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Researchers from the US and Argentina have analysed fossilised leaves and presented a new theory as to why the southern hemisphere recovered faster following the asteroid strike that killed the dinosaurs 66 million years ago.

Ecosystems in North America took 9 million years to recover from the asteroid, whilst in South America, insect life bounced back only after about 4 million years. This is the conclusion of the join US-Argentine research team that has published the results of its study in the journal Nature Ecology & Evolution.

Previous evidence had suggested that the asteroid strike - which killed all non-avian dinosaurs and a large number of other species - had a less severe impact on the southern hemisphere and one theory had argued that this was because it provided a sort of refuge for species. However, this new research points to a different explanation, being that ecosystems recovered much more quickly than in the north.

'This extinction is very important - it is one of the major extinctions in the history of the Earth,' commented lead researcher Michael Donovan of Pennsylvania State University. 'The biodiversity patterns we see today, where things are living, may be related to what survived - so it is important to learn about what was happening around the world at this time.'

To understand why life in the southern hemisphere appeared to fare better following the asteroid strike, the researchers studied fossil leaves for insect damage at a site in Patagonia, Argentina. They analysed 3 646 fossil leaves dating from around a million years before the impact to around four million years after it. The analysis focused on comparing damage produced by the nibbling of insects to that previously seen in fossil leaves from the western interior of North America. 'There aren't many insect body fossils," Donovan explains. 'So we can use insect damage on fossil leaves as a sort of proxy for the diversity of plant-eating insects that were around.'

The leaves in North America showed that the diversity of damage, and thus the variety of insects that were feasting on the leaves, decreased dramatically at the time of the asteroid impact, with insects that fed on just a small selection of plants being the worst hit.

Similar trends were seen in the fossil samples from Patagonia but before and after the asteroid strike, the diversity of damage was much higher. This suggests that Patagonia was rich in a huge variety of insects and the diversity of the leaf damage returned to pre-impact levels more quickly than in North America, taking just 4 million years.

Additional insights were gathered by examining the damage produced by insects that burrow between leaf layers. This analysis revealed that even though the same type of insects existed in Patagonia before and after the asteroid strike, they were unlikely to be of the same species. According to Donovan, this could seem like there was a lower rate of extinction, but it was actually new insects that were making this damage.

Although it is not clear why ecosystems recovered faster in Patagonia, Donovan believes one factor could be its greater distance from the site of the asteroid strike in Chicxulub, Mexico, but this would unlikely be the primary reason. Even though it seemed to have recovered at a quicker rate, this study seems to indicate that diversity in the southern hemisphere was also severely affected by the asteroid impact and was not, as previously thought, to be a 'safe haven' for species.