December 8, 2009
I love discarded hypotheses for the extinction of the non-avian dinosaurs. Some ideas, such as a global pandemic, sound at least somewhat reasonable, but others seem to have come out of left field. One particular paper, published in the Journal of Research on the Lepidoptera in 1962 by entomologist S.E. Flanders, falls into the latter category.
Flanders saw the “Age of Dinosaurs” as a time when food was unlimited. The planet was clothed with lush forests providing an all-you-can-eat buffet to herbivores and hence plenty of food on the go for predatory dinosaurs. The herbivores stuffed themselves with plants and the carnivores kept the populations of herbivores in check, as so the “circle of life” kept cycling on.
But what if something destabilized this cycle? Flanders proposed that two factors, working together, utterly changed the dinosaur’s world. The first was the evolution of seed-producing plants. Flanders envisioned the world of the dinosaurs as one carpeted in ferns and other archaic plants, and these new kinds of plants provided ample food for caterpillars.
As a result of his research in agricultural science Flanders was well aware of the damage caterpillars could do to plants if their populations were left unchecked. The insatiable hordes could quickly denude an entire forest of its foliage, thus depriving other herbivores of food. This is just what happened at the end of the Cretaceous, Flanders argued. The caterpillars were too numerous and multiplied too quickly, consuming all the plant food before the herbivorous dinosaurs could get any for themselves. And as the herbivorous dinosaurs died out, so did their predators, leaving behind only small reptiles such as crocodiles and turtles that found their food by different means.
This might sound reasonable—animals have to eat to survive after all—but Flanders’ hypothesis suffers from some serious flaws. First, we now know that the group of insects that have caterpillar larvae (the Lepidoptera) probably evolved during the Jurassic, right in the middle of the “Age of Dinosaurs.” If they were such a blight on the Earth’s plants then, why was the extinction delayed? Second, herbivorous dinosaurs did not all feed on trees. Like living herbivorous mammals, different dinosaurs fed on various types of plants, from those available on the ground to the branches high up in trees. Caterpillars would not have attacked all the available plants from the ground up, so it would be expected that at least some types of herbivorous dinosaur would have survived.
Most importantly, though, the extinction at the end of the Cretaceous wiped out more than just the dinosaurs. The spiral-shelled molluscs called ammonites, the sea-going mosasaurs, the flying pterosaurs and many types of small mammals (just to name a few) all disappeared, too. Were caterpillars to blame for all these extinctions, even in the oceans? Of course not. A much more powerful mechanism for extinction was required, one that severely affected life in the sea as well as on land, and at the present is appears that the most powerful extinction trigger was the impact of an asteroid in the area of what is now Central America about 65 million years ago. That, I think, is a much more reasonable hypothesis than an invasion of inchworms.
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