Surfin’ USA – Walrus ancestors roamed Californian coast | Polarjournal
Walruses prefer flat beach sections due to their size and way of life, on which they can crawl up and where they can rest from their dives. The depth of the sea is also a prerequirement: not too deep and with many buried mussels for food. Mostly they lie on the beach in groups for socializing and as protection. Picture: Michael Wenger

Walruses are the icons of the Arctic alongside polar bears. The largest Arctic seal species is one of the highlights for all Arctic travellers thanks to its size, its original appearance and its seemingly relaxed way of life. Within the seals, the walrus occupies a special position between the eared and the true seals and officially, there exists only one species. But researchers, using fossil finds, have discovered that the diversity of walrus ancestors was much greater millions of years ago, and they must have been at home even on California beaches.

A research group led by study leader and geology professor James Parham of Cal State University Fullerton has discovered three new species in Orange County, California. In their paper, published in the journal Journal of Vertebrate Paleontology, first author geology student Jacob Biewer and his colleagues examined fossil skull remains of twelve specimen of walrus ancestors. They came to the surprising conclusion that among them were three new species that were previously unknown to science. Another surprise: the fossils included both male and female and young animals. “Orange County is the most important area for fossil walruses in the world,” explains Jacob Biewer. Most of the fossils studied by the researchers came from the collection of the Natural History Museum of Los Angeles County.

The map shows the sites of various species, which are counted among the ancestors of the walruses. The three newly discovered species are all from Orange County, a district south of Los Angeles that is best known for its beaches, PC hardware industry, and theme parks. Picture: Courtesy of Cal State Fullerton

The research showed that the animals had lived about 5-10 million years ago, but not all at the same time. The degree of kinship of the species is also different: while Pontolis kohoni probably originated from its predecessors Pontolis barroni, the species Osodobenus eodon is somewhat remote in development. The evolution of the walruses, which led to the only species Odobenus rosmarus today, is very patchy. Especially the distinctive tusks of the animals should help to shed more light onto the evolution. To this end, the fossils discovered by Biewer and his colleagues are very helpful. Because only the newly discovered species Osodobenus eodon has something like prolongated teeth at all. The remaining species had the usual predator set of teeth. “This research further emphasizes how tusks were a later addition to the history of walruses,” says Biewer. In addition, the researchers found a great variability in their dental setup. “Scientists assumed you could identify certain species just based on the teeth, but we show how even individuals of the same species could have variability in their dental setup,” Jacob Biewer explains.

The picture shows a skull of a young modern walrus and its lower jaw (right). The tusks of adult walrus ancestors look very similar to those of modern young walruses. But the remaining teeth are either shaped differently or missing. Photo: Annina Egli

According to the researchers, the development of the distinctive tusks occurred only later. The fossils found show an extension of the canines only in Osodobenus eodon. “Osodobenus eodon is the most primitive walrus with tusk-like teeth,” explains study leader James Parham. The differences between this species and the other species were so large that the researchers even developed a new genus. “This new species demonstrates the important role of feeding ecology on the origin and early evolution of tusks.” The researchers assume that the early walrus relatives did not have the same diet as the animals living today. “The majority of walrus species were fish eaters and adapted to catching fish, rather than using suction feeding on mollusks like modern walruses,” Biewer continues. The scientists assume that the group of walruses was slow to evolve initially. It was only about 7 million years ago that the evolution of tusks began, in which Osodobenus eodon had played an important role.

Cal State Fullerton researchers Jacob Biewer (left) and James F. Parham (right) were able to add an important piece of the puzzle to the evolution of walruses. Especially that the distinctive tusks developed at a time when the waters off California began to warm up, resulting in a larger and more diverse food supply. Picture: Courtesy of Cal State Fullerton

But the question still arises as to why a region like California was so important to the evolution of walruses. The researchers in their study speculate that an increase in water temperatures off the coast had increased food supply and food diversity. Especially easy-to-catch bottom-dwelling animals, which lived as filter-feeders and found more plankton thanks to the warmer water, may have appeared more numerous and thus became an easier food source for the walrus ancestors. This could have been the starting signal for the suction-feeding diet and the change in the function of the canines.

Walruses live throughout the Arctic. In the past, hunting, today pollution of habitat, shipping and especially climate change are causing enormous pressures on the large seal species. The animals, which weigh up to 1 tonne, need ice and intact mussel banks in the soft seabed for their survival. Both are dwindling due to climate change. Picture: Michael Wenger

Today, however, the walruses are threatened by rising water temperatures in the Arctic, as their base, sea ice, is rapidly disappearing and pushing the animals out of their habitat. And as the evolution of walruses has shown, they only react shabby to changes. And these are coming into their habitat now very fast, perhaps too fast for these Arctic giants.

Dr Michael Wenger, PolarJournal

Link to the study: Jacob N. Biewer , Jorge Velez-Juarbe & James F. Parham (2020): Insights
on the dental evolution of walruses based on new fossil specimens from California, Journal of
Vertebrate Paleontology, DOI: 10.1080/02724634.2020.1833896

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