In Part 1, we saw that cetaceans are the descendants of ancient artiodactyls that returned to the water in the Eocene, and it is those earliest ancestors that we’ll be examining today.
First Steps: Indohyus
The first step in the evolution of whales is Indohyus major, a primitive artiodactyl that lived in India approximately 48 million years ago. It belongs to a group called the raoellids, the extinct sister group to cetaceans, and it is likely that the first whales were very similar to it.
Indohyus is a small animal, about the size of a raccoon, and would have looked something like a modern mouse deer with a long tail in life. While its anatomy is ostensibly that of a terrestrial animal, it has a couple unique features that suggest otherwise. First, it has dense limb bones, a feature seen in modern hippos that acts as ballast to keep them submerged. Second, it has an involucrum – a cup of thickened bone around the inner ear. This is an adaptation for hearing underwater, and has only ever been found in cetaceans. Its teeth, however, directly contradict these features, with isotopic analyses showing that it fed on terrestrial plants. What’s going on here?
The most likely answer is that Indohyus was something like the modern water chevrotain, a small species of artiodactyl related to deer that escapes from predators by diving into water, where it remains submerged until the danger has passed. Indohyus probably lived a similar lifestyle, foraging near shore and bolting into water to avoid being eaten, able to walk on the bottom while still having some basic hearing. I decided to lean in to this modern analogue for my reconstruction, basing the coloring and spots off of chevrotains and mouse deer.
From Prey to Predator: Pakicetus
Living in Pakistan at roughly the same time as Indohyus, 49-million-year-old Pakicetus inachus is one of the earliest cetaceans. In fact, for a long time it was the oldest-known whale, but despite its importance, it was very poorly-understood, with the type specimen consisting only of a braincase and jaw fragment. Despite being fragmentary, its teeth and ear bones were distinctively cetacean, so much so that it was imagined as a seal-like animal caught halfway between land and sea. This interpretation was way off; a much more complete specimen was described in 2001, which revealed Pakicetus as a largely terrestrial animal, looking something like a cross between a wolf and a hippo.
While it has dense limb bones and webbed feet, Pakicetus is proportioned like a terrestrial mammal. It was probably a poor swimmer, unable to do much more than a doggy paddle, and most likely walked along the bottom like a hippo. With its eyes located near the top of its head, it could hunt visually, grabbing fish that swam overhead or small animals onshore. While it does have an involucrum, its lower jaw lacks the fatty pads found in modern whales, so its hearing would have been very limited underwater.
The real Pakicetus, then, is even more bizarre than the creature it was initially thought to be. It’s a whale built like a hippo, a terrestrial creature with only the most basic adaptations for life in water. I tried to reflect this in my reconstruction, which imagines it with short fur (plus a crest along its neck and shoulders) and coloration based on hippos and pigs, punting its way along the bottom and only capable of awkward swimming. Those of you who are fond of the outdated version need not fret, however; while not reflective of Pakicetus itself, it is very similar to more derived whales that were discovered only a few years later. We will meet them next time.
- Bajpai, S. G. M., et al. “The Origin and Early Evolution of Whales: Macroevolution Documented on the Indian Subcontinent.” Journal of Biosciences, vol. 34, no. 5, 1 Nov. 2009, pp. 673–686.
- Gingerich, Philip D., and D. E. Russel. “Pakicetus inachus, A New Archaeocete (Mammalia, Cetacea) from the Early-Middle Eocene Kuldana Formation of Kohat (Pakistan).” Contributions from the Museum of Paleontology, University of Michigan, vol. 25, no. 11, 31 Dec. 1981, pp. 235–246.
- Thewissen, J. G. M., et al. “Skeletons of Terrestrial Cetaceans and the Relationship of Whales to Artiodactyls.” Nature, vol. 413, no. 6853, 20 Sept. 2001, pp. 277–281.
- Thewissen, J. G. M. “Hans”. The Walking Whales: From Land to Water in Eight Million Years. University of California Press, 2014, pp. 137-154, 199-204.