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 of Tyrannosaurus 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.

In sharks and ichthyosaurs, the backbone extends into one of the tail lobes, and based on what he saw in the fossils and the living animals, Lindgren believes the same structure — a two-lobed, crescent tail — evolved in mosasaurs over time.

His research also documented other changes: The tail became regionalized, with sections of vertebrae adapting to serve a particular purpose, becoming more robust at the base of the tail to anchor it, for example. Other changes, including the shortening of the bodies of the vertebrae, made the tail more powerful and less flexible. In addition, their extremities became less like feet and more like paddles.

The tail would've resembled that on whales, sharks and some ichthyosaurs — another fishlike marine reptile that disappeared from the Cretaceous seas as mosasaurs arrived, according to Lindgren and his colleagues.

Mike Everhart, adjunct curator of paleontology at the Sternberg Museum of Natural History in Hays, Kan., said the study added detail to what was already known about mosasaurs' adaptation, but he did not entirely agree with Lindgren's conclusion.

"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."