Fossilized skulls of semi-aquatic thallatosuchian species (A and B) next to a fully aquatic metriorhynchid skull (C). Credit: Sven Sachs
Fear of the oceans, or thalassophobia, is an understandable emotional reaction. The environment is dark, inhospitable to humans, and contains some of the planet’s biggest and weirdest creatures. But you can take some small comfort in the knowledge that, at the very least, evolution saved us from deep sea crocodiles.
Crocodilian ancestors known as thallatosuchians roamed the middle Jurassic and early Cretaceous eras (roughly 191-113 million years ago) and comprised two primary groups, teleosauridae and metriorhynchidae. While fossil evidence indicates the former resembled today’s crocodiles, the latter evolved streamlined, dolphin-like bodies and flippered appendages that allowed them to thrive in marine environments. That said, fossil evidence indicates they never managed to reach the same depths as dolphins and other cetaceans routinely. And thanks to dozens of recent skull scans, experts may now know why—thallatosuchian snouts couldn’t handle the pressure.
The enlarged snout sinuses of species like suchodus durobrevensis prevented them from travelled too deep into ocean waters. Credit: University of Southampton
An international research team led by paleobiologists at the UK’s University of Southampton recently used computed tomography to examine 11 thallatosuchian skulls in comparison to six other fossil species, as well as 14 species of modern crocodile. Using the scans, experts paid close attention to two sinus systems—the bone-enclosed braincase sinuses and those located in their snouts. Their results are detailed in a study published on October 30th in the journal, Royal Society Open Science.
The images indicated that, similar to dolphins and whales, thallatosuchians evolved smaller braincase sinuses as they ventured further into water, likely to aid with diving, hunting, and buoyancy. But unlike cetaceans, there came a point when thallatosuchian snout sinuses (particularly those of metriorhynchids) started to expand instead of shrink.
[Related: This ancient crocodile looked kind of like a dolphin.]
“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,” Mark Young, the paper’s lead author, said in a statement on Wednesday. Although modern cetacean sinus systems help to mitigate ocean pressure around their skulls, a metriorhynchid’s larger snout sinuses would have done the opposite.
“[A]t greater depths, air within [metriorhynchid] sinuses would compress, causing discomfort, damage, or even collapse in the snout due to its inability to withstand or equalize the increasing pressure,” Young explained.
A fossil of a metriorhynchid thalattosuchian. Credit: Sven Sachs
The limit to thalattosuchian deep dives wasn’t without purpose, however. While whales and dolphin kidneys are extremely efficient at filtering salt from sea water, marine reptiles and birds instead rely on specialized glands to rid their systems of the potentially harmful minerals. Iguanas, for example, sneeze to release excess salt from their ducts. Given the metriorhynchid’s larger and more complex snout sinuses, paleobiologists theorize the crocodilian ancestor likely did the same thing.
“Birds, like metriorhynchids, have sinuses that exit the snout and pass under the eye and when their jaw muscles contract, it creates a bellows-like effect within their sinuses,” Young said. “For metriorhynchids, when the sinuses were subjected to this effect, it would have compressed the salt glands within the skull and created a sneeze-like effect, similar to modern marine iguanas.”
Unfortunately for thalattosuchians, however, their evolutionary luck ran out during the Cretaceous period, making way for their modern descendents. Because of this, Young said experts will never know with any certainty if more time would have allowed them to evolve in ways to further resemble whales and dolphins—or if humans would ever need to add “deep sea crocodiles” to their list of thalassophobia worries.
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Source : Popular Science