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New species of extinct walrus discovered from UK fossils
A predecessor of modern walruses has been named from a fossil found in Suffolk.
Dated to roughly two million years ago, the jawbone of Ontocetus posti helps to reveal how the animals became experts at sucking molluscs out of their shells.
A new species of ancient walrus once called the North Sea home.
While present-day walruses spend most of their time in the Arctic, those living in the distant past had a much wider range. Fossils found on the coast of the UK and Belgium reveal that a previously unknown species, named Ontocetus posti, lived in northern Europe for as long as half a million years.
This new species has captured the attention of scientists because of its unusual combination of characteristics from ancient and modern walruses. In particular, it appears that Ontocetus posti is ‘midway’ on the evolutionary journey to becoming a specialised suction feeder.
Dr James Rule, an expert on seals and their relatives (pinnipeds) at the Natural History Museum who was not involved in the study, says the findings offer fresh insights into how this unique way of eating developed.
“Walruses are suction feeders, manipulating pressure to suck molluscs right out of their shells,” he explains. “This is a very specialised way of life, and the jaw of Ontocetus posti captures a fascinating moment in its evolution.”
“There are still signs of a tusk in the lower jaw, but its small size and peg-like shape suggests that it was in the process of being lost to make the walrus a more efficient suction feeder. This has implications for the unique feeding style of modern walruses.”
The findings of the study were published in the journal PeerJ.
The jawbone of Ontocetus posti reveals it was partway towards becoming a suction feeder, with a fused jaw similar to a modern walrus. © James Ashworth
Under pressure
While a variety of different marine mammals have become suction feeders, including cetaceans and seals, walruses are among the most specialised. By forming a tight seal between their lips and a shell, they can quickly pull back their tongue to create a vacuum that sucks a mollusc out of its shell.
This ability didn’t evolve overnight. It took millions of years, beginning with an otter-like ancestor that had many more teeth than its descendants.
The process started with the loss of teeth, as ancient walruses relied less and less on being able to tear their prey apart. The remaining teeth shrank to form a flattened surface, allowing the walrus to form a more effective seal with the shells.
Some species took this to extremes, with Valenictus chulavistensis losing almost all of its teeth to become a highly specialised suction feeder. The only teeth that remain are the upper canines, which developed into long tusks for sexual display.
Ontocetus posti isn’t quite as well-adapted for suction feeding as Valenictus but would still have been an efficient mollusc predator.
“Ontocetus posti has fused both sides of its jaws together, which would have helped it to withstand the pressure that suction feeding generates,” James explains. “This characteristic isn’t seen among other walruses that lived at the same time, like Ontocetus emmonsi, but is a feature of their living relatives.”
By comparing Ontocetus posti to its relatives, researchers have also got a deeper insight into the evolution of walruses, an area in which we know surprisingly little about.
While today's walruses only live in cold waters, fossil walruses used to live in much warmer areas of the world. ©Wildnerdpix/ Shutterstock.
Where do walruses come from?
While walruses are the largest known group of pinnipeds, their evolutionary history is tricky to interpret. A 2024 paper suggests that these animals have historically been very quick to form new species, but also to go extinct.
James adds that this has been compounded by a “notoriously incomplete” fossil record, which makes it hard to know where walruses come from.
“Most research into the evolution of walruses takes place in the northern hemisphere, where their fossils are poorly preserved,” he explains. “The presence of glaciers over the continents during ice ages would wear away the rock underneath them, eroding any fossils too.”
As a result of these difficulties, it’s not entirely certain where and when the first walruses evolved. High numbers of fossils found in Japan and the USA suggest that it was probably somewhere in the North Pacific, with their descendants spreading out across the world.
“When fossil walruses were alive, North and South America were still separate continents,” James says. “This allowed them to migrate through a body of water known as the Central American Seaway, which lay between the Americas, and enter the Atlantic Ocean.”
“These walruses spread further and eventually reached what is now the North Sea, where they diversified into a variety of species, including Ontocetus posti.”
While walruses thrived for a time, everything changed around three million years ago. The isthmus of Panama formed between North and South America, cutting off the flow of water, and walruses, between the Pacific and Atlantic.
Wider changes in the climate meant that the Earth became much cooler over hundreds of thousands of years, causing sea levels to drop as the water froze to become the polar ice caps. Most of the world’s walruses died out as their habitats drained away, including Ontocetus posti.
“Walruses like Ontocetus depend on shallow water to find food in, so as sea levels fell these habitats would have slowly disappeared,” James says. “The wider climate changes are also thought to have affected mollusc populations, leaving these specialists with less and less to eat.”
“Eventually, this meant that Ontocetus posti became extinct around 1.7 million years ago. Its more generalist relatives clung on for longer in the southern USA, but eventually disappeared as well.”
The extinction of its relatives leaves the modern walrus as the only living member of its family today. Even though it never lived alongside Ontocetus posti, the lifestyle of both animals would have been recognisable to each other, even with more than a million years between them.
