Archaeopteryx played a pivotal part in revealing one of the greatest transitions in the history of life when dinosaurs took to the skies and fluttered away. There are not many transitions that can quite rival that evolutionary moment but, if there is one, it is the journey that the terrestrial ancestors of whales made into the sea. How the largest mammals on the planet came to lead the lives that they do today is a matter that is much shrouded in mystery. More specifically, how the blue whale and its kin ended up filtering tiny organisms out of the water with fibrous baleen when they started with sharp and pointed teeth is a subject of tremendous debate. Now a new fossil find is doing for baleen whales what Archaeopteryx did for birds by providing the first glimpse of an animal that was in the midst of a remarkable transition.
To say that the origins of baleen are controversial is an understatement. One hypothesis suggests that teeth were lost during a suction-feeding stage of whale evolution and that baleen evolved thereafter. The other suggests that baleen evolved before teeth were lost. The new fossil solidly supports the second argument. The new species, named Coronodon havensteini, dates to the Oligocene epoch roughly 30 million years ago and has an astonishing mouth. While most of its teeth indicate that it captured large prey, its broad lower molars frame narrow slots that look like that had to have been used for filter-feeding. This notion is further supported by the fact that, structurally, the rest of specimen looks like an ancient relative of the filter feeding branch of the whale family. Given this, the researchers are arguing that filter-feeding was preceded by predatory feeding, and that suction-feeding (which is seen in a few whales) evolved separately within a group that was removed from modern baleen whales. You can read more in The Economist article that I wrote on this here.