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From Feet to Flippers, Part 2: The Walking Whales

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.

From so humble a beginning: tiny Indohyus and its kin are cetaceans’ closest relatives.

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.

A water chevrotain living up to its name.

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.

Pakicetus as it was once imagined: a seal-like blubber nugget already well on its way to being fully-aquatic. Alas, it was not to be…

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.

Pakicetus as it actually was: an animal adapted for hunting in water, but otherwise built for life on land.

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.

Sources

  • 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.

From Feet to Flippers, Part 1: Introduction

In May, I graduated from the University of Iowa with a BA in Art and Cinema, with Honors in Art. In order to earn credit for honors in my major, I had to create and exhibit a semester-long, research-based art project. Having fallen in love with 2D animation in a previous class, I decided to create a short video depicting the evolution of one of my favorite groups of animals: cetaceans (whales, dolphins, and porpoises). What was originally supposed to be a 1-minute video showcasing the transition from land to sea quickly ballooned into a much more elaborate video that started with the very earliest cetacean relatives and ended in modern times with the blue whale, the largest animal that has ever lived. You can watch it below:

Each animal depicted in the video is the end result of many hours of research, plus correspondence with Dr. Annalisa Berta (I can’t thank her enough for her generosity and feedback). Since that process can’t be shown in the final product, I thought it would be interesting to explore it here instead. Over the next few weeks, I’ll be doing a deep dive (pun very much intended) into all ten species that appear in my project – what we know about them, where they fit into the cetacean family tree, what modern animals can guide our understanding of them, and how all these factors came together in the final reconstruction. But first, some context…

A Brief History of Whaleontology

The evolutionary history of cetaceans has only recently become well-understood. Though they were recognized as mammals in the 18th Century, their adaptations for aquatic life are so extreme that it was impossible to link them to a specific group. The extinct cetaceans known at the time weren’t much help either; the earliest known from relatively complete remains were already fully-aquatic, giving little clues to their terrestrial origins. Their teeth, however, were similar to those of mesonychians, an extinct group of carnivorous ungulates (hoofed mammals) related to modern artiodactyls (even-toed ungulates). With no other obvious candidates, the consensus became that cetaceans very close relatives of mesonychians, if not their direct descendants.

A generic mesonychian. While they looked somewhat like dogs, mesonychians were actually hoofed mammals – veritable sheep in wolves’ clothing.

Things finally started to turn around at the end of the 1970s, when a slew of fossil discoveries from India and Pakistan revealed the earliest terrestrial cetaceans, finally giving us a clear picture of their transition from land to sea. Combined with genetic studies, we can also finally place them on the mammalian family tree. It turns out that cetaceans are actually artiodactyls, and their closest living relatives are hippos! Their earliest ancestors first entered the water some 50 million years ago, and it’s those early pioneers that we will meet next time.

A cladogram showing the relationship between the cetaceans in my project and their modern relatives.

Sources

  • Prothero, Donald R., and Mary Persis Williams. The Princeton Field Guide to Prehistoric Mammals. Princeton University Press, 2017. pp. 149-150.
  • Thewissen, J. G. M. “Hans”. The Walking Whales: From Land to Water in Eight Million Years. University of California Press, 2014, pp. 129–134.
  • University Of California, Berkeley. “UC Berkeley, French Scientists Find Missing Link Between The Whale And Its Closest Relative, The Hippo.” ScienceDaily. ScienceDaily, 7 February 2005. https://www.sciencedaily.com/releases/2005/02/050205103109.htm.