Researchers solve fossil croc mystery

Assumptions about the evolutionary descent of a type of crocodile found in India have been shown to be false after researchers found a similar-looking fossil species to be only very distantly related.

The gharial (Gavialis gangeticus) and its genetically close relative the false gharial (Tomistoma schlegelii) have trademark long, thin pincer-like jaws well adapted for eating fish.

Genetic analysis suggests that the gharials are a relatively recent evolutionary line, diverging from other crocs about 30 million years ago.

This finding, however, threw up a tricky problem. A fossil croc known as a thoracosaur, dating to 70 million years ago, had very similar jaws, leading some researchers to infer a direct line of descent between the two. Genetic and morphological evidence, thus, did not agree, leaving a 40-million-year gap.

Now, however, researchers Michael Lee, from Flinders University in Adelaide, and Adam Yates, from the Museum of Central Australia, have resolved the impasse. They did so using an analytical method that combined body-shapes, fossil age, and molecular evidence.

The similarity between thoracosaurs and gharials, the results indicate, is down to coincidence. More accurately, it is a strong example of “convergent evolution”, the process by which unrelated species come to share characteristics because of similar environmental pressures.

“The DNA of living gharials indicates they are a young group, which evolved well after the dinosaurs – but then why are there gharial-like fossils older than T. rex?” says Lee.

“Either the DNA evidence is wrong, or we’ve misinterpreted these ancient thoracosaurs. Our work suggests we’ve got the fossils wrong, after being misled by convergent evolution.”

The research is published in the journal Proceedings of the Royal Society B.