A literal sea change disrupted the balance of aquatic predators during the middle Cretaceous. Before that period, during the Late Jurassic and Early Cretaceous, fierce aquatic reptilian predators prowled the oceans, including pliosaurids with jaws over six feet long, toothy thalattosuchia crocodyliforms, and fast, fish-like ichthyosaurians.
Then, during the middle Cretaceous, something happened. Ichthyosaurs, thalattosuchians, and pliosaurids disappeared rather abruptly. But the change wasn’t a uniform mass extinction event. Mosasaurs, plesiosaurs, and sharks diversified and expanded.
So what caused this abrupt, unequal shift? The highest spike in temperatures within the last 541 million years likely altered the oceans’ chemistry, according to a study that will soon be presented at the General Assembly of the European Geosciences Union. Oxygen levels dropped, while carbon dioxide increased. The rapid heat change also disrupted the levels of nutrients like sulfur and iron within the oceans.
Top Ocean Predators
This transition shifted the composition of top predators in the ocean, essentially changing the oceanic food webs of the Late Cretaceous, according to Valentin Fischer of the Université de Liège in Belgium and his colleagues. Those changes did not affect all species equally, and were relatively short-lived.
“Changes in temperatures, sea levels, forcing modifications in trophic chains and which favors some groups and will be detrimental to some others,” says Fischer. “These events are not a mass extinction event but what we call a strong biotic turnover.”
Fischer and his colleagues combed through two massive data sets to reach their conclusions. First, they pooled and analyzed data on the phylogenetic relationships of hundreds of marine reptile lineages. They aimed to determine how distinctions were distributed throughout the tree of life. Then, they analyzed the largest-ever-assembled collection of 2D and 3D images of marine reptiles to compare the predatory capabilities of those creatures over time.
Read More: 6 Ancient Mega-Predators that Once Ruled the World
Evolutionary Impacts
Their analysis showed that the rapid shift in conditions — called the Cenomanian-Turonian transition — disproportionately affected some of the larger, faster predators higher on the food chain. Those changes triggered some relatively quick evolutionary impacts, such as fairly drastic differences in some aquatic reptiles’ skull shapes and sizes.
“I was surprised that the effects of the turnover were so strong and so clear,” says Fischer.
As a young scientist in training, Fischer started with the extinction of ichthyosaurs. Then he examined the disappearance of other aquatic lizards that had evolved along with them. As his research methods expanded, he began to cast wider and wider nets — which took him to the Cenomanian-Turonian transition. That work left him with a clear conclusion about this past extinction event — and may pose as a warning to the future.
“Ancient climate change forced a clear shift in marine top predators,” Fischer says.
Read More: Why Were Prehistoric Marine Reptiles So Huge?
Article Sources
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General Assembly of the European Geosciences Union. How mid-Cretaceous events affected marine top predators
Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
