In the distant past, all whales had teeth. Now, some feed using baleen instead. A new analysis of previously-discovered fossils has provided a fresh insight into the evolution of that baleen, suggesting that for a while, some whales simply sucked down their food.

Baleen whales feed by lunging forward and gulping seawater, then straining that water out through the fibrous baleen plates in their mouths. Those plates trap tiny crustaceans called krill, which the whale subsequently swallows. Theories vary as to whether or not certain types of prehistoric toothed whales evolved straight through to having baleen, or went through a toothless and baleen-less stage in between.

In the 1970s, the fossilized skeleton of a 15-foot (4.6-m) whale known as Maiabalaena nesbittae (illustrated below) was discovered in Oregon. It lived about 33 million years ago, and while it definitely didn't have teeth, it might not have had baleen either – scientists have remained uncertain on this point, due to the fact that baleen doesn't fossilize well.

Now, using CT-scanning technology, a team from the Smithsonian Institute's National Museum of Natural History has taken a more in-depth look at Maiabalaena's skull. Lead scientist Carlos Mauricio Peredo determined that while the whale's upper jaw was too thin and narrow to support baleen, muscle attachments on its throat bones suggest that it did have strong cheeks and a retractable tongue.

This would allow it to suck water into its mouth, along with small prey items such as fish and squid. No teeth would be required for this suction-feeding method, which could have set the stage for the subsequent evolution of baleen.

Interestingly, a 2017 study somewhat refutes the notion that baleen whales went through a toothless and baleen-less phase in their evolution. Upon analyzing the fossilized remains of a toothed whale called Coronodon havensteini, a team from the New York Institute of Technology determined that its teeth were evolving to serve a baleen-like filter-feeding purpose.

This implies that over time, Coronodon's teeth could have actually become what we now know as baleen. Peredo, however, believes that the jury is very much still out on that theory.

"Coronodon is on one of the earliest branches of the baleen whale family tree. Its ability to filter feed has not been tested in a robust way; proposed living analogs such as crabeater seals and leopard seals remain understudied and probably do not feed the way fossil whales did," he tells us. "Most importantly, Maiabalaena is at the right place on the baleen whale family tree for this transition; Coronodon represents a much earlier branch that is unlikely to inform us about the origin of baleen."

A paper on the research – which also involved scientists from Virginia's George Mason University, Texas A and M University, and the Burke Museum of Natural History and Culture in Seattle – was recently published in the journal Current Biology.

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