One of the enduring mysteries of paleontology, the demise of a highly successful group of dolphin-like marine reptiles called ichthyosaurs that flourished in Earth's seas for more than 150 million years, may finally have been solved.
Scientists on Tuesday attributed their extinction 94 million years ago to a combination of global warming and their own failure to evolve swiftly enough.
The comprehensive research undercut previous notions that ichthyosaurs had been in decline for tens of millions of years and had been outcompeted by other predators such as the fearsome ocean-going lizards called mosasaurs that were just arriving on the scene.
The study showed large mosasaurs in fact appeared only after ichthyosaurs went extinct.
"We found ichthyosaurs were very diversified during the last part of their reign," said paleontologist Valentin Fischer of Belgium's University of Liege, noting that several species with various body shapes and ecological niches existed although ichthyosaur evolution had become relatively stagnant.
"We find that the extinction was abrupt, not gradual," added University of Oxford paleontologist Roger Benson.
Ichthyosaurs had arisen about 248 million years ago, becoming key players in marine ecosystems while dinosaurs ruled the land.
With a streamlined, dolphin-like body, they were fast, efficient, air-breathing swimmers with muscular flippers and vertical tail flukes like sharks. They had unusually large eyes to spot prey like fish and squid in deep or turbid waters. They bore live babies rather than lay eggs.
Some measured less than one metre long, but others, like Shonisaurus, which lived about 210 million years ago, had reached about 21 metres.
The researchers thoroughly examined the ichthyosaur fossil record, reconstructing the group's evolutionary diversity, and scrutinised evidence of climate change coinciding with their extinction.
Earth had warmed rapidly, approaching the hottest times of the previous 250 million years, triggering strong fluctuations in sea levels and temperatures. For a time large swathes of seafloor became depleted in the oxygen necessary for animals to live.
Fischer said those changes likely altered ichthyosaur migratory routes, food availability and birthing places.
Other marine creatures, including squid relatives and reef-building clams, had also suffered major losses.
The research was published in the journal Nature Communications.
