The sinuses of ancient relatives of today's crocodiles prevented them from becoming deep divers, research by the University of Southampton has found.

A study, published on October 30 in "Royal Society Open Science," suggests that large snout sinuses hindered the thalattosuchians, which lived during the time of the dinosaurs, from exploring deep waters like whales and dolphins.

Thalattosuchians lived during the Jurassic and Cretaceous periods, with two main types, one similar to modern-day gharial crocodiles and another more adapted to the sea.

Unlike whales and dolphins, which evolved reduced bone-enclosed sinuses and developed external sinuses and air sacs, these ancient marine reptiles' snout sinuses expanded as they became fully aquatic.

Dr Mark Young, lead author of the paper from the University of Southampton, said: "The regression of braincase sinuses in thalattosuchians mirrors that of cetaceans, reducing during their semi-aquatic phases and then diminishing further as they became fully aquatic.

"Both groups also developed extracranial sinuses. But whereas the cetacean’s sinus system aids pressure regulation around the skull during deep dives, the expansive snout sinus systems of metriorhynchids precluded it from diving deeply.

"That’s because at greater depths, air within the sinuses would compress, causing discomfort, damage, or even collapse in the snout due to its inability to withstand or equalise the increasing pressure.

"A major problem for animals with salt glands is ‘encrustation’, where the salt dries and blocks the salt excreting ducts. Modern birds shake their heads to avoid this, while marine iguanas sneeze to force the salt out."

The international team of paleobiologists, including researchers from the University of Southampton and the University of Edinburgh, conducted the study using a special kind of scan to examine the sinuses of 11 thalattosuchian skulls, along with the skulls of 14 modern crocodile species and six other fossil species.

They discovered that while braincase sinuses reduced, snout sinuses expanded.

The researchers believe the complex snout sinuses helped in draining salt glands, similar to modern marine iguanas.

The study highlights how major evolutionary transitions unfold, influenced by species' anatomy, biology, and evolutionary history.