It’s been awhile since we’ve had a good, solid transitional fossil. Transitional fossils are fossils that show a combination of primitive or plesiomorphic traits, along with newer, derived traits, which illustrate the progression of evolution in the fossil record. The last good transitional fossil we showed off here was the famous Archaeopteryx. But my favorite transitional fossil was from the Devonian Period, the Paleozoic Medal of Honor recipient, the indomitable Tiktaalik. Tiktaalik was a pioneer in learning to breathe air and using his flippers and brand-new wrists to make some strides on land for the first time. This changed everything.
So if an ancient fish-tetrapod intermediate worked so painfully hard to make it possible for vertebrates to live on land, why would any vertebrate ever go back to the water?
For some, it was an easier opportunity to catch horses.
CENOZOIC ERA, PALEOGENE PERIOD. EOCENE PERIOD
56 – 34 MYA:
Ambulocetus means “walking whale”, and he possesses a variety of both plesiomorphic traits of terrestrial mammals and derived traits of modern marine mammals. Some of his plesiomorphic traits you can obviously see for yourself- he’s got legs, for one. He also has a nose, rather than a blowhole. Other evidence that Ambulocetus had terrestrial origins are still present in modern whales, such as a complete set of wrist bones and bones of the hands and feet. Unlike whales though, Ambulocetus’s wrists were visible on the outside. Also, the alignment of Ambulocetus’s spine only allows vertical undulation. All fish, salamanders, lizards and snakes (if you drop one into water) move through the water by undulating their bodies sideways, whereas all marine mammals- including seals, whales, mermaids, and dolphins move through the water by arching their body up and down. If whales had never once been terrestrial, their spines would be configured more like fish since the vertical spine articulation was an adaptation for terrestrial life.
The more derived traits that made Ambulocetus well-adapted for aquatic life include his lack of external ears and his periotic bones, similar to those of whales, that help him hear underwater. He can walk around on land but he’s more agile in the water, since his legs aren’t well-suited to running fast or climbing. However, he lacks a few adapations found in true whales that make him not quite ready for life at sea. Instead, Ambulocetus preferred to wander the wetlands and hunt in a fashion similar to that of a crocodile- that is, he would hover around shallow water and wait for an unsuspecting mammal, such as a horse, to come along for a drink. Much like the Paleocene epoch, the Eocene epoch was not a good time to be a horse. They were still small and insignificant back then.
This seems like a decent way for an early carnivorous mammal to make a living, but it doesn’t explain why Ambulocetus evolved to aquatic life and returned to the water after his ancestors had been living on land since the Paleozoic era. What happened was that at the end of the Paleocene epoch, the earth experienced one of the most rapid and extreme global warming events in history (except maybe for the one happening right now- we’ll see), called the Paleocene-Eocene Thermal Maximum. The earth’s temperature shot up by 6°C (11°F) in just 20,000 years, with a number of results. Of course, as in every point in natural history when the earth underwent major changes, there were extinctions, which resulted in new ecological niches to fill.
The water levels rose all over the earth, which was nearly uniformly warm. Temperate forests extended all the way to the poles and the tropical regions extended to 45 degrees latitude- along the northern Vermont/Canadian border. The earth now had far more forests and swamps than it used to. This is bad news for animals that preferred dry, open grasslands with large stretches to chase down prey- the terror bird survived, but his habitat was reduced and he was no longer the uncontested apex predator. By this time there were carnivorous mammals on land- we’ll meet one next- but the strategy of Ambulocetus was to exploit the new niche that could be found hunting along the shores and shallows.
This niche was already occupied by the crocodile, of course. That guy never goes anywhere. But with increasing competition for hunting on dry land, and with the new abundance of wetlands, there was plenty of room for both Ambulocetus and crocodiles to fill the same role without running into eachother and competing. Good news for Ambulocetus. Bad news for horses.('’) delicious