"A comparison I suppose could be made to monitor lizards and komodo dragons," Polcyn said. "Those animals are on the land most of the time but swim once in a while. They may be the closest thing [to Dallasaurus] living today."
The Dallas Museum of Natural History now has on display a life-size model of Dallasaurus, which sits next to a model of one of its descendantsa 30-foot (9-meter) mosasaur with sharp teeth and a massive jaw that lived some 75 million years ago.
Land-based lizards, which have a nearly 150 million-year-long history, were the marine reptiles' ancestors.
Mosasaurs moved into the ocean about 90 million years ago, a time of hot-house temperatures and rising sea levels, and developed skills for life in the water.
"You have an explosion of evolution occurring," Polcyn said.
Mosasaurs eventually evolved into the top predator of their domain before becoming extinct some 65 million years ago, at the same time the dinosaurs disappeared.
Dallasaurus represents a split near the beginning of the mosasaur family tree. Later mosasaurs have been grouped into three major lineages; the new study shows that Dallasaurus forms the basis for one of those three later forms.
It is the first time that paleontologists have been able to show that mosasaurs evolved fins from limbs within a single lineage.
"It fills in a huge gap in our understanding of the early evolution of a very dominant group of marine reptiles, the mosasaurs," said Fiorillo, of the Dallas Museum of Natural History.
Later mosasaurs grew as large as their dinosaur brethren, some reaching up to 45 feet (14 meters) in length. The Dallasaurus lineage later produced the prognathodon, a fearsome marine reptile that Polcyn calls "the T. Rex of the sea."
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