Mosasaurs weren’t born on a beach after all

It’s fair to say that most people probably wouldn’t confuse a bird skull with that of a 50 foot sea monster from the late Cretaceous period. A Mosasuarus, an apex predator living in the middle of the Western Interior Sea should clearly stand out from a bird fossil, right? It turns out that this wasn’t the case, largely thanks to some erroneous expectations about how mosasaurs were born.

Mosasaurs were marine reptiles, living in the ocean but breathing air like a whale or sea turtle. With that staring point it was assumed that, like a modern sea turtle, mosasaurs must have occasionally scooted up onto a beach to lay eggs. No eggs had been found, but there was nothing really disputing that pattern.

Skulls hidden under erroneous labels

This hypothesis began unraveling when Aaron LeBlanc of the University of Toronto was re-examining some supposed bird skulls that had been discovered and cataloged 100 yeas ago. The small, toothed mouth fragments were fairly beak-like, and were labeled as primitive birds. But the realization that they were baby mosasaur skulls, and the fact that they were found in what would have been the middle of the sea, instead of near an egg-appropriate shoreline, sparked a reevaluation of the marine reptiles’ births.

A more logical birth-plan for a mosasaur

All the pieces fit together much more easily if mosasaurs gave live birth, similar to a modern whale. As in a whale, the hip bones remaining from their terrestrial, tetrapod ancestors were not attached by any solid bone, and thus could not have borne weight out of the water. Combined with the depth of their rib-cage and vertical tail fluke, a live birth out in the ocean avoids all of the mechanical issues that would have arisen if a 50 foot-long, fish-shaped animal tried to heave itself up on dry land to lay eggs. And egg-laying isn’t universal among modern reptiles either, making it all the easier to see how this could have evolved in the Mesozoic as well.

My kindergartner asked: Did they find one skull labeled as a bird, or more than one? They found more than one, which were all collected in a frantic period called the Bone Wars, where paleontologists were hastily rushing to collect as many flashy specimens as possible. These skulls may just be the first correction to be made, and so further checks are being made into collections for other mislabeled animals.

Source: Gigantic Marine Lizards Had Live Births by Bob McDonald, Quirks & Quarks