Climate change was what triggered the age of the dinosaurs over 250 million years ago – not mass extinction of other species – according to new research.
For a long time, the explosion in diversity of reptiles during the end of the Permian and start of the Triassic period has been attributed two of the biggest mass extinction events, around 261 and 252 million years ago.
These events were thought to have wiped out reptiles’ competition, allowing them to flourish and diversify into a dizzying variety of abilities, body plans and traits.
However, a new study by Harvard researchers challenges this theory, by reconstructing how the bodies of ancient reptiles changed and comparing it against millions of years of climate change.
It shows that the diversification of reptiles not only started years before these mass extinction events, but that they were directly driven by what caused them in the first place – rising global temperatures due to climate change.
Artistic reconstruction of a massive, big-headed, carnivorous erythrosuchid (close relative to crocodiles and dinosaurs) and a tiny gliding reptile about 240 million years ago. The erythrosuchid is chasing the gliding reptile and it is propelling itself using a fossilised skull of the extinct Dimetrodon (early mammalian ancestor) in a hot and dry river valley.
‘We are suggesting we have two major factors at play,’ said Tiago R. Simões, a postdoctoral fellow in the Pierce lab at Harvard University and lead author on the study.
‘Not just this open ecological opportunity that has always been thought by several scientists but also something that nobody had previously come up with.
‘Climate change actually directly triggered the adaptive response of reptiles to help build this vast array of new body plans and the explosion of groups that we see in the Triassic.
‘Basically, rising global temperatures triggered all these different morphological experiments – some that worked quite well and survived for millions of years up to this day, and some others that basically vanished a few million years later.’
The findings are based on photos, scans and analysis of over 1,000 reptile fossils in more than 50 museums around the world.
The researchers used this data to create an ‘evolutionary time tree’ revealing how early reptiles – including the forerunners of crocodiles and dinosaurs – were related to one another, when their lineages first originated, and how fast they were evolving.
They then combined this with global temperature data from millions of years ago, to examine correlations between the two.
They found that diversification of reptile body plans started about 30 million years before the Permian-Triassic extinction, making it clear these changes weren’t triggered by the event as previously thought.
Moreover, rises in global temperatures, which started at about 270 million years ago and lasted until at least 240 million years ago, were followed by rapid body changes in most reptile lineages.
Graphic showing the evolutionary response from reptiles to global warming and fast climatic changes. The most intensive period of global warming coincided with the fastest rates of evolution in reptiles, marking the diversification of reptile body plans and the origin of modern reptile groups
Some reptiles evolved to live in water so they could cool down more easily, such as the plesiosaur (pictured), a giant, long-necked marine reptile once thought to be the Loch Ness monster
For instance, some of the larger cold-blooded animals evolved to become smaller so they could cool down more easily, while others evolved to live in water for the same reason.
The latter group included some of the most bizarre forms of reptiles that would go on to become extinct such as a giant, long-necked marine reptile once thought to be the Loch Ness monster, a tiny chameleon-like creature with a bird-like skull and beak, and a gliding reptile resembling a gecko with wings.
It also includes the ancestors of reptiles that still exist today like turtles and crocodiles.
Smaller reptiles, which gave rise to the first lizards and tuataras, went on a different path than their larger reptile cousins. Their evolutionary rates slowed down and stabilised in response to the rising temperatures.
The researchers believe this was because the small-bodied reptiles were already better adapted to the rising heat, since they can more easily release heat from their bodies compared to larger reptiles when temperatures got hot very quickly all-around Earth.
The findings have implications for today, as temperatures continue to rise.
The Harvard researchers pointed out that the rate of carbon dioxide released into the atmosphere today is about nine times what it was during the Permian-Triassic mass extinction 252 million years ago.
Reptiles underwent a massive explosion in diversity during the Triassic, leading to the appearance of crocodiles (pictured), lizards and turtles
Some animals have already started shape-shifting to cope with weather-linked stresses.
Recent research by the University of Sheffield found birds are adapting to global warming by developing bigger beaks, which help keep them cool.
Meanwhile, Australian scientists revealed elephants and rabbits are coping by turning into ‘Dumbo’ and ‘Bugs Bunny’ – they are growing bigger ears.
Elephants use them as fans, while an extensive network of blood vessels contract in rabbits to cool them down.
‘Major shifts in global temperature can have dramatic and varying impacts on biodiversity said co-author Professor Stephanie Pierce, of Harvard University in the US.
‘Here we show that rising temperatures during the Permian-Triassic led to the extinction of many animals, including many of the ancestors of mammals, but also sparked the explosive evolution of others, especially the reptiles that went on to dominate the Triassic period.’
The researchers are now planning to investigate the impact of environmental catastrophes on evolution of organisms with abundant modern diversity, such as the major groups of lizards and snakes.
The study is published in the journal Science Advances.
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