Episode 502: Back to the Ocean

Show transcript: Welcome to Strange Animals Podcast. I’m your host, Kate Shaw. Life arose in the oceans and only much later spread to the land. But some land animals returned to the ocean and eventually became fully marine animals. That’s our topic this month, animals that returned to the sea. Vertebrates that evolved on land are called tetrapods, meaning four-footed. It applies even to animals like birds that have two legs and two wings, and snakes that have no legs, because all the ancestors of tetrapods had four legs. Returning to marine life as a tetrapod eventually results in an animal developing certain characteristics. Even animals like sea turtles become more streamlined, which helps the animal swim more easily, and arms and legs develop into flippers for the same reason. Sometimes limbs are lost completely, like the rear legs of cetaceans and sirenians, because they’re not needed. One interesting note is that as far as we know, all marine tetrapods still have to breathe air, which requires them to come to the surface of the water periodically. The ancestors of marine tetrapods probably all started out as partially aquatic and gradually became more and more at home in the water. That’s true even for animals as well adapted to marine life as whales. Some of the earliest whale ancestors were the ambulocetids, which probably looked sort of like a cross between a hippopotamus and a crocodile. Ambulocetid means “walking whale,” but there’s some evidence that it might have already evolved to be fully aquatic 49 million years ago. It had short legs that might not have been strong enough to support it out of water. It might have been covered with short fur like a seal but it might have had little to no fur, more like a hippopotamus. Either way, it had long toothy jaws, strong legs with webbed toes that it used to propel itself through the water, and a tail that probably resembled that of an otter. It grew about ten feet long, or 3 meters. Ambulocetids lived in shallow coastal areas where the land was swampy, and it may have spent part of its time in fresh water. It wouldn’t have looked or acted much like a whale to us, but it was already developing features now found only in whales. By around 41 million years ago, the basilosaurids and their close relations had evolved, and were definitely fully aquatic. Their nostrils had moved almost to the location of modern whales’ blowholes. Their forelegs were basically flippers with little fingers, their hind legs had almost disappeared, and they had tail flukes. They were also much bigger than their ancestors. Basilosaurus could grow up to 60 feet long, or 18 meters, and probably looked more like a gigantic eel than a modern whale. It was long and relatively thin, and may have mostly lived at the ocean’s surface instead of diving for food. Dakosaurus wasn’t a dinosaur despite its name, although it lived at the same time as the dinosaurs, about 145 million years ago. It was a crocodylomorph that grew up to about 16 feet long, or 5 meters, and looked kind of like a shark with legs—but also a lot like a crocodile wearing a shark costume. Its tail had a fin near the end that made it resemble a shark’s tail, and its legs were flipper-like but still obviously legs. It had a whole lot of big serrated teeth in a deep skull with strong jaws. Because its jaws and teeth were so strong, in fact, scientists think Dakosaurus probably ate other marine reptiles instead of just fish. While Dakosaurus was streamlined and well adapted to life in the ocean, we don’t know if it left the water to lay its eggs on land the way sea turtles still do or if it gave birth in the water. We don’t have any nest sites or young specimens one way or another. Other marine reptiles definitely gave birth in the water, including ichthyosaurs. Ichthyosaurs and their close relations were incredibly successful, first appearing in the fossil record around 250 million years ago and last appearing around 90 million years ago. Most ichthyosaurs grew around 6 and a half to 11 feet long, or 2 to 3.3 meters, depending on species, so while they were pretty big animals, they weren’t enormous. They would have been fast, though, and looked a lot like fish or dolphins, with four flippers, a dorsal fin, a long tail with fins, and a long thin rostrum with lots of little teeth. We know they gave birth to live young because we have fossils of female ichthyosaurs who died in the process of giving birth, with the baby’s tail outside of its mother but the rest of it still in the birth canal. We have other fossils of ichthyosaurs with partially developed young in what would have been the mother’s uterus. We also have lots of ichthyosaur coprolites that show us what it ate, since the coprolites fossilized with pieces of undigested food in them. This includes fish scales and squid beaks. Of course, if you want to talk a