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What killed off the vicious ichthyosaur?


Well before the dinosaur-killing meteorite, the ocean's top predators – ichthyosaurs – up and died. New research has uncovered the culprits that worked together to force the ichthyosaurs' demise: sluggish evolution and climate change. Viviane Richter reports.


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The last ichthyosaurs. Two ichthyosaurs (Pervushovisaurus bannovkensis) wander in a middle Cenomanian ecosystem with high sea level and sea temperatures, reefs (Ichthyosarcolites, Hippurites), neoselacian sharks and acanthomorph fishes (Aipichthyoides).
Andrey Atuchin

The dolphin-like ichthyosaur was king of the ocean, reigning at the top of the food chain. But some 90 million years ago, well before the mass extinction that wiped dinosaurs off the face of the planet, they died out.

Why the ferocious “sea dragon” disappeared has been a mystery – until now.

An international team of scientists ploughed through the fossil record and discovered slow evolution and climate change combined to form a lethal one-two punch that the top predator simply could not survive.

They published their findings in Nature Communications.

One theory for the ichthyosaur’s early demise was competition – researchers believed the number of different ichthyosaur species dwindled towards the end of their reign, leaving them easily outcompeted for resources by other marine hunters.

The authors of the current study surveyed ichthyosaur fossils from other papers as well as museum collections. They reconstructed the evolution of the ichthyosaur over the final 120 million years of their lifetime, from the Late Triassic to the Late Cretaceous.

The authors discovered ichthyosaurs were, in fact, “highly diverse” before their extinction – comprising many species with different body shapes, which appeared to feed on different prey. This blew the competition theory right out of the water.

On the other hand, the data showed the ichthyosaur’s evolution rate – the rate at which they could develop new body plans – slowed drastically.

And when digging through climate data of the time, the team discovered the ichthyosaur’s extinction corresponded to gradual but significant changes in sea temperature and level.

The planet looked very different all those millions of years ago. Scientists believe there was no polar ice, and seas were higher, warmer and contained little oxygen.

While the rising temperatures may not have directly affected the ichthyosaurs, says lead author Valentin Fischer, a geologist at Belgium’s University of Liege, “related factors such as changes in food availability, migratory routes, competitors and birthing places are all potential drivers, probably occurring in conjunction to drive ichthyosaurs to extinction”.

The authors also believe the ichthyosaur extinction was part of a much larger event. During the ichthyosaur’s demise, some microplankton, ammonites and reef builders also took a hit, while other organisms flourished.

The team are now further investigating this turnover, which they say gave “rise to the highly peculiar and geological brief Late Cretaceous marine world”.

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Viviane Richter is a freelance science writer based in Melbourne.
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