When amateur fossil hunter Van Turner discovered a small vertebra at a construction site near Dallas 16 years ago, he knew the creature was unlike anything in the fossil record.
Scientists now know the significance of Turner's fossil as the origin of an extinct line of lizards with an evolutionary twist: a land-dwelling species that became fully aquatic.
Turner took the remains to paleontologists at the Dallas Museum of Natural History, but it took several years before scientists dubbed the find Dallasaurus turneri.
The creature is just now being described publically in a special issue of the Netherlands Journal of Geosciences by Southern Methodist University paleontologist Michael Polcyn and Gordon Bell Jr. of Guadalupe National Park in Texas.
Dallasaurus was a 3-foot lizard that lived 92 million years ago in shallow seas and shores of what is now Texas. It is a missing link in the evolution of a group of creatures called mosasaurs, prehistoric animals that started out on land, but evolved in the seas and dominated the oceans at the same time dinosaurs ruled the land.
Dallasaurus retained complete limbs, hands and feet suitable for walking on land, whereas later mosasaurs evolved their limbs into flippers, the new study reports.
"This is pretty close to the beginning of the mosasaur family tree," says Dallas Museum of Natural History Earth Sciences Curator Anthony Fiorillo. "It is the most complete mosasaur retaining all of its limbs found in North America."
Mosasaurs lived and became extinct alongside dinosaurs. Later mosasaurs grew up to 45 feet in length. Until the discovery of Dallasaurus, however, only five primitive forms with land-capable limbs were known, all of them found in the Middle East and the eastern Adriatic.
"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," Polcyn said Wednesday. "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."
The Late Cretaceous period was a time of very warm temperatures and rising sea levels.
"As the Earth warmed and the seas rose, small land-dwelling lizards took to the oceans and developed increasing levels of seagoing capabilities, and over 30 million years, eventually evolving into the top predator of their domain before becoming extinct some 65 million years ago" Polcyn said.
Although a small number of primitive mosasaur have been known to retain land-capable limbs, they were thought to be an ancestral group separate from the later fin-bearing forms. Dallasaurus represents a clear link to one lineage of the later forms and the first time researchers can clearly show mosasaurs evolved fins from limbs within the different lineages of mosasaurs.
With the aid of computer science and SMU's visualization laboratory, Polcyn has been able to simulate what Dallasaurus looked like, and how, based on skeletal remains, he would swim and move from land to sea. An artist has taken Polcyn's visualization work even one step further by creating a life-sized model of Dallasaurus, a work that is soon to be on display at the Museum along with the computer simulation.
Meanwhile Van Turner , the amateur, has his legacy securred in the form of the creature's last name, turneri.
"Not all major discoveries are made by highly trained paleontologists," said Fiorillo. "The observant individual, even kids, can still make an important find."