Marine turtles, as we know them today, got an evolutionary leg-up (or flipper-up) when a huge number of ancient crocodile ancestors died out 145 million years ago, according to new research.
A paper published in Proceedings of the Royal Society B uncovered a link between a mass extinction of crocodyliforms – the name given to crocodiles, alligators and their ancient ancestors – and the rise of the marine turtle.
This extreme shift took place during the Jurassic/Cretaceous boundary – a dramatic time in prehistory in which 80% of the world’s species were wiped out.
The researchers, led by Jonathan Tennant, a PhD student at the Imperial College London, studied 1,200 crocodyliform fossil records, revealing a steep decline during the Jurassic/Cretaceous boundary.
This time period is characterised by a significant drop in sea levels around the world. According to the study, this shift cut off homes and hunting grounds – lagoons and swamps – frequented by the ancestors of today’s crocs, and may also have increased sulfur toxicity of ocean waters while depleting them of oxygen.
When sea-dwelling crocodyliforms started to disappear, the researchers suggest this probably eased the environment for smaller, more gentle species such as turtles.
“This major extinction of crocodyliforms was literally a case of out with the old and in with the new for many species,” Tennant explains.
“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.”
The demise of the croc-like hunters may also have paved the way for other predator species such as sharks, and varieties of long-necked plesiosaurs. Once these species found their niche, marine crocodyliforms struggled to recover their numbers.
Land-dwelling crocodyliforms were less affected by the shifting environment. According to the fossil record, many populations bounced back following the Jurassic/Cretaceous boundary.
These populations diversified into a number of species including eusuchians, which later evolved into today’s crocodiles.
Amy Middleton is a Melbourne-based journalist.
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