Walruses are perhaps best recognized for their iconic tusks. But one thing that makes them unusual among modern marine mammals is the way they use suction to eat.
That ability appears to have been missing among many of the animal’s ancient relatives. But fossils now show that a newly named species developed that skill — perhaps as a way to adapt to a changing climate, according to a report in PeerJ Life & Environment.
This finding is both exciting and unusual because it showcases a potential example of two different species, separated by time and space, making similar adaptations — a phenomenon sometimes called evolutionary convergence.
“It’s really something quite rare in the field of fossil marine mammals, to be able to observe an evolutionary convergence with these large animals,” says Mathieu Boisville, a paleontologist at the University of Tsukuba in Japan and an author of the paper.
A Case of Evolutionary Convergence?
In this case, showing convergence came down to comparing teeth and jaws of multiple ancestors of the contemporary walrus, Odobenus rosmarus. That was a more complicated and challenging task than one might think.
Since the 1870s, paleontologists have discovered hundreds of walrus-like fossils from around the Northern Hemisphere, ostensibly categorized among the genus Ontocetus. Unfortunately, the various species were based on sometimes inconsistent comparisons. Some classifications were based on skull shape, others on jaws, and still others on other bones.
These classifications were reevaluated in 2008 and all the variations were grouped into a single species, Ontocetus emmonsi, based on the shape of the tusks. However, while the earlier Ontocetus classification might have been too jumbled, the O. emmonsi label may have been too broad.
Read More: What Is the Difference Between Convergent and Divergent Evolution?
Comparing Ancient Walrus Fossils
When Boisville and his colleagues re-examined some fossils from England and Belgium labeled O. emmonsi, they focused more on skull shape — especially the jawbone. They noticed a number of differences in the teeth and jawbones, compared to many of the fossils labelled O. emmonsi.
However, detailed analysis of the mandibles revealed features distinguishing it as a new species they named Ontocetus posti. The scientists noted that these features meant how O. posti ate differently than O. emmonsi.
“Such anatomical characteristics suggest that O. posti was quite well adapted to suction-feeding, somewhat similar to its modern relative, the walrus,” says Boisville.
So why did O. emmonsi and O. posti develop differently? Probably due to climate change. Global cooling intensified over the last 5 million years. That altered ocean salinity, currents, and sea levels, which in turn impacted the animals living in that changing environment. Walruses, due to their specific ecological niche, were especially affected by this phenomenon.
Read More: The 5 Mass Extinctions That Have Swept Our Planet
Different Feeding Techniques
O. emmonsi thrived in warmer water and didn’t specialize in suction-feeding. O. posti may have tried to adapt using suction-feeding techniques, says Boisville. But they didn’t appear to adapt fast enough and went extinct during the Early Pleistocene. All warmer-tolerant walruses living at lower latitudes also disappeared by then, leaving only the modern, cold-tolerant walrus Odobenus rosmarus.
The loss of one suction-feeding walrus species and the rise of another resembles evolutionary convergence, according to Boisville. But a more accurate characterization would be that the two species filled similar ecological niches at different times.
“This proves that it’s also possible that evolutionary convergence is perhaps something more common than we might think. I hope this will pave the way for more studies on this phenomenon,” says Boisville.
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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
Source : Discovermagazine
