A team of paleontologists has unearthed a previously unknown species of extinct walrus that roamed the Earth around 5.3 million years ago. The species, named Ontocetus posti, exhibits remarkable similarities in feeding adaptations to the modern walrus, Odobenus rosmarus. This fascinating discovery, published on August 13 in the open-access journal PeerJ Life & Environment, highlights an intriguing case of convergent evolution among these large marine mammals.
Millions of years ago, a variety of walrus species inhabited the planet. Today, only one main species remains, with two subspecies: the Atlantic and Pacific walruses. The fossilized remains of Ontocetus posti were discovered in Norwich, England, and Antwerp, Belgium. Study co-author Mathieu Boisville, who uncovered the Belgian specimens while working on his thesis in 2020, played a crucial role in identifying this new species.
"I noticed that some fossil walrus mandibles differed from what was previously known," says Boisville, now a PhD paleontologist at the University of Tsukuba in Japan, in an interview with Popular Science. "Furthermore, the holotype—the reference fossil for the species—had never been described, although it had been mentioned by some researchers."
The research team initially believed the mandible belonged to the species Ontocetus emmonsi. However, a detailed analysis revealed that the new specimen had a unique set of features, including post-canine teeth, a larger lower canine, and a fused, short mandibular symphysis. These characteristics suggest that Ontocetus posti was well-adapted to suction-feeding, much like its modern relatives.
While modern walruses didn’t appear in the North Atlantic until the Middle Pleistocene, roughly 200,000 years ago, extinct species like Ontocetus emmonsi were already present in the region during the early Pliocene, around 5.3 million years ago. Boisville explains, "Modern walruses inhabit the cold waters of the Arctic and have developed a unique suction-feeding technique. However, this ability and cold-temperature tolerance only evolved later in the walrus lineage."
The migration of these ancient walruses was likely aided by the Central American Seaway, a vital oceanic passage that once separated North and South America before being closed off by the Isthmus of Panama. The global cooling that followed during the Late Pliocene likely played a role in the extinction of Ontocetus posti, paving the way for cold-adapted modern walruses to dominate the Arctic waters.
These findings underscore the significant influence of environmental changes on the evolution and survival of marine mammals. The similarities in feeding adaptations between Ontocetus posti and the modern walrus demonstrate how evolutionary processes can lead to convergent adaptations across different eras and ecosystems.
The research team hopes to discover additional fossils of this species, which could provide further insights into past climate changes and the adaptations of marine mammals. Walruses and cetaceans—such as whales and dolphins—are key to understanding how marine mammals have historically adapted to environmental shifts. Although walruses were once more diverse and widespread, they continue to serve as important indicators of climate change.
Boisville emphasizes the importance of public awareness regarding the impact of global climate change on marine mammals. "The modern walrus is significantly affected by human activity, including hunting and global warming, which threatens its habitat and prey. We hope this discovery will strengthen the call for the conservation of these beautiful yet vulnerable animals," he says.
This new species not only provides a glimpse into the past but also reinforces the ongoing need to protect marine life in the face of a rapidly changing climate.