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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.

A 65-million-year-old Plesiotylosaurus cressidens, a mosasaur reptile found in California, is suspended from the ceiling of the Dinosaur Hall at the Natural History Museum of Los Angeles County, Exposition Park.


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At a time when dinosaurs ruled the land, mosasaurs, a type of swimming reptile related to modern Komodo dragons, came to dominate the seas. Within the span of roughly 27 million years, these predators transformed from an animal with limited swimming ability and limbs still meant for walking into a sleek, fishlike form.

Now, a new study reveals the evolutionary details behind this transformation, which turned the mosasaurs into swimming machines and fearsome predators, the marine equivalent ofTyrannosaurus rex, that may even have decimated the large ginsu sharks of the time. [T.-Rex of the Seas: A Mosasaur Gallery]

Since the discovery of the first mosasaur in the late 18th century, they have been generally depicted as slender, serpentine animals with narrow, straight tails, like that of the modern sea snake, said Johan Lindgren, the lead researcher and a paleontologist at Lund University. While mosasaurs appear to have started out this way after their ancestors first arrived in coastal waters, they did not keep this form.

A mosasaur tale

Lindgren and his colleagues charted anatomical changes in fossils from the tails of four types of mosasaurs at different stages of adaptation to their ocean life, from the small Dallasaurus, still largely built for life on land, to Plotosaurus, which had ridges on its small scales to channel water and a dolphin-shaped body, according to Lindgren.

They also looked at modern animals — lizards, sea snakes and sharks. While mosasaur fossils have been found around the world, preserved soft tissue from their tails is virtually unknown, so modern animals helped the researchers fill in the gaps.


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