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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 about big marine reptiles, you have to talk about mosasaurs. They lived in the late Cretaceous and went extinct at the same time as the non-avian dinosaurs. The earlier species of mosasaur are pretty small, but after ichthyosaurs went extinct, mosasaurs got really really big. Some estimates for the largest species range up to almost 50 feet long, or 15 meters, and maybe even longer. Mosasaurs looked kind of like streamlined iguanas, with four flippers, a shark-like tail, but a very lizard-like head. They probably ate other marine reptiles, including smaller species of mosasaur, along with sharks and other fish, dinosaurs that made the mistake of getting in the water, and sea turtles. Honestly, mosasaurs could eat whatever they could catch, and they could probably catch just about anything. Their jaws were double-hinged, which allowed them to swallow prey whole like snakes do.
We even have some skin impressions of mosasaurs that show smooth scales similar to those of snakes, and one study suggests that mosasaurs might have had forked tongues too. Mosasaurs were distantly related to snakes, but they were more closely related to monitor lizards that also have forked tongues.
Speaking of snakes, we’ll finish with an animal that people don’t usually remember when they think of animals that have returned to the ocean: sea snakes.
Not all sea snakes are fully marine. One genus spends part of its time on land and still lays eggs, but fully marine sea snakes give birth to live young. Researchers aren’t sure if the sea snakes that are fully marine were able to completely stop returning to the land because they already gave birth to live young or if that developed after the snakes started spending part of the time in the water.
The sea snake has a single lung, like most snakes, but unlike snakes that live on land this lung extends almost the length of the whole body (but not the tail). The snake’s nostrils are at the top of its snout so it can breathe without sticking too much of its head above water, and its extra-long lung can store lots of air so the snake doesn’t have to breathe too often. Some sea snakes can stay underwater for up to about two hours, partly because of the air stored in the lung, but also because they can absorb some oxygen from the water through their skin. Studies have shown that a sea snake can absorb almost 25% of the oxygen it needs through its skin.
Sea snakes are pretty big, with the largest growing up to 10 feet long, or 3 meters. Most are about half that length. They live in warm tropical oceans, usually in shallow water, and eat fish and other small animals. The sea snake’s body is slightly flattened side to side and its tail is much more flattened like a paddle, which is how it swims. In most cases, if a sea snake is washed ashore, it’s unable to crawl and is pretty much helpless. Terrestrial snakes have enlarged ventral scales that the snake can move up and down, allowing the edges of the scales to grab dirt or leaves or whatever the snake is moving over. This gives it a purchase, and as the scales move they push the body forward. Most sea snakes don’t have these specialized ventral scales and have regular smooth scales instead.
But while most sea snakes can’t move on land, if you see one stranded, don’t try to pick it up to help it. All sea snakes are extremely venomous. (Use a long paddle to push the sea snake back into the water, or better yet, call an expert who can safely return the snake to the water.) Most sea snakes are brightly colored as a result, to warn potential predators away. Many have stripes and are absolutely gorgeous.
Generally, if you encounter a sea snake while you’re swimming, the snake isn’t going to bite you. You’re too big for it to swallow and it doesn’t want to waste its venom on you. Don’t bother it and it won’t bother you unless it feels threatened. Just swim calmly away and be glad it’s not a mosasaur.
Thanks for your support, and thanks for listening!