There are few animals in the world that share the morphological peculiarities of turtles, and the physical likenesses that do exist between these shelled wonders and other animals are so ambiguous as to be considered one of the last great obstacles to a more complete understanding of not only the turtle's evolutionary past but also the whole of vertebrate evolution.
But scientists are now a step closer to achieving that understanding, thanks to a recent genome-scale analysis that revealed turtles' deepest molecular secret—that they are very closely related to the archosaurs, the reptilian clade that contains modern birds and crocodiles. The study
, published in the May 2012 issue of Biology Letters
, was the first to explore turtles' evolutionary position at the genomic level as part of a sampling of representative species from the major reptile lineages.
The team of U.S. scientists who conducted the study compared more than 1,000 segments of DNA known as ultraconserved elements from the genomes of various reptiles. Ultraconservative elements are powerful tools for assessing the evolutionary relatedness of species, because they presumably are found—and are nearly identical—in all species of animals. Their similarity across divergent groups is a function of persistence, having maintained their place in animal genomes, with very little change, for millions of years, likely since before the evolutionary tree sprouted separate mammalian and avian branches. Since the number of sequence differences that exist in ultraconserved elements increases with distance from each element’s core, comparing the locations and number of variations can provide valuable genetic insight on species relatedness.
The finding that turtles are close relatives of birds and crocodiles contradicts the results of a study
reported in 2011, which concluded that turtles belong on the branch of the vertebrate evolutionary tree occupied by lizards and snakes, otherwise known as lepidosaurs. That study was based on an analysis of microRNA, or very short RNA sequences, gathered from RNA libraries (collections of sequences), which are incomplete for reptiles. The authors of the latest study suggest that, because microRNA expression is specific to tissue and developmental stage and because the selection of sequences from libraries is a biased sampling process, it is likely that the microRNAs that had been found only in turtles and lepidosaurs also exist in other reptile families but were either missed in the microRNA analysis or are as yet undiscovered.
Although the new study seems to definitively align turtles with archosaurs, much remains to be understood concerning the morphological traits of turtles. For example, turtles lack temporal openings in their skulls, which suggests that they are anapsids, all other representatives of which are extinct. And then, of course, there is the shell. Turtles, tortoises, and terrapins are the only reptiles to possess shells, and while awkward to port around, shells serve as ready-made hiding places, enabling their carriers to escape the outside world at a moment's notice.