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Tracking the diversity of turtles over 230 million years is not only revealing how they responded to the asteroid that killed the dinosaurs, but could also give us clues as to how they might respond to increasing pressures in the future, including climate change.
Turtles are a group of reptiles found on every continent except Antarctica. Including sea turtles, terrapins and tortoises, they are comprised of species that live in both salt and fresh water, as well as those that make a living on the land.
Having been around for hundreds of millions of years, turtles have weathered multiple catastrophic mass extinctions, including the one that killed the dinosaurs. But their future is threatened by yet another, as humans continue to radically alter the environment.
Dr Terri Cleary, who completed her PhD at the Museum and is now at the University of Birmingham, led a study looking at how the diversity of turtles has changed over time.
'We looked specifically at non-marine turtles,' explains Terri. 'Tracking their diversity from when they first appear in the Triassic around 230 million years ago, to the end of the Oligocene about 23 million years ago.
'By using this long time period we could look at how they changed through time and whether we can track what is going on with turtles in response to the climate.
'Studies such as this are really important because in order to understand what is going to happen in the future, you need to understand what has already occurred.'
The paper has been published in the journal Palaeontology.
Turtles are known to have originated around 230 million years ago during the Triassic Period, but what exactly they evolved from is still debated.
Some scientists argue that turtles are more closely related to lizards and snakes, while others believe that turtles should in fact be placed within the archosaurs, a group that contains crocodiles, pterosaurs, dinosaurs and birds.
'We still disagree on where they fit in the family tree,' says Terri. 'They probably originate from some sort of reptile that gradually expanded its ribs out and those became its shell. But we don't have that many informative transitional fossils.'
Turtles survived a mass extinction at the end of the Triassic Period which killed large swathes of organisms and led to the rise of the dinosaurs. As this was happening, turtle diversity was low.
It was not until the Cretaceous, beginning 145 million years ago, that the number of turtle species began to ramp up. It remained high right up until another mass extinction, one of the most dramatic the world has ever seen. This catastrophe, thought to have been caused by an asteroid hitting Earth, killed the dinosaurs and thousands of other animals - but not only did turtles survive, their diversity seemed to increase.
Terri says, 'Some turtles species did go extinct at this point in time, but many didn't. The number of species that survived is really good when compared to groups like the dinosaurs or lizards.
'In the next five million years there was a massive peak in the amount of turtle genera.
'There is something about turtles that meant a lot of them survived this catastrophic event, and immediately following diversified really quickly.'
It is important to note that this pattern was only observed in turtle species found in North America, probably because the fossil record there is particularly strong and continuous.
Interestingly, it does show some parallels with another group that survived and then rapidly diversified after the asteroid strike: crocodiles.
'We speculate this might be down to their habitat,' explains Terri. 'Aquatic turtles might not have been as heavily affected by the extinction event because there is some evidence suggesting that crocodiles are also not as affected as some of the other groups. But it is very hard to test these sorts of things.'
Turtles continued to have a relatively high diversity following the asteroid strike despite the cooling climate.
'Turtles are ectotherms, so they rely on temperature for a lot of their ecology. It even determines how many are born male or female,' says Terri. 'But the study shows that turtles don't seem to be as affected by cooling global temperatures as we'd expect.'
This might be because factors other than temperature may also influence turtles, something which could be relevant to the conservation of modern species as the climate crisis continues to change average temperatures around the world.
'It seems from other studies that they might be more tied to the availability of water, with some influence from temperature,' Terri continues. 'Aspects such as rainfall - which determines where water basins are - are potentially more important to their survival.
'We do have to remember, though, that there is not going to be any strict one-to-one comparison between the past and present, because what is happening today is much more rapid, the present climate is very different to the deep past, and there are a lot of new factors.'
These include the simple fact that humans are reducing the amount of available habitat for turtles. While in the past, turtles may have been able to migrate if conditions became unfavourable, they are less likely to be able to do so in the modern era.
But studies such as this one could help us focus on what may be the best hope for the protecting turtles, such as making sure that wetlands and river deltas are preserved, as this may be the key to helping them survive.