An international team of paleobiologists have found that the sinuses of ocean-dwelling relatives of modern-day crocodiles prevented them from evolving into deep divers like whales and dolphins.

A new paper published today in Royal Society Open Science suggests that thalattosuchians, which lived at the time of the dinosaurs, were stopped from exploring the deep due to their large snout sinuses.

Whales and dolphins (cetaceans) evolved from land-dwelling mammals to become fully aquatic over the course of around 10 million years. During this time, their bone-enclosed sinuses reduced and they developed sinuses and air sacs outside of their skulls.

This would have alleviated increases in pressure during deeper dives, allowing them to reach depths of hundreds (dolphins) and thousands (whales) of meters without damaging their skulls.