Sinuses Were What Held Prehistoric Crocodiles Back From Being Deep Divers Like Dolphins And Whales
The sinuses of the ancient relatives of modern-day crocodiles prevented them from becoming deep divers like dolphins and whales, according to this research.
A study has suggested that large snout sinuses stopped thalattosuchians, which lived during the time of the dinosaurs, from venturing into deep waters.
Dolphins and whales (cetaceans) evolved from land mammals to fully aquatic creatures over the course of 10 million years or so. Their bone-enclosed sinuses reduced, and they developed sinuses and air sacs outside of their skulls.
These external features would have aided in pressure regulation during deep dives, protecting their skulls from damage.
Thalattosuchians lived during the Jurassic and Cretaceous periods. They are categorized into two main groups—Teleosauridae, which likely lived in coastal waters and estuaries and were similar to modern-day gharial crocodiles—and Metriorhynchidae, which were more adapted to life at sea.
Researchers from the University of Southampton, the University of Edinburgh, and other institutions wanted to see if thalattosuchians made sinus adaptations similar to dolphins and whales as they evolved from the land to the sea.
The team used 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.
They found that as thalattosuchians became more aquatic, braincase sinuses decreased, similar to dolphins and whales. The team thinks this may be linked to buoyancy, diving, and feeding reasons.
However, they also discovered that the snout sinuses of thalattosuchians expanded compared to their ancestors once they became fully aquatic.
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“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,” said Dr. Mark Young, the lead author of the study from the University of Southampton.
Both the cetaceans and thalattosuchians developed extracranial sinuses. But the cetacean’s sinus system alleviated increases in water pressure during deeper dives, while the expansive snout sinus system of the metriorhynchids prevented it from diving deep.
“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 equalize the increasing pressure,” said Dr. Young.
Furthermore, the kidneys of dolphins and whales can filter out salt from seawater. But marine reptiles and birds rely on salt glands to excrete salt from their systems.
The researchers believe that the larger snouts of metriorhynchids helped to drain salt glands, much like modern marine iguanas.
“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,” said Dr. Young.
For metriorhynchids, their sinuses would have compressed the salt glands within the skull to generate a sneeze-like effect, according to Dr. Young.
Overall, the study highlights the unique ways in which thalattosuchians adapted to ocean life. The research team’s paper was published in the Royal Society Open.
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