water, have turned out to be undertracks of the front limbs that preserved more deeply than the hind footprints. The only definitive dinosaur swim tracks paleontologists know of so far were made by carnivorous theropods – the sort of dinosaur paleontologists used to think of as aquaphobic – and these are usually long scratches made by claws on three-toed feet that pushed the little predators along.

The hippo tracks could help paleontologists come up with a better idea of what to look for while pondering swimming sauropods. While hippos are adapted to an amphibious life and sauropods were terrestrial animals that were probably really lousy at swimming, there’s no reason to think that Diplodocus and company totally avoided water. When they crossed lakes, rivers, or lagoons, what sort of tracks did they make? If the dinosaurs pushed off with their tippy toes like hippos, they’d probably leave weird divots and semi-complete tracks on the bottom in a pattern that would be relatively difficult to follow as a trackway (unless the dinosaur was pushing off at at regular interval along a particular path).

There are two complications with using hippos as guides, though. First, Camarasaurus and kin had smaller, crescent-shaped front feet and much larger, broader hind feet that bore big claws. In general shape, elephants may offer better approximations of sauropod feet. If anyone happens across swim tracks made by prehistoric elephants – as they must have created as they swam to some of the islands where dwarfed species evolved – those prints may refine the search image for swimming sauropod clues.

Then again, sauropods had something that hippos and elephants don’t. While dinosaurs like Apatosaurus were large and had column-like limbs, much like elephants, the front half of their bodies was invaded by air sacs that extended from the lungs. This would have made them relatively more buoyant and affected their swimming technique. And even though birds have these air sacs, too, they’re so anatomically different from sauropods that avian dinosaurs aren’t good models for how Sauroposeidon might have swum. It’s really difficult to find a workable sauropod substitute.

Nevertheless, paleontology is a comparative science. Finding fossils and properly interpreting them relies on mental images of what to look for in the first place. Having swim tracks to look at will help scientists pick out additional specimens. Now that Bennett, Morse, and Falkingham have given us the first-known fossil impressions of a big swimming mammal, who knows what other traces will suddenly snap into focus as paleontologists continue to scour the fossil record?

Reference:

Bennett, M., Morse, S., Falkingham, P. 2014. Tracks made by swimming Hippopotami: An example from Koobi Fora (Turkana Basin, Kenya). Palaeogeographic, Palaeoclimatology, Palaeoecology. 409: 9-23