clidastes

Medium-size mosasurs, swimming lizards that lived at the time of dinosaurs, in comparison with a modern-day person. From top to bottom, they are:
Platecarpus, from the Late Cretaceous of the United States,
Clidastes, from the Late Cretaceous of the United States and Europe,
Plioplatecarpus, possibly a deep-diving hunter with large eyes, from the Late Cretaceous of the United States and Europe,
Halisaurus, a small-bodied form from the Late Cretaceous of the United States.

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Fossil Shows Prehistoric Reptile Gave Birth in Open Ocean

by Michelle Z. Donahue

Collected in Kansas in the late 1800s from the famous Niobrara Formation, the baby mosasaur bones strongly resemble the jaws of unrelated toothed birds in size and shape, and were found in an area where other fossilized birds had been collected. They’d probably been mistaken as bird bones simply due to their size.

Daniel Field’s expertise with birds and prior experience with mosasaurs enabled him to identify a distinguishing bulge on the jawbone samples, a structure absent in prehistoric toothed birds. The jawbones belong to a species of mosasaur called Clidastes. Adult specimens have been found 9 feet in length, but these jawbones came from babies no more than 12 inches long.

Fossils of baby mosasaurs have been found before, but all were etched and pitted as if by acid, indicating they’d been transported to their final resting places in the belly of a predator. The Yale bones showed no such evidence, meaning they’d died of other causes. Field’s discovery fills a puzzling gap in the fossil record…

(read more: Smithsonian Magazine)

illustrations: Julius Csotonyi and Daniel Field