Once Upon A Time in the Eocene

Living on Earth is a little bit like riding a horse: sometimes it throws you to the ground.

“[A]lmost the entire drive of human history has been an attempt to get as far away from Nature as possible.” — Pratchett and Gaiman, in “Good Omens.” (Image: Nick Damico, CC BY 2.0)

This upset can happen to Earth life from some local event, say, an earthquake or a drought.

Or it may come from harsh complexities that are severe enough to cause global effects up to and including a major mass extinction.

The last time one of those happened was about 66 million years ago, when the non-avian dinosaurs — ones, like T. rex, that weren’t related to birds — as well as many other land and sea animals and plants went away forever.

You have probably heard about the impact of a space rock that occurred around that time. It was more powerful than the blast that would have resulted if every nuclear weapon in the world’s arsenal at the height of the Cold War had gone off at once! (Prothero)

No human actually witnessed it, of course. There are competing hypotheses about what occurred on and after that terrible day.

Geochemists and geophysicists, who know about impact dynamics, sometimes come up with truly hellish global scenarios for what it did to the planet. Paleontologists and other biology experts, however, point out that groups highly vulnerable to those effects — for example, amphibians, insects, crocodilians, and fresh-water fish — made it through the K/T extinction. (Prothero)

And (at least in rare parts of the Northern Hemisphere where there’s a readable fossil record from those times) dinosaurs still ruled the land afterwards. But these early possible apex predators now were avian dinosaurs, a/k/a birds: “terror cranes” of various kinds, almost seven feet tall, with ridiculously small arms but very powerful legs and a huge, nasty beak. (Prothero)

Current thinking is turning towards the idea that these birds might not have been carnivorous.

Still, there they were, and all the rest of it, too. Clearly there was more to that mass extinction than just the Chicxulub impact, large as it was. Many questions about the whole event remain, but I want to talk about something else today: whales.


Because of this recent adorable video of two sea-going mammals — a curious young whale and awestruck human beings:

Wow! Those two mammal groups — cetaceans and primates — took very different paths after the K/T extinction! (More on gray whales here.)

Only there weren’t whales 65 million years ago. They first show up in the rocky archives 11 million years later, as the Eocene epoch began, so it’s difficult to bring them in.

It’s interesting, though:

  • Life “getting back on the horse”after being knocked down for 10 to 12 million years by the K/T extinction.

    Family Felidae, by the way, arrived around the start of the Miocene. (Image: Thanaben via Wikimedia, CC BY-SA 3.0)

    This recovery period is called the Paleocene epoch.

    It was followed by the epoch during which recognizable modern orders like Cetacea (today’s members: whales, dolphins, and porpoises) first appeared — the Eocene, meaning ‘Dawn of the Recent.’

  • Paleocene mammals starting to take advantage of the breaks, despite being small and still having to dodge terror cranes, crocodilians, and other beasts. There were lots of open niches around!

    As they adapted to their new world, mammals tried some options that proved to be keepers, like the even number of hoofs on herbivores called artiodactyls, still seen today in hippos, camels, pigs, cattle, goats, and giraffes (groups with an odd number of hoofs, perissodactyls like the horse, came a bit later).

    Other Paleocene mammal trials — like artiodactyl hoofs, instead of claws, on the rather wolf-like mesonychid predators — worked at the time but eventually died out or were replaced with something else (in the mesonychid example, clawed meat-eating mammals like creodonts and carnivorans took over; only the carnivorans are here now, but creodonts were also successful for millions of years).

    Our primate group, by the way, was around then but lived up in the trees (and resembled lemurs — apes and monkeys weren’t a thing yet). (Prothero) Hoofs and claws didn’t suit that arboreal lifestyle, but flat nails made out of the same material (keratin) on long fingers were very helpful and also have lasted down through geologic time. (Hamrick)

  • Evolution kicking into overdrive, or as it’s formally called, an adaptive radiation, starting in the Paleocene. And as part of that, somehow, a group of mammals forsaking the land for that open marine predator niche (according to Prothero, the biggest underwater predators, after the K/T extinction had wiped out giant marine reptiles, were small sharks).

The why of it seems easy enough — Lots of food! Little to no competition! Plenty of room to roam (a tropical seaway, Tethys, circled the globe in those days)! — but how did they do it?

That they did do it is obvious, but it wasn’t until the late 1990s that fossil evidence started coming in, particularly from Pakistan and northern India, which was about to collide with Asia 54 million years ago, to show the various stages of what is now one of the best documented evolutionary transitions ever.

Once upon a time in the Eocene…

…Some hungry little hoofed animal, living on the shores of long-vanished Tethys when it was a seaway separating India and Asia, in the vicinity of what is now Pakistan, discovered that dead fish are good food and, as part of its learning process, waded out into the water to catch some more.

This little critter couldn’t swim at first but eventually got to paddling around with strong hind legs, like an otter. As time passed and the creature became more streamlined, the hind legs got smaller and smaller and finally couldn’t be seen any more.

The creature was now a whale — an archaic one, but nonetheless a full-on cetacean. (The modern groups first appeared as the world cooled down tens of millions of years later).

Figuring it out

That’s a tale, based on this layperson’s understanding of information written by experts (see source list at end).

From a science perspective, for centuries taxonomists could not agree on where whales fit into the placental mammal group (yes, besides breathing air, whales do nourish the growing calf before birth via a placenta and afterwards with specialized mammary glands).

Experts did realize that whales were descended from artiodactyls — those even-hoofed mammals mentioned earlier. However, it was an open question whether that ancestor was a mesonychid or an herbivore.

The skulls of some early whales resembled those of mesonychids and the most ancient known whale group was also wolf-like, with a tail and long, slender legs. (Prothero)

However, molecular studies pointed instead to the herbivores, with hippos appearing to be the whale’s closest living non-cetacean relative!

Gingerich, in the reference list, sums it all up nicely, and as of 2015, it appears that whales eventually evolved from a common ancestor that wasn’t a mesonychid and which also produced hippos, as well as the cute little deer-like critter shown in the video below.

Life is full of surprises!

Featured image: Andrea Quartarone/Shutterstock


Agustí, J., and Antón, M. 2002. Mammoths, sabertooths, and hominids: 65 million years of mammalian evolution in Europe. New York and Chichester: Columbia University Press.

Bajpai, S.; Thewissen, J. G. M.; and Sahni, A. 2009. The origin and early evolution of whales: macroevolution documented on the Indian Subcontinent. Journal of Biosciences, 34(5): 673-686.

Gingerich, P. D. 2015. Evolution of whales from land to sea. Great Transformations in “Vertebrate Evolution,” KP Dial, N. Shubin, EL Brainerd, Eds.(University of Chicago Press, Chicago, IL, 2015), 239-256.

Hamrick, M. W. 2001. Development and evolution of the mammalian limb: adaptive diversification of nails, hooves, and claws. Evolution & Development, 3:355363. http://onlinelibrary.wiley.com/doi/10.1046/j.1525-142X.2001.01032.x/abstract

Marx, F. G., and Uhen, M. D. 2010. Climate, critters, and cetaceans: Cenozoic drivers of the evolution of modern whales. Science, 327(5968): 993-996.

Prothero, D. R. 2006. After the Dinosaurs: The Age of Mammals. Bloomington and Indianapolis: Indiana University Press. Retrieved from https://play.google.com/store/books/details?id=Qh82IW-HHWAC

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