What makes a whale a whale?
Flippers, the need to breath air, the ability to give milk, and a streamlined shape. There are plenty of signs that let us immediately distinguish whales from fish and the other creatures of the sea. Yet, alone or in combination, the traits I just mentioned aren’t unique to whales. Seals and sea lions, for example, are also flippered, air-breathing, milk-giving, streamlined mammals. If you truly wish to draw out leviathan, you have to look beyond the blubber to a clue made of bone.
The secret is in the skull. Being mammals, whales have domes of bone on their skulls that enclose the middle ear. These are called tympanic bullae. But whales have a peculiar modification to these bony bulges. The inner edges of these bulbs are so thick and dense that they get their own name – the involucrum.
It’s not just modern whales that display this structure. An involucrum is a very, very ancient cetacean trait. When Philip Gingerich and Donald Russell first described the 55 million year old Pakicetus in 1981, for example, they were able to immediately recognize it as an early whale because of a partial skull preserving the thickened tympanic bullae on the bottom. Living and fossil, whales are united by a thick wall of bone over their ears.
But here’s where things get a little complicated. In 2007 Hans Thewissen and colleagues announced that Indohyus – a small, hoofed, 48 million year old mammal found in India – had an involucrum, too. A happy accident had broken open the beast’s middle ear and revealed a dense, thick margin just like that in whales. And while Indohyus wasn’t technically a whale – it belonged to a group called raoellids – the mammal was nevertheless an extremely close relative of the very first whales, providing paleontologists a proxy for what the ancestors of whales were like. Whales had inherited the involucrum from their deer-like ancestors and have kept it throughout their history, a rind of bone that whispers of the distant past.
Gingerich, P., Russell, D. 1981. Pakicetus inachus, a new archaeocete (Mammalia, Cetacea) from the Early-Middle Eocene Kuldana Formation of Kohat (Pakistan). Contributions from the Museum of Paleontology. 25 (11): 235-246
Thewissen, J., Cooper, L., Clementz, M., Bajpai, S., Tiwari, B. 2007. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature. 450: 1190-1194. doi: 10.1038/nature06343