Some 2 million years ago, a large mammal swimming through the North Atlantic Ocean hauled its bulk up onto a beach, its belly full of clams. It had blubber, tusks and more than likely, whiskers.
Sounds like a walrus, right? In fact, the extinct animal was a member of a different marine mammal genus entirely, as different from a modern walrus as a fox is from your dog. For all the evolutionary differences between the animals, researchers say in a study published on Tuesday in the journal PeerJ Life & Environment, this relative of the walrus may have used a very similar foraging strategy: suction feeding.
Mathieu Boisville, a paleontologist at the University of Tsukuba in Japan and an author of the study, said that “It’s really something quite rare in the field of fossil marine mammals,” for two large animals to develop such similar adaptations, a phenomenon known as convergent evolution.
The false walrus, which Dr. Boisville and his colleagues named Ontocetus posti, belongs to a group of marine mammals that first appeared around five million years ago on the Pacific Coast of North America, before spreading into the Atlantic Ocean via the not-yet closed seaway between North and South America.
Most remains from animals in the group are found around the southeastern shores of the United States. But over several decades, amateurs and paleontologists found scraps of skull and a lower jaw along the coasts and seabed of the North Sea, Dr. Boisville said, not far from regions around the towns of Antwerp in Belgium and Norwich in England. For a long time, the remains were classified as a separate species, Ontocetus emmonsi.
When Dr. Boisville’s team studied the fossils, however, they noticed that the jaws were more walrus-like than those of their southern relatives. Like its close kin, Ontocetus posti had a compliment of teeth, including lower incisors and a large lower canine. But its heavy chin bones were fused — like those of a modern walrus.
The anatomical similarities are notable because of the particular way that modern walruses get their prey: rustling up shelled mollusks from the mud with their lips, using their tongue like a piston to pull out the flesh, and slurping it down. The specific anatomy of walrus jaws — an arch at the top of their mouths and fused bones — help them manage the physical strain that the suction places on the skull, Dr. Boisville said. While the southern species, Ontocetus emmonsi, wasn’t an effective suction feeder, Ontocetus posti appeared to have evolved to favor the technique.
Not everyone is sure that Ontocetus posti’s jaws represent a case of convergent evolution, however. The combination of traits seen in modern walruses and an earlier Ontocetus species might be a sign of a “direct evolutionary transition between the two,” said Robert Boessenecker, a marine mammal paleontologist who was not involved in the study.
And while the new study proposes that Ontocetus posti preferred warmer waters than modern walruses do, this might be a fluke of the fossil record. Wild changes in sea levels around 2.6 million years ago left few viable fossil sites from the period in the North Atlantic, and that has rendered marine mammal evolution at the time a “black box,” Dr. Boessenecker said.
“In other words, Ontocetus could have lived in the Arctic Circle and we’d never know from the fossil record,” Dr. Boessenecker added.
The same instability in sea levels that distorted the fossil record wrought havoc on marine ecosystems, leading to shake-ups among whales and pinnipeds and the disappearance of creatures like the marine sloth, the “ultimate birds” of Japan, and the fearsome giant shark Otodus megalodon. Between growing glaciers, expanding Arctic ice caps and changes in the diversity of mollusks, the North Sea also became an inhospitable place to live. By 1 million years ago, Ontocetus posti had disappeared, hundreds of thousands of years before modern walruses, or Odobenus rosmarus, arrived in the North Atlantic and swam smoothly into the same niche.
“The beautiful thing about walruses is that they can tell us about climate change,” Dr. Boisville said. “It’s also amazing to observe that in the past, animals evolved to try and survive in the face of past climate change, in this case, rapid global climate cooling.”
“Only those species that were better able to adapt to these rapid climatic changes had any chance of surviving,” he added.
