If you're a fan of whales, then you probably already known that some of the largest ones feed mainly on tiny crustaceans known as krill. They do so by lunging forward and filling their mouths with water, then straining the krill out of that water as they expel it, using fibrous plates in their mouth called baleen. Now, scientists claim to have come a step closer to understanding how that baleen came to be.

Although there are currently both toothed and baleen whales, the fossil record indicates that the prehistoric ancestors of today's baleen whales just had teeth. Even now, baleen whales pass through a phase in which they start developing teeth while still in the womb, but then stop and grow baleen instead.

This means that at some point in their evolution, baleen whales transitioned from catching larger prey by snagging it with their teeth, to filtering smaller prey out of the water. What hasn't been clear is whether they used their teeth to filter-feed, or instead went through a toothless stage of sucking prey down whole, before later evolving baleen.

That's where Coronodon havensteini comes in.

A type of mysticete – a toothed prehistoric precursor to baleen whales – it lived about 30 million years ago, in a period between that of purely toothed whales and the appearance of the first modern baleen whales. Its fossilized remains were discovered in the early 2000s by scuba diver Mark Havenstein, while exploring South Carolina's Wando River. In a recent study conducted by the New York Institute of Technology, associate professors Jonathan Geisler and Brian Beatty led a team that examined its teeth.

One of the things they discovered was that as compared to other ancient whales, its molars were unusually large – about the size of an adult human's palm. Unlike its front teeth, which were presumably used to snag prey, they showed no evidence of having performed shearing or cutting. Wear was visible, however, on cusps bordering openings between them.

"The transition from teeth to baleen is widely contested, but our research indicates that ancient toothed whales relied on the spaces between their complex and enormous teeth for filtering," says Geisler. "It appears that over millions of years, the teeth were retained until baleen became sufficiently large and complex to take over the role of filter feeding."

Source: New York Institute of Technology