You'd never expect a turtle to jump from one branch to another, but a paper in today's Science has them leaping to a new section of the reptile evolutionary tree. An extensive comparison of gene sequences among turtles and other reptiles suggests that the hard-shelled animals arose much more recently than scientists had thought, and that they are most closely related to the group that includes crocodiles.
For at least 80 years, most paleontologists had assumed that turtles had no close evolutionary relatives among other reptiles. Several lines of fossil evidence--including a tally of the number of holes in reptile skulls that showed turtles have no extra holes behind the eye socket while all other reptiles have two--suggested that ancestral turtles branched off from other reptiles roughly 300 million years ago, near the base of the reptilian tree. In the last few years, several gene sequence comparisons have contradicted this view, but these studies included only a few genes and showed a low degree of confidence.
After finding sequences for two previously unanalyzed turtle genes, evolutionary biologists Blair Hedges and Laura Poling of Pennsylvania State University in University Park decided to take another stab at where to put turtles in the animal kingdom. They combined their results with sequence data for 22 other nuclear genes and nine mitochondrial genes, then compared the turtle sequences to those for genes in every reptile group: crocodiles, squamates (a group that includes snakes and lizards), birds, and a spiny-backed, iguanalike reptile called a tuatara, which is considered its own class. The researchers found that turtles are most closely related to crocodiles, and least similar to snakes and lizards. "Every which way you analyze the data, you get the same result," Hedges says. Because any one gene mutates at a constant rate, the researchers concluded that turtles must have branched off from crocodiles around 210 million years ago.
Hedges hopes that paleontologists will reexamine fossils to "see if there are some patterns they may have overlooked" that would support his genetic findings. But don't expect them to come rushing to revise the reptile family tree: The new findings are sure to be controversial, says Olivier Rieppel, a paleontologist at the Field Museum in Chicago. Still, Rieppel says, "I think there is enough there now to really have a serious discussion about this."