In the lizard kingdom, animals are classified in five separate groups of specific types of lizards, such as geckos and skinks. One is the Iguanian group, which contains iguanas and their close relatives, totalling some 1,700 species.
These iguanas are, in turn, divided into two groups: acrodontan, with the teeth fused to the top of their jaws, and pleurodont, with teeth fused to the side of the jaw.
The acrodontan group contains chameleons and bearded dragons, and their distribution is restricted exclusively to the Old World, which covers Africa, Europe and Asia. The latter group, the pleurodonts, are iguanas, distributed throughout the New World, which covers the Americas and the Caribbean.
The reason you now know more about lizard dentistry than you ever thought you needed to is because the remains of a new 80-million-year-old acrodontan lizard have been found in South America, the land of the pleurodonts. This creates a link between the two types of lizards where previously none had been.
The lizard has been named Gueragama sulamericana, and described in a paper published in the journal Nature Communications.
"This fossil is an 80-million-year-old specimen of an acrodontan in the New World," explained study co-author Michael Caldwell, a professor of biological sciences at the University of Alberta in Canada.
"It's a missing link in the sense of the paleobiogeography and possibly the origins of the group, so it's pretty good evidence to suggest that back in the lower part of the Cretaceous, the southern part of Pangaea [the prehistoric supercontinent that broke up and formed the continents as we know them today] was still a kind of single continental chunk."
Pangaea's evolution across the Paleozoic, Mesozoic and Cenozoic eras has been traced by the distribution of plant and animal life over the millennia. Fossils of particular animals dating back to the same period, for instance, have been found across multiple continents. By the time the Late Cretaceous era rolled around in the second half of the Mesozoic, Pangaea's break-up was moving towards its final stages.
The presence of Gueragama sulamericana in South America indicates that the South American iguanas may have evolved from acrodontan lizards.
"This Gueragama sulamericana fossil indicates that the group is old, that it's probably Southern Pangaean in its origin, and that after the breakup, the acrodontans and chameleon group dominated in the Old World, and the iguanid side arose out of this acrodontan lineage that was left alone in South America," Caldwell said.
"South America remained isolated until about five million years ago. That's when it bumps into North America, and we see this exchange of organisms north and south. It was kind of like a floating Noah's Ark for a very long time, about 100 million years. This is an Old World lizard in the New World at a time when we weren't expecting to find it. It answers a few questions about iguanid lizards and their origin."
Because the lizard pushes the evolution of iguanas back further than had been previously supposed, the next step in the research is to look for even older fossils.
"Each answer only rattles the questions harder," Caldwell said. "The evolution of the group is much older than had been previously thought, which means we can push an acrodontan to 80 million years ago in South America. We now need to focus on much older units of rock if we're going to find the next step in the process."