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Late Jurassic and Early Cretaceous oceans are known for enormous and fierce predators like pliosaurids with 2-meter-long jaws, toothy thalattosuchia crocodyliforms, and fast, fish-like ichthyosaurians. Then, during the middle Cretaceous, the fossil record shows an abrupt change . Ichthyosaurs, thalattosuchians, and pliosaurids disappeared. Meanwhile, mosasaurs, plesiosaurs, and sharks diversified and expanded. What changed to cause species that had ruled the oceans for millions of years to suddenly die out and new species to evolve?
According to a new study to be presented at the General Assembly of the European Geosciences Union next week, it was likely a period of ocean anoxia and climate volatility related to the hottest interval of the last 541 million years. This interval, known as the Cenomanian/Turonian transition, experienced the highest carbon dioxide concentrations during the Cretaceous, as well as disturbances in nutrients like sulphur and iron in the oceans. This transition is associated with a shift in top predators, creating the unique and somewhat short-lived oceanic food webs of the Late Cretaceous, according to Valentin Fischer of the Université de Liège in Belgium and his colleagues.
As Fischer and the team reported on 01 May, they combined data on the phylogenetic relationships of hundreds of marine reptile lineages to analyze how extinctions were distributed in the tree of life. Then, they used the largest sample of 2D and 3D data on marine reptiles ever assembled to analyze the effect of these extinctions on the predatory capabilities of Cretaceous marine reptiles.
“Our analyses showed that the Cenomanian-Turonian transition is associated with elevated rates of extinction and that these extinctions disproportionally targeted some groups of large and fast predators, in a stepwise manner,” Fischer says.
For example, skull shapes of predators were significantly different before and after the transition, “notably resulting in distinct bite force,” he says.
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