About 66 million years ago, a six-mile-wide space rock struck Earth near a place now called Chixulub. As you may have heard, the dinosaurs (and a whole lot of other things) did not survive.
The resulting devastation is hard to imagine. What did the planet look like the day after? The year after? 100 years later? 1,000? How long was Earth a blackened, horrible place to live? How long was it just sad? We can’t know for certain, but we do know that the destruction was so profound that life did not reach former levels of diversity for millions of years. Think about that for a moment.
The last ice age, generally agreed upon by humans as lasting a heckuva long time, endured for a measly 100,000 years, give or take. In North America, the smoking wasteland that rode the skirts of the asteroid gave way to some sort of life-impoverished void that, although certainly greening and pepping up sooner, did not recover to pre-impact diversity levels for 9 million years. At least, that is, according to a somewhat peculiar proxy: insect leaf damage.
Leaves are good fossil fodder. They are generally flat and somewhat resistant to decay, improving their odds of preservation. The leaves dropped by plants in the millennia following Chixulub tell an interesting story – not by their own abundance and diversity, or at least not entirely by that – but by the damage done by leaf-chewing, skeletonizing, piercing, sucking, mining, gall-forming, and egg-laying insects. The presence or absence of the scars incurred by these animals can tell us about the diversity of herbivorous insects, even when we don’t have their bodies.
Until now, most of the studies of this type have been done in an area referred to by the authors of a new study published today in Nature Ecology and Evolution as “WINA”, or Western Interior North America. WINA is, as most people know, fossil heaven, due to the abundance of ancient rock layers laid bare to the elements by the dry climate and mountainous terrain.
It’s not that there’s anything wrong with fossils from WINA. It’s just that it's in the north, and so few people have looked in the south. Chixulub is in the Yucatan Peninsula of Mexico which, as it is now, was located in the Northern Hemisphere at the business end of the Cretaceous. WINA is relatively close by. But what was going on down under? Was the southern hemisphere a refuge for species – a place where they could avoid the desolation of the asteroid? And did it later serve as a base from which life could recolonize the planet? Some studies have suggested that.
Ocean plankton in the southern hemisphere went extinct at a lower rate than those in the north, and then recovered almost right away. Some Mesozoic plants survived well past the impact in Australia, and pollen data from Patagonia and New Zealand also indicate little plant extinction. On the other hand, marine invertebrate species around Antarctica bit it in large numbers.
Until now, no one has studied plant leaves through time across the K/T boundary in the southern hemisphere, the authors say. But fossil leaves offer an easy way to gauge the diversity of plant-feeding insects. Today, the diversity of insect-feeding damage on tropical rainforest leaves correlates with the diversity of insects causing the damage: more damage, more diversity.
In North America, insect leaf damage -- especially damage caused by insects that tend to specialize on just one or a few plant species like leaf miners and gall-formers -- plummeted after the impact. Diversity remained low for a long time before finally rebounding during a warming period about 9 million years later.
In this study, the scientists classified insect damage on fossil leaves in four locations in Patagonia, Argentina. This is one of them:
They focused especially on leaf mines because of their builders’ host specialization, the rich morphological detail mines provide, and because these were also the focus of leaf studies in WINA (I’m really loving this WINA thing. I think that’s what I’m going to say from now on when people ask where I live.)
Overall, the scientists found more diverse leaf damage in Patagonia both before and after the end-Cretaceous mass extinction – revealing a previously unknown insect diversity hot spot, the authors say. This is particularly remarkable because so many more specimens have been studied in WINA over a longer time period and a larger spatial area, and the results of this single study already top those totals.
However, the south did not escape unscathed. It too suffered a dip in diversity following the end-Cretaceous impact. The diversity of all insect leaf damage decreased by 21.7% between the end-Cretaceous and the earliest Paleogene. However, recovery was, relatively speaking, twice as fast in the southern hemisphere. Damage diversity rebounded by 15.4% from about three-quarters of a million years post-impact to two (below left), reaching near pre-impact levels in only, ahem, two million years.
Leaf mine diversity – again, a more sensitive measure of highly-host specialized and thus more heavily evolved insects -- plummeted 30% between half a million years before the impact and three-quarters of a million post, but approached end-Cretaceous levels within four million years, a good five million years sooner than leaf mines in the north (above right).
When scientists looked very carefully at the mines, they determined that none of the Cretaceous Patagonian leaf miner species seem to have survived the impact, the same pattern seen in WINA. The idea of a southern hemisphere refuge for leaf-munching insects does not seem to be supported by these fossils.
However, following the extinction, recovery and diversification seem to have happened much faster in Patagonia. In WINA, leaf mines are rare and of a single type following the extinction. In Patagonia, on the other hand, diverse leaf mines appeared relatively quickly following the extinction, with at least 11 different types appearing by four million years later.
As a result, these data support the idea that the southern hemisphere recovered much more quickly from Chixulub than the northern. Of course, Patagonia is but one site among many in the souths, but as previously mentioned, pollen data from other southern locations seem to support these conclusions. Was it simply that the southern hemisphere was farther from the impact site, or were some other unknown forces also at play?
Looking out my window at WINA today, it is hard to envision the landscape 65 million years ago in the aftermath of the strike. Perhaps the speed with which the planet started to look presentable would surprise me. But the terrible collision was so profound, evidently, that it erased more than just dinosaurs, annihilating even tiny insect species that drive tunnels in leaves.