Archives of The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 33, Number 5;   May, 2006

How "Dinosaurs of the Ocean" Evolved

When amateur fossil finder Van Turner discovered a small bit of an animal backbone at a construction site near in Texas 16 years ago, he knew the creature was unusual.

Scientists say Turner's find sheds light on a group of lizards that moved into the sea and became top predators during the dinosaur era -- in a sense, dinosaurs of the ocean.

The researchers describe Turner's fossil, dubbed Dallasaurus turneri in his honor, as a specimen of one of the first of these animals, called mosasaurs.

"Lizards had nearly 150 million-year-long history on land; then in the Late Cretaceous, the final stage of the age of dinosaurs, one group moved into the sea and rose to the very top of the food chain," said Michael Polcyn of Southern Methodist University in Dallas, Texas.

"Starting out as small animals like Dallasaurus, they mastered their new marine environment and rose to become the top predator in their ecosystem, the T. rex of the ocean."

Mosasaurs, every bit as prolific, fascinating and nearly as big as some dinosaurs, are becoming more popular for paleontologists to study, according to Polcyn and his colleagues. Some later mosasaurs grew as large as their dinosaur brethren, reaching up to 45 feet (14 m) in length.

They lived and died out alongside dinosaurs, but few paleontologists specialize in them.

Dallasaurus represents a missing link in mosasaur evolution, Polcyn said, because research shows it retained limbs, hands and feet suitable for land walking. Later mosasaurs evolved their limbs into flippers.

Thus, Dallasaurus "is pretty close to the beginning of the mosasaur family tree," said Anthony Fiorillo of the Dallas Museum of Natural History and Southern Methodist University.

Similar primitive mosasaurs had been found before, but all of them in the area of the Middle East, he said, none in North America.

Polcyn and Gordon Bell Jr. of Guadalupe National Park in Texas describe new research on Dallasaurus in a special issue of the Netherlands Journal of Geosciences.

Dallasaurus, a three-foot (91 cm) long lizard, lived 92 million years ago in shallow seas and shores of what was then a stretch of Texas mostly under water, they said. The authors pieced together an account of Dallasaurus history from Turner's fossils and some matching bones at the Texas Memorial Museum at the University of Texas in Austin.

The Late Cretaceous period was a time of hot temperatures and rising seas, the researchers said.

"As the earth warmed and the seas rose, small land-dwelling lizards took to the oceans," Polcyn explained. It took them around 30 million years to reach the summit of the food chain, he added.

Advanced fin-bearing mosasaurs have been grouped into three major lineages. Dallasaurus represents a clear link to one of these, and the first time researchers can clearly show mosasaurs evolved fins from limbs within the different mosasaur lineages, Polcyn contends.

Polcyn produced computer simulations of what Dallasaurus might have looked like, and how he would swim and move from land to sea. An artist took this work further by creating a life-sized model of Dallasaurus, to go on display at the museum.

Major dinosaur finds are often the result of creatures dying in groups through flooding or drought. Such situations lend themselves to better preservation of their bones, and slower deterioration. Mosasaur fossils, in contrast, are rarely found in large groupings, the scientists said, so they are rare.

"Not all major discoveries are made by highly trained paleontologists," noted Fiorillo. "The observant individual, even kids, can still make an important find." He added that as more people "begin to recognize what mosasaurs are, we'll be finding more and more."