For 150 million years during the age of the dinosaurs, a group of dolphin-like reptiles called ichthyosaurs ruled the ocean – until everything started to go wrong.
After multiplying into about 100 known species, ichthyosaurs began to disappear from the oceans around the middle of the Cretaceous period, a time of turbulent changes in the environment The aquatic reptiles went extinct 28 million years before the rest of their dinosaur cohort.
Scientists have been unable to find a singular cataclysmic event, such as a volcanic eruption or meteorite strike, that could explain ichthyosaurs' abrupt disappearance from the fossil record.
Now researchers have new explanation for the lizard fish's demise: They had lost their ability to adapt to a swiftly changing climate.
The sea creatures hadn't always been so rigid. The Ichthyosauria order included a wide range of animals that had evolved to fill unique ecological niches. Some ichthyosaurs had small curved teeth for eating soft, squishy animals; others had giant sharp teeth to help them tear up larger prey, including sea turtles.
All of them had flippers as their front limbs and elongated bodies terminating with a shark-like tail fin. They also had long snouts and some of the largest eyes ever seen in the animal kingdom, said Valentin Fischer, a paleontologist who studies ichthyosaurs at the University of Liege in Belgium.
The aquatic lizards thrived throughout the early years of the dinosaurs in the Triassic and Jurassic periods. Then came the Cretaceous.
That was a turbulent time to live on Earth. The poles were nearly ice-free, causing sea levels to reach great heights. Temperatures were among the hottest the planet has seen in the last 250 million years.
Scientists have blamed the demise of the ichthyosaurs on their inability to keep up with other predators or the loss of key prey species. But a new study by Fischer and his colleagues says neither of these explanations could account for the scale and speed of the die-off.
Writing in the journal Nature Communications, Fischer and his colleagues argue that the unstable climate contributed to the disappearance of ichthyosaurs during a 5-million to 6-million-year window in the Cretaceous known as the Cenomanian stage.
While the rising temperatures and higher seas wouldn't have had a direct effect on the ichthyosaurs, they could have played an indirect role in their decline, Fischer said. The changing climate could have made their food more scarce, or disrupted migratory routes and birthing places, among other consequences.
Despite their variety of body shapes and survival strategies, ichthyosaurs were unable to keep up with all of these changes, the study authors wrote.
They reached this conclusion by comparing the geological record of the Cretaceous to a newly reconstructed history of ichthyosaur evolution based on museum specimens and a review of information in previously published studies.
What became clear was the ichthyosaurs' extinction coincided with global climate shifts and the animals' slower rate of evolution, which left them unable to adapt as their environment changed, the study found.
"They were probably very well adapted to their niches, and their environments were probably pretty stable," Fischer said. "That could have lowered the pressure of natural selection and thus lowered their rates of evolution."
Think of the Cenomanian stage as a transition period.
The changing climate "profoundly reorganized marine ecosystems," the study said, leading into the geologically brief and ecologically weird Late Cretaceous.
During this phase, not only were temperatures soaring and seas rising, but the oceans were being choked of oxygen.
Ichthyosaurs weren't the only species that faced challenges in adapting. Other animals, including cephalopods
and some groups of clams, mollusks and microplankton, saw declines in diversity too.
The downfall of the ichthyosaurs cleared the way for another group of marine reptiles, called mosasaurs, to spread throughout the oceans. They ruled the seas for about 20 million years.
The Late Cretaceous also saw an explosion of recognizably modern sharks and species of bony fish, whose ancestors are still with us today.
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