A major drop in sea levels drove many crocodile ancestors to extinction around 145 million years ago at the boundary between the Jurassic and Cretaceous periods, according to new findings published in Proceedings of the Royal Society B. And this massive decline was a boon for early marine turtles. 

Crocodyliforms used to be hugely diverse, and they have a much richer evolutionary history than you’d expected judging from their living descendants, which include today’s alligators and crocodiles. Some swam in the ocean and grew up to 12 meters long (39 feet long), while others were much smaller and lived on land. Based on fossils, researchers have known about the extinction event at the Jurassic/Cretaceous (J/K) boundary for decades, but the effects of crocodyliform declines remained largely unstudied. 

So, a trio of researchers led by Jonathan Tennant of Imperial College London turned to a digital archive called the Paleobiology Database to look for changes in longer-term trends in crocodyliform fossil records spanning 201 million to 66 million years ago. 

They found a huge biodiversity crash across the J/K boundary: More than 75 percent of genera living on land and at sea were lost. While non-marine biodiversity partially recovered, marine biodiversity remained low throughout the Early Cretaceous. The remaining crocodyliform species diversified into new groups such as the now-extinct notosuchians, which were about 1.5 meters long (5 feet long). The eusuchians came to prominence after the extinction event, and they led up to today's crocodiles.

This massive decline was mostly caused by a drop in sea levels, which closed off shallow marine environments like the lagoons and coastal swamps where many crocodyliforms lived and hunted. Additionally, a change in ocean water chemistry also increased sulfur toxicity and depleted oxygen. 

Furthermore, the team found that the timing of the mass crocodyliform extinction coincided with the origin of modern marine turtles. Their ancestors were common prey for ancient marine crocs. "This major extinction of crocodyliforms was literally a case of out with the old and in with the new for many species," Tennant says in a statement. "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." 

However, the drop in marine croc diversity also paved the way for their ecological replacements, including sharks, plesiosaurs, and other large predators.