A lizard that lived 92 million years ago in the shallow seas and shores has been identified as the ancestor of the mosasaur, US scientists said on Wednesday in their paper published in the latest issue of the Netherlands Journal of Geosciences.
Fossil of the lizard, which is now named Dallasaurus, was discovered by amateurish fossil finder Van Turner 16 years ago. Scientists now said Dallasaurus was the origin of an extinct line of lizards with an evolutionary twist: a land-dwelling species that became fully aquatic.
Michael Polcyn, a paleontologist at the Southern Methodist University, and Gordon Bell Jr. of Guadalupe National Park in Texas, said they pieced together an understanding of the anatomic and natural history of Dallasaurus from the bones.
Based on the skeletal remains and with the aid of computer visualization, the researchers were able to simulate what Dallasaurus looked like, and how it would swim and move from land to sea.
Dallasaurus represents a missing link in the evolution of 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, according to the researchers.
Dallasaurus retained complete limbs, hands and feet suitable for walking on land, whereas later mosasaurs evolved their limbs into flippers, the paper said.
Mosasaurs lived and became extinct alongside dinosaurs, but few paleontologists specialize in them. Later mosasaurs grew as large as their dinosaur brethren, reaching up to 15 meters 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," explained Polcyn.
"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 advanced fin-bearing mosasaurs have been grouped into three major lineages. Although a small number of primitive mosasaurs 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 for the first time researchers can clearly show mosasaurs evolved fins from limbs within the different lineages of mosasaurs, according to Polcyn.