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Dino-Era Lizard Is Missing Link to Swimming Reptiles, Experts Say

Stefan Lovgren
for National Geographic News
November 21, 2005
 
A lizard whose fossilized bones were discovered near Dallas, Texas, 16 years ago is a missing link in the evolution of extinct swimming reptiles known as mosasaurs, a new study says.

The ancient lizard, named Dallasaurus turneri, measured three feet (about a meter) long and lived 92 million years ago in shallow seas that covered what is now Texas.

As an early mosasaur, Dallasaurus is unusual, because it had tiny feet and hands suitable for walking on land. Later mosasaurs developed fin-like limbs and came to dominate the seas at the time when dinosaurs ruled the land.

"This study paints a much more complex picture of the evolution of [mosasaurs] than previously thought," said Michael Polcyn, a paleontologist at Southern Methodist University in Dallas.

Polcyn led the research, which is reported this month in the Netherlands Journal of Geosciences.

Reconstructing Anatomy

Van Turner, an amateur fossil hunter, found the Dallasaurus fossils in 1989 while searching through dirt turned up by bulldozers at a construction site near Dallas.

Remains of early mosasaurs have been difficult to find. They are only found in areas once covered by water and are quick to deteriorate.

This is the first well-preserved early mosasaur found in North America. Only five primitive specimens have been found before, all of them in the Middle East and along the Adriatic Sea.

"To have this discovery in our own backyard is a terrific find," said Anthony Fiorillo, curator of the Dallas Museum of Natural History and an SMU paleontology professor.

The discovery included more than 100 identifiable skeletal pieces, composing about 80 percent of the animal.

Polcyn, who heads SMU's visualization laboratory, used computers to simulate what Dallasaurus looked like and how it swam and moved from land to sea.

"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 descendants—a 30-foot (9-meter) mosasaur with sharp teeth and a massive jaw that lived some 75 million years ago.

"Huge Gap"

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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