A team of paleontologists have uncovered a new species of extinct walrus that lived roughly 5.3 million years ago. Ontocetus posti bears some surprising similarities in feeding adaptations to the modern, living walrus Odobenus rosmarus. The new walrus is described in a study published August 13 in the open-access journal PeerJ Life & Environment and highlights an intriguing case of convergent evolution in these large marine mammals.
Millions of years ago, there were several walrus species on Earth. Now, there is only one main species with two subspecies–the Atlantic Walrus and Pacific Walrus. The fossilized remains of Ontocetus posti were discovered in Norwich, England and Antwerp, Belgium. The Belgian specimens were uncovered by study co-author Mathieu Boisville while working on his thesis in 2020.
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“I noticed that some fossil walrus mandibles differed from what was previously known,” Boisville, now a PhD and paleontologist at the University of Tsukuba in Japan, tells Popular Science. “Furthermore, the holotype (i.e. the type fossil that takes the species as its reference) had never been described, although it had been mentioned by some researchers.”
In the study, the team conducted a detailed analysis of the mandible, since they initially thought it was the species Ontocetus emmonsi. They found that the new specimen had a unique combination of features. It has post-canine teeth, a larger lower canine, and a fused and short mandibular symphysis. According to the team, these anatomical characteristics suggest that Ontocetus posti well adapted to suction-feeding similar to its living relatives.
“The modern walrus won’t arrive in the North Atlantic until the end of the Middle Pleistocene [about 200,000 years ago], yet a now-extinct walrus species (i.e. Ontocetus emmonsi) was already present in the North Atlantic at the beginning of the Pliocene (5.3 million years ago),” says Boisville. “Modern walruses live in the cold waters of the Arctic, and use a unique feeding technique (i.e. suction-feeding). However, this cold-temperature tolerance and suction-feeding technique was only developed late in the walrus lineage.”
Their migration was likely facilitated by the Central American Seaway. This crucial oceanic passage once separated the continents of North America and South America, but was eventually closed off by the Isthmus of Panama. The resulting global cooling during the Late Pliocene significantly impacted marine life, likely contributing to Ontocetus posti’s extinction allowing the cold-adapted modern walruses to emerge and eventually dominate the landscape.
The findings emphasize how changes in the environment shaped the adaptations and survival of marine mammals. This convergence of feeding adaptations between Ontocetus posti and the modern walrus shows how constantly shifting evolutionary processes occur across different time periods and environments.
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The team hopes to find more fossils that belong to this species, so that they can learn more about past climate changes and adaptations. Walruses and cetaceans–like whales and dolphins–are some of the animal lineages that can tell us a great deal about how marine mammals adapted to changes in the past. Walruses used to be more diverse and covered a wide geographical area, but still remain key indicators of climate change.
“We believe it is important for the public to realize the impact of global climate change on marine mammals, as well as on fauna and flora in general,” says Boisville. “The modern walrus is strongly impacted by human activity, whether it be hunting or global warming, affecting its habitat and thus its prey. We hope that this discovery can reinforce the idea behind conservation of these beautiful but vulnerable animals.”
