Baby Mosasaurs Were Born Out at Sea

If you want to find sea monsters, there’s hardly a better place to look than western Kansas. Not that you’re going to see any live ones slithering around. They’ve been dead for more than 66 million years. In rocks deposited by a warm, shallow sea that once washed over the middle of North America, paleontologists have pulled dozens of marine reptiles that ruled the marine realm while non-avian dinosaurs stomped around on land and pterosaurs soared through the air. And among the most impressive of all were the great mosasaurs.

Enormous aquatic lizards related to today’s monitor lizards, mosasaurs like Tylosaurus and Platecarpus were totally at home in the sea. Descendants of terrestrial reptiles, their limbs had become modified into flippers, their tails took on a downward kink that supported a vertical tail fluke for more push with each sweep, and their scales became streamlined to help them better cut through the water. The largest of their kind reached lengths of 50 feet. But the discovery of many large mosasaurs from the ancient ocean rock of Kansas raised a prehistoric puzzle. Where were the baby mosasaurs?

Early 20th century marine reptile expert Samuel Wendell Williston proposed a handful of different explanations for the absence of wee sea lizards. Maybe mosasaurs had special pupping grounds that were nearer to the shore than the sites preserved in the Kansas stone, or perhaps the gravid mothers swam up rivers to drop their broods in the relative safety of fresh water. Then again, Wiiliston suggested, perhaps mosasaurs clambered back onto beaches to lay eggs in the way that sea turtles do today.

What Williston didn’t consider was that baby mosasaurs had already been found. At least two were in the collections of the Yale Peabody Museum of Natural History by the time Williston wrote his 1904 report on mosasaur behavior. The trouble was that the little lizards had been mislabeled.

The exact circumstances of when the fossils were collected is unclear. It seems that they were found in the approximately 88 million year old rock of western Kansas sometime in the late 19th century, at the height of the “Bone Wars.” The skull bones were so light and delicate, however, that they were cataloged as “Aves indeterminate” – seemingly fragmentary examples of the toothed birds Yale’s Othniel Charles Marsh described from rocks of about the same age.

Yale paleontologist Daniel Field and his coauthors have now corrected the misattribution. In the pages of Palaeontology, the researchers briefly describe the fossils as baby mosasaurs that probably stretched just a little over two feet long, or about 22% of expected adult size.

The fossils add to the growing body of evidence that – in the Western Interior Seaway, at least – baby mosasaurs were born out in the open ocean. The deposits the youngsters were found in was hundreds of miles from the nearest Cretaceous shore, meaning that they were not hanging out in a nursery. Added to other fossils – such a purportedly pregnant mosasaur that has yet to be formally described and a forerunner of mosasaurs called Carsosaurus found with young inside – the Yale babies suggest that mosasaur mothers gave live birth in open water.

And the new study offers another lesson. During the Bone Wars era and decades of fossil expeditions that followed, many paleontologists were concerned with acquiring the biggest and most impressive fossils. The grand halls of eastern museums needed petrified celebrities to fill their scientific trophy rooms. Little fossils were either ignored in the field or, if collected, were given little interest in the lab. But now it seems that “missing” parts of prehistoric creature’s lifecycles and many hitherto unknown species are being recovered amongst the meeker fossils within museum collections. We know about the giants. Now it’s time to learn the stories of the small.


Field, D., Leblanc, A., Gau, A., Behlke, A. 2015. Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs. Palaeontology. doi: 10.1111/pala.12165

3 thoughts on “Baby Mosasaurs Were Born Out at Sea

  1. So mosasaurs finally get to join the other great sea reptiles in the live birth club, eh? That’s pretty cool!

  2. Any thoughts on why returned-to-water reptiles developed fishlike side-to-side tails while mammal flukes go up/down?

  3. p.a. – It’s generally thought that it reflects the way the precursor animals moved on land. Running mammals tend to flex their spines vertically, whereas running reptiles flex their spines horizontally because of the angle their legs are attached to their body. So an early aquatic reptile would tend to swim with a horizontal tail stroke, making a vertical fluke more useful, whereas a proto-whale would swim with a vertical tail stroke which would be enhanced by horizontal flukes.

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