Lizard diversity was determined long ago, according to Texas, Oklahoma researchers

May 3, 2005

AUSTIN, Texas—The diversity of present-day lizards might have been determined 200 million years ago when two groups of lizards diverged and not come more recently as the result of environment and competition, according to researchers from The University of Texas at Austin and the University of Oklahoma.

The researchers used quantitative analyses of what lizards eat to determine that the most striking dietary divergence between the iguania and sceroglossa groups of lizards came in the late Triassic.

Eric Pianka, professor of integrative biology at The University of Texas at Austin, and Laurie Vitt, professor of zoology at the University of Oklahoma, detailed the study in a paper published this week in the Proceedings of the National Academy of Sciences. They each have studied lizards for almost 40 years.

Members of scleroglossa include gekkos, snakes and monitor lizards. They have prehensile jaws and actively seek out foods with sensory capabilities that help them find it. Iguania, which include iguanas, chameleons and Texas horned lizards, lack these characteristics and wait for the food to come to them. They capture food with their sticky tongues

Back 200 million years ago, the scleroglossa switched from tongue prehension to jaw prehension. That, in turn, freed their tongues to evolve in new ways.

“When the tongue evolved in these new ways, scleroglossan lizards became much better at finding prey by chemical means,” Pianka said, by smell or sensing it with their tongues.

“This gave these lizards a real leg up. They had access to all kinds of prey that iguanians could never get. That’s why we think they became so diverse. They radiated out into all kinds of new niches.”

Pianka and Vitt proposed that those differences allowed scleroglossans access to new resources and might account for why there are about 6,000 species of scleroglossans but only about 1,230 iguanians.

They used data on lizard diets that they have collected over their careers. The data come from 184 species in 12 families on four continents.

They ran the data through a quantitative analysis to find where striking divergences of diet between the two major groups of lizards occurred.

There were significant dietary shifts at six major points, which reduced dietary variance by almost 80 percent. The most striking divergence of almost 28 percent occurred in the late Triassic (about 200 million years ago), when iguania and scleroglossa split.

“Twenty-seven percent of the variation in lizard diets today can be tracked back 200 million years,” Pianka said. “We were just stunned. The impact on biodiversity is that iguanians produced only one-fifth as many species as scleroglossans and we think it’s because of this fundamental shift that happened 200 million years ago.”

The results of their study changed Pianka and Vitt’s outlook.

“All of our lives we thought that what lizards eat today is due to where they live and who they’re competing with, more modern day pressures,” Pianka said. “We never dreamt that deep history was so important.”

Pianka is the Denton A. Cooley Centennial Professor of Zoology at The University of Texas at Austin. Vitt is the George Lynn Cross Research Professor in Zoology at the University of Oklahoma and curator of reptiles at the Sam Noble Oklahoma Museum of Natural History.

They are the authors of “Lizards: Windows to the Evolution of Diversity,” published in 2004 by University of California Press, which won the grand prize at the ninth annual University Co-operative Society Robert W. Hamilton Book Awards.

For more information contact: Tim Green, 512-475-6596.