ARCHAEOPTERYX, a toothy, feathered fossil found in Germany in the 19th century, hinted at a crucial moment in one of the greatest transitions in the history of life—the point when dinosaurs (which have teeth) took to the skies, and birds (which have feathers) were thus born. There are not many evolutionary journeys that can rival this, but that made by the descendants of some small, terrestrial mammals, which turned into the gargantuan aquatic krill-eaters called baleen whales, is one such. Analysis of a newly discovered toothy, finned fossil from America, just published in Current Biology by Jonathan Geisler of the New York Institute of Technology and his colleagues, promises to illuminate one part of this remarkable journey.

Mysticetes, as baleen whales are known technically, feed by sucking in large volumes of water and then forcing it out of their mouths through fibrous outgrowths known as baleen plates, to filter out small animals for consumption. These plates sit where other mammals have teeth, but rather than robust dentine, the principle ingredient of teeth, they are composed of keratin, a flexible material that also makes up hair and fingernails.

Some researchers theorise that suction-filter feeding in whales began with teeth that could be gnashed together to form a simple sieve, and that only subsequently were these teeth replaced by baleen. Others think teeth were lost entirely by ancient suction-feeding whales (which no longer needed them to capture individual prey) and that baleen arose secondarily in some of these animals, to make suction feeding more efficient. Unfortunately for palaeontologists, baleen rots rapidly, so is rarely found in the fossil record. This makes it tricky to work out which of these theories is correct.

The new fossil, named Coronodon havensteini and found in 28m-year-old sediments near the Wando River in South Carolina, has characteristics like a wide snout and short jaw bones that identify it as a member of the mysticetes. However, unlike living mysticetes, it has teeth. These are not particularly sharp. They have an apical angle of 155°, compared with the 50°-60° found in toothy hunters such as the sperm whales, which eat squid. Nor are they well supported by bone in the way that the teeth of animals which chew their food usually are. But, when Dr Geisler and his colleagues brought together the teeth from the animal’s upper and lower jaws they found the result was a sieve that would have superbly served the task of filtering out small animals from water passing through them.

This suggested that the idea of filter feeding evolving while whales still had teeth was right. But Dr Geisler knew that just because the teeth could be brought together to form an effective filter did not mean that this was how they were actually used. He therefore handed the fossil over to a colleague, Brian Beatty, to take a closer look. Dr Beatty theorised that regions near the openings in the sieve would have taken a lot of punishment if water loaded with small prey had been flushed past them routinely when the animal was alive. And they had. C. havensteini, then, does indeed seem to have been a filter feeder.

Its teeth do, though, suggest it could take individual large prey as well, given the opportunity. Such an arrangement is unknown in modern whales. It is, though, seen in leopard seals, which use some of their teeth to snatch animals like penguins and employ others to filter krill from seawater. Leopard seals even have similarly shaped snouts and jaws to those of C. havensteini. The new fossil does, therefore, seem to provide a bridge between predation and filter feeding, and thus marks an important milestone on the route to the largest animals the world has ever seen.