Evidence Points To Turtles Being Sister Group To Archosaurs
DNA studies are opening up new fields of study, in terms of understanding the relationships between reptiles, both at a specific level in terms of individual species alive today, and also the way in which different groups developed in the past
As far as reptiles are concerned, there has always been considerable uncertainty about the relationships of turtles within the Class Reptilia. A team from Boston University (BU) has recently been investigating the genomes of turtles, comparing these with other groups of reptiles and also birds.
This study has come up with some unexpected findings. It was based on an extensive investigation into 1145 ultraconserved elements (UCEs), which are recognized as relatively stable and the least changed components within the genetic code. The investigation also included tuataras, which are the remaining survivors of a now rare and ancient group confined to New Zealand.
Nick Crawford, a post-graduate researcher in biology in BU’s Graduate School of Arts & Sciences and lead author of the study, carried out this work by using computational analysis to examine regions of the different animals’ genomes.
“Turtles have been an enigmatic vertebrate group for a long time and morphological studies placed them as either most closely related to the ancestral reptiles, that diverged early in the reptile evolutionary tree, or as closer to lizards, snakes, and tuataras,” he explains.
The Way Forward
Past research had tended to suggest that turtles could be linked on the basis of anatomical features with the latter three groups, collectively known as lepidosaurs, but recent molecular studies suggested a closer relationship with crocodilians and birds (archosaurs). It is this latter view that has now been borne out by this latest research, which is the most detailed study of its type ever undertaken.
It now seems clear that turtles, crocodilians and birds once shared a common ancestor. The result also confirms once again that relying on physical features rather than genetic ties is actually counter-productive in trying to understand long-term relationships between different groups of reptiles - and animals in general. This BU study is the first to produce a map of the reptilian tree, and this approach is equally applicable right across the Animal Kingdom, so the impact of this work is potentially far-reaching, in terms of determining past relationships in other groups of creatures.
Reference: N. G. Crawford, B. C. Faircloth, J. E. McCormack, R. T. Brumfield, K. Winker, T. C. Glenn. More than 1000 ultraconserved elements provide evidence that turtles are the sister group of archosaurs. Biology Letters, 2012; DOI: 10.1098/rsbl.2012.0331