After an asteroid wiped out the non-avian dinosaurs 66 million years ago, along with much of the rest of the planet, it took a long time for life to recover. Yet new evidence suggests the recovery was substantially quicker in the Southern Hemisphere than in the North. The evidence lies in insect bites on fossilized Patagonian leaves.
Fossil deposits in North Dakota revealed that the trees that managed to survive the destruction wrought by the Chicxulub asteroid, and the darkness that followed, reaped a reward. For 9 million years after the asteroid strike, their descendants suffered fewer attacks from insects, particularly specialist species. Despite sometimes contradictory findings from neighboring Montana, this has been taken as evidence that even fast-breeding insects are slow to recover when the ecological web is rent asunder.
Until recently, no equivalent deposit was known in the Southern Hemisphere that could be used as a point of comparison. The discovery of extensively preserved plants at several sites in Chubut Province, Argentina, has changed that. Michael Donovan, a student at Pennsylvania State University, examined 3,646 leaves deposited there between 62.2 and 67 million years ago. The results are reported in Nature Ecology and Evolution.
Donovan found that plant-eating insects vanished with the asteroid impact, but new species eventually filled the same ecological niches. There are eight major ways in which insects approach leaf-eating, with more subtle variations among these, so even when the species that attacked a leaf has not been preserved, their diversity can be seen in the holes they made.
This leaf from Las Flores, Patagonia, shows the recovery of leaf-eating insects in the region by 62.3 million years ago. Michael Donovan
It was 4 million years before the variety of leaf-eaters had recovered to the pre-extinction levels seen in the late Cretaceous. We might not think of leaf-eaters as particularly important, but their tendency to specialize in a particular host species makes them a valuable proxy measure of the health of the wider ecosystem.
Four million years to recover is a soberingly long time for humanity, which is in the middle of another mass extinction, but it is less than half as long as it took in North America. Moreover, as impressive as Donovan's leaf samples sound, it represents a fraction of the numbers surveyed in North America, yet the diversity in the south was much greater both during the Cretaceous and after recovery.
Paleontologists have suspected for some time that the last mass extinction was less damaging south of the equator than in the north. However, this paper cuts against the theory that insect species were more likely to survive in the Southern Hemisphere. In the case of the leaf-eaters at least, extinction appears to have been total. On the other hand, new species were quicker to adapt to take advantage of what must have been an astonishing abundance of unchewed leaves.
Other studies are also pointing to the idea that southern ecosystems recovered more quickly from the calamaty that ended the Cretaceous, but it remains something of a mystery why this was the case.
The Palacio de los Loros in Patagonia is bleak today, but 62 million years ago, it was a rich cornicopia for leaf-eating insects. Peter Wilf