Since the early 1980s, the story of how whales walked into the sea has become one of the most celebrated of all evolutionary transitions. Pakicetus, Ambulocetus, Rodhocetus, and many, many more—these fossil whales with legs have beautifully demonstrated how land-dwelling mammals became adapted to life at sea. But between 50 million and 40 million years or so ago, whales were just going through a transition that many other vertebrate groups had gone through before. They were not the first vertebrates to return to the sea, nor were they the last, and a paper recently published in Paleobiology by paleontologists Johan Lindgren, Michael Polcyn, and Bruce Young has traced the history of how a very different group of animals got their sea legs.
Mosasaurs were formidable oceanic predators. Take a Komodo Dragon, put flippers on it, and, in some cases, blow it up until it’s over 40 feet long and you’ll have some idea of what these Cretaceous marine lizards were like. Their fossil record—stretching over 27 million years—is also relatively well known, and so the mosasaurs provided Lindgren and colleagues with a good opportunity to see how these peculiar animals evolved.
The first thing you need to know about mosasaur evolution is that the way they swam was constrained by their anatomy. Whales provide a good counterpoint. The ancestors of whales were wolf-like animals which carried their limbs under their bodies, and when they walked, their spines undulated in a vertical plane. That’s why whales swim by beating their tails up and down—their mode of swimming is the product of an anatomical precondition from when their ancestors dwelt on land. The ancestors of mosasaurs, on the other flipper, moved like lizards—that is, their spines were more flexible from side to side. It’s no wonder, then, that mosasaurs swam by beating their tails back and forth, just like fish and that other group of famous marine reptiles, the ichthyosaurs.
So mosasaurs were side-to-side swimmers, and one of the genera taken to represent the early stage of their evolution is Dallasaurus. This was not a gigantic sea monster. Dallasaurus was small—less than three feet long—and it did not have the highly modified tail and flippers of the later, open-ocean mosasaurs. For example, the upper arm elements of Dallasaurus were relatively long, preserving a more archaic anatomical construction, than the shortened upper arm elements which helped keep the flippers stable for their roles as rudders in later mosasaurs. (Similar modifications of the upper arm can be seen in whales, too. The mechanics of swimming provided the selective pressure for parts of these very difference animals to be adapted in similar ways.)
A handful of recent discoveries have helped paleontologists better understand just how much some of the later mosasaurs became modified to life at sea. Mosasaurs have traditionally been reconstructed with long, thin, lizard-like tails. They did not appear to have any specialized tail fins as seen in the shark-like ichthyosaurs. Yet evidence that some mosasaurs had such structures has now been found. Skeletons of Plotosaurus and Platecarpus appear to exhibit downward kinks in the posterior part of the tail which could have supported fleshy tail fins. (Significantly, the part of the tail which supports the tail fin kinks upwards in sharks but downwards in marine reptiles—perhaps as a result of some kind of constraint or contingency.) These mosasaurs may be another case—with ichthyosaurs and crocodiles—of marine reptiles evolving prominent tail fins independently.
Lindgren and co-authors only looked at four representative mosasaur genera—Dallasaurus, Clidastes, Mosasaurus, and Plotosaurus—but together these creatures cover almost the entire span of mosasaur history and provide a rough idea of how the lizards changed through the Cretaceous. As might be expected, earlier mosasaurs lived nearer to shore in shallow environments, whereas later, more specialized forms—such as Plotosaurus—were open ocean cruisers which have been found in deposits indicating deeper environments. The rough picture is similar to that seen among fossil whales—easing into coastal environments and only later spreading far and wide. The same is true of the way the vertebrae of mosasaurs became evolutionarily modified for swimming. In early mosasaurs, the tail vertebrae were more or less the same and unspecialized. By the time of Mosasaurus and Plotosaurus, however, the tail had become divvied up into several different functional regions which enhanced swimming ability.
There is far more detail in the paper, of course—the entire thing runs 25 pages—but what strikes me is how very different vertebrates, even those with different anatomical constrains, eased into the seas in similar ways. Early whales were up-and-down swimmers, while mosasaurs were side-to-side swimmers, yet they both started off in the shallows and underwent a sequence of modification in which their tails became specialized into specific modules suited for swimming. This is wonderful stuff—when contingency, constraint, and convergence meet together in a great transformation.
Top Image: A modern restoration of the mosasaur Platecarpus by Dmitry Bogdanov. Image from Wikipedia.
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