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The Evolutionary Path of Thalattosuchians: Insights into Aquatic Adaptations

A recent study conducted by an international team of paleobiologists reveals that the unique sinus structures of ancient ocean-dwelling relatives of contemporary crocodiles limited their ability to evolve into deep-diving creatures like whales and dolphins.

Published on 30 October in Royal Society Open Science, the research indicates that thalattosuchians, which roamed the Earth during the time of the dinosaurs, faced constraints in their evolutionary journey due to their large nasal sinuses.

In contrast, whales and dolphins (cetaceans) evolved from terrestrial mammals into fully aquatic beings over an approximate span of 10 million years. During this transformation, their sinuses, encased in bone, diminished in size, and they developed additional sinuses and air sacs outside their skulls, which helped manage the increased pressure from deep dives. This physiological change enabled cetaceans to reach remarkable depths without risking injury to their skulls, with dolphins diving hundreds of meters and some whales reaching thousands.

Thalattosuchians flourished during the Jurassic and Cretaceous periods and are categorized into two primary families: Teleosauridae, which resembled modern gharials and likely inhabited coastal and estuarine environments; and Metriorhynchidae, which exhibited extensive adaptations for life in oceanic settings, including streamlined bodies, flippers, and tail fins.

The researchers, representing institutions such as the University of Southampton and the University of Edinburgh, sought to determine whether thalattosuchians underwent similar adaptations in their sinuses compared to cetaceans as they transitioned from land to marine environments.

The team employed computed tomography (CT) scanning technology to analyze the sinus structures of 11 thalattosuchian skulls, alongside 14 contemporary crocodile species and six other fossil examples.

Sinus Evolution in Thalattosuchians

The findings revealed a reduction in braincase sinuses throughout the evolutionary history of thalattosuchians as they adapted to aquatic life, drawing parallels to the evolutionary trends observed in cetaceans. The researchers hypothesize that these changes were influenced by factors such as buoyancy, diving capabilities, and feeding strategies.

However, the study also noted a different development: once thalattosuchians became fully aquatic, their snout sinuses increased in size compared to their terrestrial predecessors.

Dr. Mark Young, the lead author from the University of Southampton, explained, “The regression of braincase sinuses in thalattosuchians mirrors that of cetaceans, reducing during their semi-aquatic phases and diminishing further as they became fully aquatic. Both groups also developed extracranial sinuses. But while the cetacean sinus system helps with pressure regulation during deep dives, the larger snout sinus systems in metriorhynchids hindered their ability to dive deeply.” He further noted that at greater depths, the air within these sinuses would compress, potentially causing injury or even collapse of the snout structure.

The Role of Salt Glands

While cetaceans possess highly efficient kidneys to filter seawater, aquatic reptiles and birds rely on specialized salt glands to manage salt excretion. The researchers propose that the extensive and intricate snout sinuses in metriorhynchids could have facilitated the drainage of these salt glands, akin to adaptations seen in modern marine iguanas.

Dr. Young elaborated, “A significant challenge for animals with salt glands is encrustation, where dried salt blocks excretion ducts. Birds employ head-shaking to mitigate this issue, while marine iguanas produce sneezing-like actions to expel salt.” He suggests that the enlarged sinuses in metriorhynchids would have aided in salt expulsion. Notably, like birds, metriorhynchids possessed sinuses that exited the snout and ran beneath the eyes; contractions of their jaw muscles could create a pressure effect, effectively expelling excess salt.

This research underlines how anatomical, biological, and historical factors shape the evolutionary adaptations of species through significant transitional phases. Dr. Julia Schwab, a co-author from the University of Manchester, remarked on the intriguing nature of these findings: “It’s remarkable to observe how ancient animals like thalattosuchians adapted to marine life through their unique evolutionary paths while showcasing both commonalities and distinctions from present-day cetaceans.”

In conclusion, Dr. Young noted, “Thalattosuchians became extinct during the Early Cretaceous period, leaving us to ponder whether, with additional evolutionary time, they might have achieved greater convergence with modern cetaceans or if their need to mechanically eliminate salt presented an insurmountable barrier to further marine specialization.”

The research received funding from the Leverhulme Trust.

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