Hedges said that while it's impossible to compare the DNA of mosasaurs with living snakes and other lizards, it is possible to compare the DNA of mosasaurs' surviving, undisputed relatives, the monitor lizards.
In the most comprehensive such study yet attempted, the pair collected difficult-to-obtain lizard DNA from sources as varied as the pet trade to remote research outposts of far-flung tropical islands. Using statistical methods to compare differences in the sequences of two genesRAG1 and C-mosamong these species, the researchers demonstrated that monitor lizards are not directly related to snakes.
Rather, monitors, such as Indonesia's fast-moving predatory Komodo dragon (Varanus komodoensis), are closely related to other groupsincluding the glass lizards of Europe and North Americanot believed to be snake relatives. The findings were recently published online in the science journal Biology Letters.
Because all lizards, apart from mosasaurs, are thought to have been land-dwelling when snakes evolved around 150 million years ago, "our study provides strong evidence that snakes evolved on land, not in the ocean," Hedges said. He noted, however, that further analysis is required to confirm which lizard group is most closely related to snakes.
"The new study is novel and interesting [and] the most comprehensive yet," commented Olivier Rieppel, chair of the department of geology at the Field Museum in Chicago, Illinois. He noted that some of the proposed relationships in the study were unusual, a problem which might be resolved if a larger number of genes were considered.
"Vidal's and Hedge's analysis is a refreshing new insight on the subject," commented Hussam Zaher, evolutionary biologist and snake expert with the University of Sao Paulo in Brazil. "Their results are relevant and seem to reject the marine theory," he said. Zaher cautions that while the study is the most complete in terms of number of lizard families reviewed, he would prefer to see a larger number of the 4,700 or so lizard species examined in future analyses.
Zaher was one of a team who recently re-assessed the proposed "missing link" fossil snakes with tiny hind limbs. His team found compelling evidence that in fact these snakes are a side-branch of snake evolution, closely related to modern boas and pythons and not mosasaurs.
The National Science Foundation has recently awarded a Tree of Life grant to researchers at the Field Museum to study the reptile family tree. The upcoming study should, Rieppel said, "generate large amounts of further data to be brought to bear on the question of snakes' origins."
SOURCES AND RELATED WEB SITES