Mosasaurs were born at sea, not in safe harbors

From Norman, Okla., at a meeting of the Society of Vertebrate Paleontology

Mosasaurs were prehistoric aquatic reptiles that cruised the world’s oceans for 30 million years or so before they and the dinosaurs on land went extinct, about 65 million years ago. Newly discovered mosasaur fossils suggest that the creatures gave birth in midocean, countering previous theories that their first days were spent on or near shore.

The evidence comes from expansive chalk deposits in Kansas that have yielded fossils of at least 10 different mosasaur species since the late 1860s, says Michael J. Everhart, a paleontologist at Fort Hays State University in Hays, Kan. Large numbers of mosasaurs, which grew up to 15 meters long, inhabited a broad, shallow seaway that covered much of what is now North America’s Great Plains, says Everhart. The fossil-bearing chalk deposits were laid down as ocean-floor ooze hundreds of kilometers from the waterway’s shores.

Because the Kansas chalk seemed to be devoid of juvenile mosasaurs, paleontologists early in the 20th century proposed that the reptiles either laid eggs on land or gave birth to their young near shore to protect them from predators. As it turns out, Everhart has found, the telltale fossils of juveniles have been there all along but were previously unrecognized or ignored. That’s because these remains are fragmentary, partially digested morsels that had been coughed up by large sharks, predatory fish, or cannibalistic adult mosasaurs.

The newfound fossils are small bits of bone and skull that probably came from young mosasaurs no more than 2 m long. These fossils, plus the mid-1990s discovery that female mosasaur fossils bore live young, strongly suggest that the lifestyles of these aquatic reptiles may never have included excursions onto terra firma.

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