A team of scientists has found that the prehistoric ancestors of crocodiles were prevented from evolving into deep divers like whales by their sinuses.
Researchers at the University of Southampton and Edinburgh discovered that ‘thalattosuchians’, which were around at the same time as dinosaurs, failed to develop the same diving capabilities as whales and dolphins.
Over the course of the 10 million years, it took cetaceans to evolve from land-dwelling mammals into ocean dwellers, their bone-enclosed sinuses reduced and they developed sinuses and air sacs outside of their skulls.
This would have alleviated increases in pressure, allowing dolphins to reach hundreds of metres under the sea and whales to reach thousands of metres without damaging their skulls.
The study has been published in the journal Royal Society Open Science. It compared thalattosuchians with the predecessors of cetacean aquatic mammals such as dolphins and whales.
Dr Mark Young, lead author of the paper from the University of Southampton, said: “The regression of brain-case sinuses in thalattosuchians mirrors that of cetaceans, reducing during their semiaquatic phases and then diminishing further as they became fully aquatic.
“Both groups also developed extra-cranial 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.”
He added that the complex snout sinuses may have developed in metriorhynchids to help drain their salt glands which whales and dolphins do not need because they have highly efficient kidneys that filter out salt from seawater.
Dr Young added: “Thalattosuchians became extinct in the Early Cretaceous period, so we will never know for sure if given more evolutionary time they could have converged further with modern cetaceans or whether the need to mechanically drain their salts glands was an impassable barrier to further aquatic specialisation.”
