It Looks Like Climate Change Drove These Giant Marine Reptiles To Extinction

It Looks Like Climate Change Drove These Giant Marine Reptiles To Extinction

A hundred million years ago, ichthyosaurs — massive marine reptiles that look like a dolphin mated with a fish — ruled Earth's oceans. But nearly 30 million years before the extinction of the dinosaurs, these majestic predators vanished. It wasn't an asteroid that killed the ichthyosaurs, so what did?

Image Credit: Andrey Atuchin

A new study argues that it was climate change.

The disappearance of the ichthyosaurs has puzzled palaeontologists for years. Last summer, a scientific paper argued that increased competition with other large marine reptiles — including spinosaurus (swimming dinosaurs) and plesiosaurs (long-necked marine reptiles) — pushed the ichthyosaurs species to its demise. But writing in Nature Climate Change today, Valentin Fischer of the University of Oxford has a different take. Rather than ecological warfare, it was a haywire climate that drove the beasts extinct.

Ichthyosaurs were a diverse group of marine reptiles that evolved some 250 million years ago during the Triassic period, from land reptiles that returned to the sea. In a remarkable case of convergent evolution, ichthyosaurs came to resemble dolphins and whales: They breathed air, gave birth to young and were probably warm-blooded. In other aspects of their anatomy, ichthyosaurs looked more like fish.

They're an undeniably fascinating group. But perhaps no aspect of the ichthyosaurs' 160 million-year history is more mysterious than how it ended some 28 million years prior to the asteroid impact that drove 75 per cent of all species extinct, marking the Cretaceous — Tertiary boundary.

To pinpoint factors that could have killed the group off, Fischer and his co-authors conducted a sweeping analysis of ichthyosaur diversity over its entire evolutionary history, along with a dataset of environmental indicators for past climate change. And the researchers found a striking pattern. Variation in the oxygen-18 isotope — which is associated with fluctuating sea level and global temperatures — is tightly associated with high rates of ichthyosaur extinction.

In fact, the researchers pinpointed two distinct bottleneck events beginning 100 million years ago that appear to be related to climate change and, together, seem to have doomed the lineage. It's possible, the researchers say, that climate change led to a cascade of other problems, reducing ichthyosaur food supply and habitat.

"Although many authors have speculated on the causes of the extinction of the ichthyosaurs, this new study presents the most thorough data set to date," Erin Maxwell, the paleontologist whose earlier work suggested ichthyosaurs died out due to direct competition with other marine reptiles, told Gizmodo. "The finding that climatic instability is an important driver of extinction dynamics in these large reptiles is consistent with our understanding of extinction processes — namely that rapid climate change is an ecological stressor that can result in extinctions in all levels of the food chain, but especially among top predators."

The new study represents a small part of a bigger story that's beginning to emerge regarding the late Cretaceous: widespread "ecological reorganisation", during which many species disappeared and others, including sharks and bony fish, rose to dominance. However dramatic the rise and fall of sea monster dynasties seemed at the time, it wound up being a blip in the history books compared with the apocalypse to come, when a six-mile-wide asteroid plunged into the Yucatan peninsula, igniting volcanoes and spelling disaster for most life on Earth.

The End Times of the ichthyosaurs were, as the researchers put it, a "highly peculiar and geologically brief" chapter in the history of our world.

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    Dinosaurs were honking great big things as were lots of lifeforms way back then - the conventional wisdom being the O2 levels were higher.. But my theory is Earth had a much denser atmosphere. .. we know at low pressure humans need more oxygen in the air mix (space flight teaches us this) and at high pressure we need far less O2 - diving teaches us this.. I was taught at school that those big winged dinosaurs could only glide because they were too heavy.. the biggest dinosaurs resided in bodies of water, they couldn't tread the land because they were too massy.

    I hear a story that Venus is so stupidly hot because of 'runaway climate change', but I know how pressure works and it's relatively easy to calculate the temperature of Venus f it had one atmosphere of pressure as we do - and if Venus were stripped of 89 atmospheres, it most certainly would not be hot - that it is currently hot is simply due to the atmospheric molecules being compressed. (ask a physicist what the actual temperature of an air molecule on Earth is.. it's REALLY hot!).. Relating this back to Old Earth it becomes conceivable that if we'd had 4 or 5 atmospheres of pressure back then those winged dinosaurs could easily fly, even I could fly in such dense atmosphere! And the big dinosaurs would have no trouble moving on land.

    I also know there's evidence of polar reversals, suggesting that while we have records of those, we'd probably have few to no records of reformations - where the poles wane but then reform in the same position.. we also do not really know how long these take to occur - nor how weak the magnetic field becomes. With weakened magnetic strength, our magnetosphere falls too, and the solar wind blows off some of our atmosphere. Mars has no effective South pole.. that's where it's atmosphere bleeds away - Maybe Mars' core has cooled a that end. Other planets - even little ones seem to have not have poles flipping and flopping about as far as we know and they have atmospheres- Maybe the Earth's moons gravitation is contributing to occasional slowing the dynamo beneath our feet and throwing the poles out of whack.

    So let's presume there's a particularly big polar shift event in the past, the Earth's magnetosphere wanes, hiccups, starts to reform, wanes again, this drags on for a bit too long and we shed a bit too much atmosphere and bang - not enough air for the vast dinosaurs who evolved under not just a very dense atmosphere, but also an effectively much more oxygen rich one. They die out rapidly, suffocating in the new Earth atmosphere. Plants too suffer the lower effective CO2 levels, but they adapt by simply growing stunted. We know there's far too little CO2 for plants and they certainly grow faster and bigger when you crank up the levels. Soluble O2 and CO2 drops with reduced pressure too, so the largest of the oceanic species will probably go first..

    I just wonder how many times the Earth has left before we loose our atmosphere altogether - will the greenies saver us with windmill farms? Or will it be high tech - This rock was a great place to evolve, but staying here might just prove our undoing.

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