by Chloe McIver of Nature magazine

Turtles should sit on the same branch of the tree of life as lizards, according to a genetic analysis that could clear up a long-standing mystery over the creature's origin.

Palaeontologists have long used morphological data, obtained by looking closely at the physical characteristics of fossils and living relatives, to show the evolutionary relationship between different species. In recent decades, however, genetic comparisons have become important.

But molecules and fossils don't always agree, and this has created a lot of confusion for the turtle. A study published today in Biology Letters could put the beleaguered reptile in its rightful place using genetic analysis.

Odd one out

Turtles have proved particularly problematic to categorize because they look so different from other reptiles, most of which belong to the Diapsida clade. Diapsids' skulls all have two holes, called temporal fenestrae, which carry muscles that attach to the jaws.

According to the fossil record, turtles first appeared in the Triassic period around 230 million years ago, and have changed little since. Crucially, turtles lack temporal fenestrae, suggesting that they are the living relatives of a more primitive reptilian form that pre-dates the diapsids. Consequently, they were set aside in their own clade, somewhere between amphibians and the rest of the reptiles.

However, there are two other possibilities that would bring turtles into the diapsid fold. One is that turtles are more closely related to birds and crocodiles than to lizards. Most of the genetic studies undertaken so far support this.

The other possibility is that turtles are much more closely related to lizards and snakes, with this group sharing a more recent common ancestor than the rest of the diapsid clade.

Disappearing act

Genetic analysis of a turtle, a lizard and an alligator1 now supports the latter hypothesis. This implies that the ancestors of turtles did once have temporal fenestrae in their skulls, but that these have disappeared as turtles evolved.

"What we've been able to provide is unambiguous evidence for one hypothesis over the others," says molecular palaeobiologist Kevin Peterson of Dartmouth College in Hanover, New Hampshire, a coauthor on the new study.

The work differs from previous efforts because it used microRNAs, short regulatory genetic molecules found in cells. They are particularly useful for solving phylogenetic puzzles because, although additional microRNAs can develop over millions of years of evolution, once established in a clade they are rarely lost.

MicroRNAs have been used to determine other similarly deep phylogenetic relationships, and most scientists agree that they are reliable tools. What makes the conclusion of this study particularly convincing is the fact that it agrees with an existing hypothesis that is also supported by morphological data.

"Over the years, as we've been trying to reconstruct the tree of life, there's been a sort of to and fro between anatomy and fossils on the one hand, and molecules on the other," says Mike Benton, a palaeontologist at the University of Bristol, UK, who was not involved in the study. For turtles, he says, this is "the first time we have anatomy and molecules seeming to agree."

This article is reproduced with permission from the magazine Nature. The article was first published on July 19, 2011.