Recent discoveries by a global team of paleobiologists indicate that the sinuses in the ocean-dwelling relatives of today’s crocodiles inhibited their ability to evolve into deep-sea divers, similar to whales and dolphins.
A new study released today, on October 30, in Royal Society Open Science, reveals that thalattosuchians, which existed during the age of dinosaurs, were limited in their capacity to dive deep due to the size of their snout sinuses.
Whales and dolphins, known as cetaceans, transitioned from land mammals to fully aquatic beings over approximately 10 million years. During this evolutionary journey, their sinuses, which were originally enclosed by bone, shrank, allowing the development of sinuses and air sacs external to their skulls.
This adaptation enabled these animals to withstand the pressure increases encountered during deep dives, permitting dolphins to reach depths of hundreds of meters and whales thousands, without suffering damage to their skulls.
Thalattosuchians thrived during the Jurassic and Cretaceous periods and are divided into two primary groups. The Teleosauridae were akin to today’s gharial crocodiles, probably residing in coastal areas and estuaries. In contrast, the Metriorhynchidae were better adapted for life in the ocean, featuring streamlined bodies, flipper-like limbs, and tail fins, along with various other marine traits.
Researchers from the University of Southampton, the University of Edinburgh, and others aimed to find out if thalattosuchians had developed sinus adaptations similar to those of whales and dolphins throughout their evolution from land to sea.
The research team employed computed tomography (CT scans) to analyze the sinuses of 11 thalattosuchian skulls alongside those of 14 contemporary crocodile species and six additional fossil types.
Sinus Adaptations
The findings indicate that the braincase sinuses in thalattosuchians diminished as they became more aquatic, paralleling the evolutionary changes seen in cetaceans. The researchers believe this change is likely linked to factors related to buoyancy, diving, and feeding.
However, they also discovered that after thalattosuchians became entirely aquatic, their snout sinuses expanded relative to those of their land-based ancestors.
“The reduction of braincase sinuses in thalattosuchians is comparable to that of cetaceans, diminishing during their semi-aquatic phases and further decreasing once they became fully aquatic,” explains Dr. Mark Young, the paper’s lead author from the University of Southampton.
“Both groups also evolved extracranial sinuses. Nevertheless, while the sinus structure in cetaceans helps manage pressure around the skull during profound dives, the large snout sinus systems in metriorhynchids limited their capacity for deep diving.”
“This limitation arises because, at significant depths, air in the sinuses compresses, causing discomfort, potential harm, or even collapse within the snout due to its inability to cope with or equalize the rising pressure.”
Salt Excretion
In contrast to whales and dolphins, which have highly efficient kidneys that remove salt from seawater, sea reptiles and birds utilize special salt glands to rid their bodies of excess salt.
The researchers propose that the more extensive and complex snout sinuses of metriorhynchids could have assisted in draining their salt glands, akin to modern marine iguanas.
“A significant issue for salt gland-bearing animals is ‘encrustation,’ where salt dries up and blocks the ducts that excrete salt. Birds shake their heads to prevent this, while marine iguanas sneeze to force out the salt,” Dr. Young mentions.
“We hypothesize that the enlarged sinuses in metriorhynchids facilitated the expulsion of excess salt. Similar to birds, these ancient reptiles possessed sinuses extending from the snout to beneath the eye, and when their jaw muscles contracted, it mimicked a bellows effect that could compress the salt glands within the skull, leading to a sneeze-like expulsion, much like what occurs in modern marine iguanas.”
This research sheds light on how significant evolutionary shifts unfold, influenced by anatomical structures, biological functions, and historical evolution.
“It’s intriguing to uncover how ancient creatures, such as thalattosuchians, uniquely adapted to marine life, displaying both similarities and differences to present-day cetaceans,” comments Dr. Julia Schwab, a coauthor of the study from the University of Manchester.
Dr. Young concludes, “Thalattosuchians went extinct in the Early Cretaceous period, leaving us uncertain whether, with additional evolutionary time, they might have evolved further in parallel with modern cetaceans or if the necessity to mechanically drain their salt glands proved to be a significant barrier to advanced aquatic specialization.”
The research was sponsored by the Leverhulme Trust.
