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How the 60-Foot-Long 'Sea Rex' Gave Birth

A new study reveals that mosasaurs gave birth in the wide open ocean.
​Restoration of Aigialosaurus bucchichi, a basal mosasaur. Image: FunkMonk (Michael B.H.)

​When we think of the late Cretaceous period, the first giant predator that comes to mind is the resplendent Tyrannosaurus rex—a quintessential hunter on land. But the oceans of the Cretaceous were also home to enormous carnivores, the most famous of which is the Mosasaur, which is sometimes referred to as a "Sea Rex."

Mosasaurs were an incredibly diverse family of marine reptiles, with species ranging in length from one meter to a hulking 18 meters (about 59 feet). They completely dominated the oceans from around 85 million to 65 million years ago, becoming so successful as a group that their fossils have been found on every single continent on Earth.

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But despite the abundance of mosasaur fossils, very little is known about how they gave birth or raised their young due to the lack of juvenile fossils. All that changed on Friday when a team of paleontologists published a study in the journal Palaeontology describing the rare remains of two newborn mosasaurs.

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Interestingly enough, the fossils of these two baby Sea Rexes were probably excavated well over a hundred years ago, but were miscategorized with the label "Aves indeterminate."

Accordingly, the fossils were passed off as being cranial remnants of birds, and it wasn't until they were recently re-examined that they were identified as extremely rare remains of neonatal mosasaurs.

As the authors of the paper note, the "superficial similarity" between the jaws of Cretaceous sea birds and small mosasaurs has "led to multiple instances of taxonomic confusion." It goes to show that sifting through existing paleontological collections is as promising a route to discovery as fossil hunting in the field.

But what's even more exciting is that these fossils were excavated in Kansas's Niobrara Formation, which provides a snapshot of a thriving pelagic—or open ocean—ecosystem. Because the specimens originate from this bed, it suggests that mosasaurs gave birth in the wide open sea, and not in shallow water clutches near the shore, as some scientists have suggested.

"The recovery of these extremely young specimens from a pelagic setting indicates that even neonatal mosasaurs occupied open oceanic habitats and were likely born in this setting," the study's authors write.

"These new specimens bolster the existing evidence for mosasaur viviparity and provide some of the first direct evidence in support of a pelagic ecology for flipper-bearing, neonatal mosasaurs," the team adds.

Mosasaurs are enjoying a bit of a cultural renaissance at the moment in the wake of the documentary film Sea Rex. It looks as if the formidable swimmer has even nabbed a role in Jurassic World, which comes out on June 12 this year.

It's easy to understand why there is waxing public captivation with these magnificent carnivores, which commanded the Cretaceous seas until they were wiped out alongside the dinosaurs 65 million years ago.

But as the authors of the new study have demonstrated, there was a gentle, maternal side to mosasaurs. A 60-foot-long torpedo with razor sharp teeth is inherently intimidating, but even the mighty Sea Rex had a soft spot for its offspring.