Turtles, not birds, have been found to be the closest relatives of crocodiles and alligators, according to an analysis of the largest available collection of reptile genes. The study's conclusions contradict decades of research based on anatomical and fossil studies, which had firmly positioned birds as the reptile group most closely related to crocodiles and alligators, a group known as crocodilians.
The surprising finding is published in the February 12, 1999, issue of the journal Science by researchers S. Blair Hedges, an associate professor of biology at Penn State, and Laura L. Poling, a graduate student.
Previous studies of gene similarities--a relatively newer tool for determining relationships between species--have never agreed with the more traditional anatomical methods on this issue. "Turtles turned out to be not where they were supposed to be on the family tree whenever their genes were included in a research study," says Hedges, who decided recently to assemble all the genetic data available in order to resolve the question.
Hedges and Poling collected new data for two nuclear genes and added this new information to all gene-sequence data available for these species in the public genetic databases worldwide. The research included 24 genes from the nucleus and 9 DNA segments from the mitochondria of reptile cells. "The results provide strong evidence that the turtle is the crocodile's closest living relative," Hedges concludes.
Because the ultimate goal of both anatomical studies and gene studies is to find the proper place of all species on the family tree, researchers would like to see agreement between the two types of studies, wherever possible. To encourage their anatomist colleagues, Hedges and Poling point to an extinct species from the Triassic era, the aetosaur, which appears to share some anatomical characteristics of both turtles and crocodilians. "We hope paleontologists will take a closer look at reptile fossils from this period to see if they can find any patterns of physical characteristics that would logically reposition the turtle on the family tree in a way consistent with the results of our large study of its genes."
This research was supported, in part, by grants from the National Science Foundation, the National Aeronautics and Space Administration, and a Howard Hughes Medical Institute grant through the Undergraduate Biological Sciences Education Program to Pennsylvania State University.
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