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Crocodiles have not always been cold-blooded, say scientists

Some prehistoric crocodiles were WARM-BLOODED enabling them to survive the Jurassic cold snap 150 million years ago, study of fossilised teeth shows

  • Ancient ancestors of today’s crocs were actually warm-blooded to survive cold  
  • British researchers analysed oxygen isotope compositions in fossilised teeth
  • At least one crocodile of the extinct metriorhynchid family was warm blooded 
  • Warm blood was key to evolving a dolphin-like body and living in open oceans 
  • Metriorhynchids thrived during a period of global cooling 150 million years ago

Some ancient ancestors of today’s crocodiles were actually warm-blooded, according to a new study.

Palaeontologists from the University of Edinburgh analysed the mineral make-up of fossil crocodile teeth from the extinct metriorhynchid family.

They discovered at least one ancient crocodile ancestor from the metriorhynchid family that wasn’t cold-blooded. 

The results indicate that they could raise their body temperature to stay warm as temperatures fell, in the same way as modern-day birds and mammals.

And it might have helped them thrive during a spell of global cooling around 150 million years ago at the end of the Jurassic period. 

Skeleton of a metriorhynchid, an extinct family of aquatic crocodyliforms - crocodile ancestors

Skeleton of a metriorhynchid, an extinct family of aquatic crocodyliforms – crocodile ancestors

WHAT ARE THE THREE GEOLOGIC ERAS? 

The Mesozoic Era is a the name given to the period from 250 million to 65 million years ago.

The era is divided into three major periods: Triassic, Jurassic and Cretaceous.

The metriorhynchids lived during the Jurassic (201.3 million to 145 million years ago) and Cretaceous (145 million to 66 million years ago) periods. 

Mesozoic was the age of the dinosaurs and lasted almost 180 million years.

The Mesozoic Era followed the Paleozoic Era, during which arthropods, molluscs, fish and amphibians all evolved.

Mesozoic was followed by the Cenozoic Era, during which the continents assumed the configuration that we know today. 

Being warm-blooded was key to metriorhynchids evolving a dolphin-like body – including flippers and a tail fin – and venturing out into the open oceans.

Today’s crocs are cold-blooded and cannot generate their own heat, meaning that they hibernate or go dormant during the colder months. 

‘This discovery helps us better understand these bizarre crocs,’ said Dr Mark Young from the University of Edinburgh.

‘They rapidly changed from animals looking similar to modern long-snouted crocodiles, to ones with flippers, a tail fin and massive, forward-facing eyes.

‘Their transition from land to sea dwellers increasingly mirrors the better-known transformation undergone by dolphins and whales millions of years ago.’

Oxygen levels in the fossil tooth enamel were affected by the animals’ body temperature and measuring them allowed researchers to find out whether they were cold or warm-blooded.

Artist's impression of the sea-loving metriorhynchid. Warm-blooded crocodile ancestors were able to maintain a body temperature higher than their environment - the sea

Artist’s impression of the sea-loving metriorhynchid. Warm-blooded crocodile ancestors were able to maintain a body temperature higher than their environment – the sea

Analysis showed that metriorhynchids could raise their body temperature above their surroundings by using their metabolism to generate heat.

While they were less efficient at heating themselves than most other warm-blooded animals, their adaptability likely helped them survive when temperatures dropped at the end of the Jurassic Period – around 150 million years ago. 

By contrast, teleosaurids, the cold-blooded cousins, struggled to adapt but ultimately survived.

Teleosaurids were cold-blooded and kept warm by sitting in the sun, the same way as modern crocodiles.

They might have struggled to stay warm when sea temperatures fell, which could partly explain why so many died out at the end of the Jurassic Period.

The study has been published in Philosophical Transactions of the Royal Society B

WHAT DO WE KNOW ABOUT THE EVOLUTION OF CROCODILES? 

Of all the reptiles alive today, crocodiles may be the least changed from their prehistoric forebears of the late Cretaceous period, over 65 million years ago 

However, the even earlier crocodiles of the Triassic and Jurassic periods sported some distinctly un-crocodile-like features, such as bipedal postures and vegetarian diets.

Along with pterosaurs – the family of flying reptiles – and their land-based relatives the dinosaurs, crocodiles were an offshoot of the archosaurs, the ‘ruling lizards’ of the early to middle Triassic period.

The earliest dinosaurs and the earliest crocodiles resembled one another a lot more than either resembled the first pterosaurs, which also evolved from archosaurs. 

What distinguished the first crocodiles from the first dinosaurs was the shape and musculature of their jaws, which tended to be much more deadly, as well as their relatively splayed limbs—as opposed to the straight, “locked in” legs of theropod dinosaurs. 

It was only well into the Mesozoic Era that crocodiles evolved the three main traits with which they’re associated today: stubby legs; sleek, armoured bodies, and marine lifestyles. 

 

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