Whales are evolutionary superstars. Over the past four decades, discoveries from all over the world have documented how their hoofed mammal ancestors proliferated at the water’s edge and slipped into the seas, turning whales from enigmas to one of the greatest examples of transcendent anatomical change. And the hits keep coming. Even though the big story of how whales went from terrestrial to aquatic beasts is known in broad strokes at this point, new finds are reminding us that there’s still far more to the cetacean story.

A new fossil whale described by paleontologist Olivier Lambert and colleagues is the latest piece of whales’ prehistoric puzzle. They’ve named it Mystacodon selenensis, and this 36.4 million year old mammal adds some new context to the origin of the great, filter-feeding baleen whales.

The remains of Mystacodon, excavated from Peru’s Pisco Basin, consist of a partial skeleton that includes the skull, teeth, vertebrae from various places in the spine, partial forelimbs, and a hip bone. The overall look of the whale was similar to Basilosaurus, the elongated predator that, until recently, was the classic image of what the most ancient whales were like. Yet, Lambert and coauthors write, specific features of the skull identify Mystacodon as the earliest known baleen whale. This cetacean is an anatomical bridge between more archaic forms like Basilosaurus and the profusion of baleen whales that followed.

The skull of Mystacodon. Credit: Lambert et al 2017

It may seem strange to think of baleen whales with teeth, but this isn’t a new concept for paleontologists. The very earliest whales – grouped together as archaeocetes – had teeth. From that broad group, the ancestors of today’s toothed whales (odontocetes) and baleen whales (split off). Baleen is a relatively recent specialization, in other words, and some fossils even seem to document how ancient baleen whales used teeth and baleen in combination before switching to baleen only. 

Of course, today’s baleen whales include some of the largest animals of all time. The blue whale can get to be over 100 feet long and weigh in excess of 200 tons. Mystacodon was quite a bit smaller, estimated to be about 13 feet long. But even though Mystacodon was quite different from today’s minkes and humpbacks, we can look back and see some features that seemed to open up a pathway for the way modern baleen whales feed.

Whales like Basilosaurus were active predators, gripping and shearing their prey with mouths fitted with differentiated teeth. But the front of the snout useful for gripping in shortened in Mystacodon, Lambert and colleagues observe, and the anatomy of the jaws seem more consistent with suction feeding. The wear patterns on the whale’s teeth seem to support this. What may have spurred the earliest mysticetes to split off from their ancestors, then, might have been the shift to suction feeding and sifting tasty morsels out of the bottom sediment. A different way of eating opened up the possibility of oceanic giants. 

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Lambert, O. Martínez-Cáceres, M., Bianucci, G., De Celma, C., Salas-Gismondi, S., Steurbaut, E., Urbina, M., de Muizon C. 2017. Earliest mysticete from the Late Eocene of Peru sheds new light on the origin of baleen whales. Current Biology. doi: 10.1016/j.cub.2017.04.026