Baleen whales, the gentle giants of the seas, have at least one fearsome skeleton in the closet. A 25-million-year-old fossil, representing a new family of the whales, was probably a small but deadly predator that--unusually for a whale--tracked its prey with enormous eyes, according to new research.
Today there are two groups of whales, which last shared a common ancestor around 35 million years ago. The mysticetes, or baleen whales such as the blue whale, feed by filtering out planktonic krill and small fish from the sea water using fibrous baleen plates, instead of teeth. The odontocetes, or toothed whales such as orcas and dolphins, hunt their prey mainly using echolocation. Many ancient mysticetes also had teeth instead of baleen, but because fossils between 23 million and 35 million years old are "vanishingly rare," scientists don't know a great deal about the earliest phases of mysticete evolution, says palaeobiologist Erich Fitzgerald of Museum Victoria, Melbourne, Australia.
The new fossil, dubbed Janjucetus hunderi, will help. Featuring a nearly complete skull, shoulder blades, a few ribs, and vertebrae, Janjucetus is "a bizarre novel side branch, more of a cousin of modern baleen whales" and not a direct ancestor, says Fitzgerald. The most striking features of the skull, he says, are the large eye sockets--12 centimeters in diameter. This suggests that Janjucetus had the largest eyes of any known whale for its size, which Fitzgerald estimates at approximately 3.5 meters in length. The teeth at the front of the jaw would have helped grasp prey, says Fitzgerald, while serrated edges on the back teeth sliced through flesh. The skull also has huge attachment sites for the muscles to clamp the mouth shut on unlucky prey. All this evidence points to a predatory Janjucetus homing in on its prey using its sharp vision before ripping it apart, quite unlike its modern filter-feeding relatives, Fitzgerald reports online 16 August in Proceedings of the Royal Society of London B.
The new find "is certainly the most bizarre" fossil mysticete to be discovered and an early experiment in baleen whale evolution, says vertebrate palaeontologist Lawrence Barnes of the Natural History Museum of Los Angeles County. The fossil suggests that early mysticete history is "more complicated than we first thought," adds vertebrate palaeontologist Alton Dooley of the Virginia Museum of Natural History in Martinsville. It's possible that filter-feeding, and baleen, evolved gradually in mysticetes after the split with the odontocetes 35 million years ago, he says, and was not the original reason the groups diverged.