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.
In the news
Two saber-toothed cats, two hunting methods.
In amber: incredible feathers unlike anything we see on birds today.
Tooth chemistry from the famous megalodon shark – was it a warm-blooded hunter?
These two pterosaurs might have been feathered. And that would be a big deal.
Meet the Mosasaurs
But this episode is about the coolest group (in David’s opinion, anyway): the mosasaurs.
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.
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.
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!
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.
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!
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!
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