A new study suggests that the decline of crocodile ancestors may have been good news for early marine turtles.

Scientists at Imperial College London and University College London have found that the mass extinction of crocodyliforms about 145 million years ago helped marine turtles thrive. Crocodyliforms comprise modern crocodiles as well as alligators and their ancient ancestors, which were major predators that thrived on Earth millions of years ago. They evolved into a variety of species, including smaller ones that lived on land through to mega-sized sea-swimming species that were up to 12 meters long. However, about 145 million years ago crocodyliforms, along with many other species, experienced a severe decline during an extinction event that took place at the Jurassic/Cretaceous boundary.

"This major extinction of crocodyliforms was literally a case of out with the old and in with the new for many species," said Jon Tennant, lead author of the study from the Department of Earth Science and Engineering at Imperial. "This major extinction of crocodyliforms was literally a case of out with the old and in with the new for many species. Marine turtles, the gentle, graceful creatures of the sea, may have been one of the major winners from this changing of the old guard. They began to thrive in oceans around the world when their ferocious arch-predators went into terminal decline."

In this latest study, the researchers looked at records of the extinction of crocodyliforms. Up to 80 percent of species on land and in marine environments were wiped out at this time. This was largely due to a drop in sea levels, which led to a closing off of shallow marine environments. In all, the researchers analyzed almost 1,200 crocodyliform fossil records.

The decline and extinction of many marine crocodyliform species not only helped prey species, but may have also paved the way for their ecological replacement by other large predatory groups such as modern shark species and new types of plesiosaurs. Plesiosaurs were long-necked, fat-bodied and small-headed ocean-going creatures with fins which then went extinct about 66 million years ago.

The findings, published in the Proceedings of the Royal Society B, show that the decline of these animals paved the way for the diversification of other species. Now, researchers are hoping to see if the analysis extended to other groups including dinosaurs, amphibians and mammals to learn more about the effects of the Jurassic/Cretaceous boundary on their biodiversity.