Southampton scientists discover new information about prehistoric crocodiles evolution

Undated handout photo issued by the University of Southampton of an artists' impression of Tyrannoneustes, a metriorhynchid thalattosuchian. These lived at the time of the dinosaurs, with the predecessors of cetaceans such as whales and dolphins. Scientists have found that these cetaceans evolved from land-dwelling mammals, and that the sinuses of the prehistoric ancestors of crocodiles prevented them from evolving into deep divers like whales and dolphins. Issue date: Wednesday October 30, 2024.
The researchers compared thalattosuchians, which lived at the time of the dinosaurs, with the predecessors of cetaceans such as whales and dolphins. Credit: PA / University of Southampton

A team of scientists has discovered new information about the way the prehistoric ancestors of crocodiles evolved over time.

Researchers at the University of Southampton and Edinburgh found that 'thalattosuchians', which were around at the same time as dinosaurs, could not develop the same diving capabilities such as whales and dolphins.

The study, published in the journal Royal Society Open Science, compared thalattosuchians with the predecessors of cetacean aquatic mammals such as dolphins and whales.

The scientists found that when dolphins and whales evolved over a 10 million-year process, they developed sinuses and air sacs outside their skulls.

This would have alleviated increases in pressure during deeper dives, allowing dolphins to reach hundreds of metres under the sea and whales to reach thousands of metres without damaging their skulls.

Despite thalattosuchians evolving in a similar way, as they became fully-aquatic, their snout sinuses expanded, which meant they would not be able to withstand increasing pressure when diving at greater depths.

The team used a special kind of scan, known as computed tomography, to measure the sinuses of 11 thalattosuchian skulls, as well as the skulls of 14 modern crocodile species and six other fossil species.

The study explains that thalattosuchians, which lived during the Jurassic and Cretaceous periods, fell into two main groups.

These were teleosauridae, which were similar to modern day gharial crocodiles and likely living in coastal waters and estuaries, and the metriorhynchidae, which were more fully adapted to life at sea, with streamlined bodies, flipper-like limbs and tail fins.

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'll 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."

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