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Mosasaur: How a reptile came to dominate the seas

Mosasaur: A new study finds how the mosasaur came to reign the oceans some 90 million years ago.

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"There is no evidence I have seen for an upper tail lobe that would make them more ichthyosaur-like than we currently envision them to be," Everhart said. "We know they were well adapted to living in the ocean. … They basically took over the ocean."

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The rise to the top

The changes in mosasaurs' bodies were crucial to their rise to the top of the late Cretaceous marine food chain, according to Lindgren.

Dallasaurus was small, roughly 5 feet (1.5 meters) long, and swam like an eel or sea snake, its spine curving like a sine wave to either side. This type of swimming works for ambush predators, because it allows them a burst of speed, but it's not useful over sustained periods.

Mosasaurs' anatomical changes added new efficiency to their swimming by allowing them to use just part of their body, the tail, to propel them through the water. This allowed them to chase down their prey.

Mosasaurs' ancestors were most likely similar to marine iguanas of the modern Galapagos Islands, land animals that went into the ocean to feed, according to Everhart. They gave rise to mosasaurs roughly 90 million years ago in waters already dominated by sharks, capable of feeding on whatever they wanted, including mosasaurs.

Within a few million years, mosasaurs got larger, and the roughly 22-foot (6.5- to 7- meter) ginsu shark, Cretoxyrhina mantelli, disappeared. There is no smoking gun, Everhart writes on his website, Oceans of Kansas Paleontology, but based on modern sharks' vulnerability to fishing, it is possible that mosasaurs, which grew as large as 56 feet (17 meters), may have eaten young ginsu sharks, and the population was unable to recover.

Their reign didn't last, however. Mosasaurs died out with the dinosaurs during the Cretaceous-Tertiary extinction 65.5 million years ago.

The study appears in the Summer 2011 issue of the journal Paleobiology.

You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry.

[Editor's note: The headline on the original version mischaracterized the mosasaur as a dinosaur. It was a marine reptile.]

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