An analysis of hundreds of fossilized skulls and jaws reveals that Mesozoic sea monsters – plesiosaurs, ichthyosaurs, and mosasaurs – burst onto the scene and rapidly dominated. The findings are published in Paleobiology this week.
Reptiles were top predators in marine ecosystems during the Mesozoic Era, which is made up of the Triassic, Jurassic, and Cretaceous periods. The era lasted from 252 million to 66 million years ago. Previous studies found that marine reptiles were wildly diverse and displayed a variety of specialized feeding modes during the Middle Triassic in the aftermath of the Permian-Triassic extinction. But their origins and early rise to the top has been a mystery. And until now, most of these studies were based on estimates of biodiversity (or species numbers) over time.
University of Bristol’s Thomas Stubbs and Michael Benton wanted to study variation in the shape and function of marine reptile jaws and teeth. In one of the largest comparative and quantitative investigations of evolution in Mesozoic marine reptiles, the team examined the skulls of 354 species and the jaws and teeth of 206 species.
"We show that when marine reptiles first entered the oceans in the Triassic period, they rapidly became very diverse and had many morphological adaptations related to feeding on varied prey," Stubbs said in a statement. "Within a relatively short space of time, marine reptiles began feeding on hard-shelled invertebrates, fast-moving fish, and other large marine reptiles."
This range of eating-related adaptations of Triassic marine reptiles was never exceeded again later during the rest of the Mesozoic. "They expanded into nearly every mode of life, indicated by their feeding habits and range of body sizes, really much faster than might have been imagined," Benton added. However, just 30 million years after this initial evolutionary burst, most groups of marine reptiles were wiped out during Late Triassic extinction events. For the most part, Jurassic marine reptiles failed to expand into those vacated roles.
(A) Pliosaurus, (B) Tylosaurus, (C) Ophthalmosaurus, and (D) Placochelys. Scale bars on the jaw illustrations represent 20 centimeters (A-C) and 5 centimeters (D). Tom Stubbs