Episode 51 – Mosasaurs

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The Mesozoic Era was home to many successful marine reptiles, but the best ones (that is to say, David’s favorites) were the carnivorous, fully-aquatic, extremely successful lizards of the sea: the Mosasaurs.

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Meet the Mosasaurs

The seas of the Mesozoic Era were home to a diversity of reptiles: sea turtles, aquatic crocs, sea snakes, porpoise-shaped ichthyosaurs, four-flippered plesiosaurs, and more.

But this episode is about the coolest group (in David’s opinion, anyway): the mosasaurs.

The well-known mosasaur, Clidastes, in the flesh (art by Dmitry Bogdanov) and out of it (photo by Greygirlbeast). Images from Wikimedia Commons.

Mosasaurs were lizards – actual, true lizards – fully adapted to an aquatic lifestyle. Their arms and legs had evolved into flippers, with long hand and foot bones forming large paddles. They used these flippers for steering while propelling themselves with very long tails ending in vertical fins.

This entire group – as far as we can tell – were carnivores. Like many lizards, they had flexible skulls that allowed them to bend their jaws around large prey, and the rows of sharp teeth on the roof of their mouths meant that once something tasty ended up in those jaws, it wasn’t getting back out.

Like many marine tetrapods, mosasaurs developed flippers with short arm bones and very long hand bones. Photo by Wilson44691

Decades of study have taught us some remarkable things about the lives of mosasaurs: they were probably warm-blooded, like many marine reptiles; they gave live birth out in the open ocean; and they swam with hypocercal tails (that means the lower “lobe” of the tail fin was larger than the upper lobe).

Unlike ichthyosaurs and plesiosaurs, which evolved back in the Triassic Period, mosasaurs were only around in the Late Cretaceous for about 25 million years. But in that time, they achieved a global distribution, from the Western Interior Seaway of North America to the Antarctic Peninsula.

Some mosasaurs, like this Platecarpus, are known from such exquisite specimens that we can confidently reconstruct them in great detail. Top: photo from Lindgren et al 2010. Bottom: art by Dmitry Bogdanov. Images from Wikimedia Commons.

Mosasaur Evolution

Mosasaurs are lizards, but their place on the lizard family tree has been debated for a long time. Generally, paleontologists agree that they are close relatives of monitor lizards, beaded lizards, and snakes. And indeed, mosasaurs look a lot like monitor lizards, particularly in the skull!

Even more closely related were the extinct aigialosaurs. This was a group of aquatic lizards that was around for a time in the Late Cretaceous, and probably gave rise to true mosasaurs. There’s even evidence that aquatic adaptations might have evolved multiple times in mosasaurs!

Aigialosaurus and its kin were aquatic lizards, but not adapted so extremely as mosasaurs. They were probably close relatives or ancestors of mosasaurs. Art by FunkMonk via Wikimedia Commons.

A World of Mosasaurs

In their short time on Earth, mosasaurs achieved an incredible diversity.

Mosasaurs like Platecarpus were rather sea serpent-shaped, while others like Plotosaurus evolved a body plan a lot like an ichthyosaur. Small mosasaurs like Clidastes grew “only” 3 or 4 meters (10-15 feet) long, while the largest species of Mosasaurus and Tylosaurus, at around 15 meters (50 feet), were some of the largest marine reptiles of all time.

Tylosaurus proriger was one of the largest mosasaurs – and indeed one of the largest animals – of all time. Top: Photo by MCDinosaurhunter. Bottom: image from Levi bernardo. Images from Wikimedia Commons.

From shallow seas to open oceans to freshwater, mosasaurs ate a wide variety of foods. Fossilized gut contents of mosasaurs have included the remains of fish, sharks, birds, turtles, plesiosaurs, and even other mosasaurs. Some mosasaurs were specialists, like Globidens, whose ball-shaped teeth were adapted for a durophagous diet (that is, hard foods like shells), and which has been found with crushed-up clams in its stomach!

Left: Most mosasaurs, like Hainosaurus, were probably generalists that fed on all sorts of soft, swimming prey. Art by Dmitry Bogdanov. Right: Globidens was a specialist, feeding on hard-shelled food, as evidenced by its ridiculous ball-teeth. Photo by Smokeybjb. Images from Wikimedia Commons.

More Mosasaurs

Mike Everhart’s website, Oceans of Kansas, is loaded with great mosasaur information: non-technical and technical alike.

Some technical resources:
Lindgren et al. 2010 describes that incredible Platecarpus specimen.
Field et al. 2015 presents research on mosasaur reproduction.
Simões et al. 2017 discusses the oft-debated evolution of mosasaurs.

This episode’s Patron question came from Lydia. Thanks, Lydia, for giving us something interesting to think about!

If you enjoyed this topic and want more like it, check out these related episodes:

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