Did Insects Kill the Dinosaurs?

By now, scientists have a pretty good idea of what conditions were like in the Cretaceous period, which started about 135 million years ago, and came to a sudden end 70 million years later, with the death of the dinosaurs. Or rather, they think they do — but two new sets of research results suggest there's a lot more to learn.

The first has to do with the period's cataclysmic close. In lots of people's minds, the mystery of what killed the dinosaurs and other species — paving the way for the rise of mammals — was solved a couple of decades ago: a giant asteroid or comet slamming into the Earth, resulting in a dust cloud that shrouded the sun, cooled the planet dramatically and killed off plants and animals wholesale. It's a compelling story, but plenty of scientists never completely bought it. The dinos died pretty quickly, they admit, but not quite abruptly enough to be explained this way. So alternate theories — the dinosaurs succumbed to allergies, from the rise of flowering plants, or to world-shaking volcanoes in what's now India, or to disease — have always bubbled around the periphery of the conventional wisdom. We wrote about one of these hypotheses a couple of months ago.

Now the disease theory has gotten another boost, in the form of a book titled What Bugged the Dinosaurs, from the Princeton University Press. Authors George and Roberta Poinar (George is a zoologist at Oregon State University and a former World Health Organization consultant on infectious disease) specialize in ancient insects preserved in amber (a key plot element in the movie Jurassic Park) and also in fossilized dinosaur poop. Among other things in their lode, they've found ticks, nematodes, biting flies and all sorts of other nasties, including intestinal parasites, dating back to the Cretaceous period. From some of the insects, the Poinars have extracted microbes that cause leishmania and malaria — evidently new pathogens back then, against which dinosaurs wouldn't have had much resistance.

The authors aren't arguing that the dinos all died in a massive epidemic; rather, the constant wear and tear of illness weakened the dinosaurs so that other catastrophes, like comets and volcanoes, could have finished them off. Still, the Poinars couldn't resist a bit of made-for-Hollywood drama. One great quote from the book: "The largest of the land animals, the dinosaurs, would have been locked in a life-or-death struggle with [insects] for survival."

The other recent challenge to conventional Cretaceous wisdom comes from a paper in the journal Science, published Thursday. It's pretty certain from many lines of evidence that the world was much hotter then (which is why a post-comet cold snap would have been pretty tough on the dinosaurs). During a period called the Turonian, about 90 million years ago, things got especially toasty: In some places, during what's often called the "super-greenhouse" years, the ocean's surface temperature approached 100 degrees F, and alligators thrived in the Arctic.

So, how come there were massive glaciers in Antarctica at the time? Paleo-climate experts have seen hints of this oddity before, but the new Science paper nails it down much more firmly. Andre Bornemann, of the Scripps Institute of Oceanography, along with several colleagues, got their information by analyzing the amount of the isotope oxygen-18 in foraminifera, tiny, shelled sea dwellers that thrived at the time. It turns out that when water evaporates from the sea but doesn't return (implying that it's trapped up on land somewhere, frozen), the ratio of oxygen-18 to oxygen-16 in seawater changes (O-18 is heavier, so it evaporates less). The foraminifera aren't picky; they just incorporate oxygen into their shells, in whichever form.

What researchers found was a stretch of a few hundred thousand years during which foraminifera shells were unusually rich in oxygen-18, suggesting the presence of glaciers. Though changes in ocean temperature can also alter the oxygen balance, sea-bottom temperatures don't vary much no matter what's happening up top, yet the bottom-dwelling foraminifera still exhibited an oxygen imbalance, implying that the ice effect was more likely. Nobody can explain how you can have glaciers in a superhot world. But then, nobody can really explain how the world got quite that hot in the first place.

Taken together, the hothouse glaciers and the sickly dinosaurs suggest a conclusion that should serve nicely as the new conventional wisdom about the paleontological past: Don't take conventional wisdom too seriously.