An ancient, hairy, eight-legged monster—one of first animals to colonize land—may be the oldest known silk-spinning bug, says Cary Easterday, a graduate student in paleontology at Ohio State University. If true, the discovery could mean that several types of arachnids independently evolved the ability to produce silk.
All modern spiders produce silk, which has led some paleontologists to conclude that ancient spiders did as well. When and how this skill evolved is not clear, however. Spiders are thought to have evolved at least 370 million years ago, but silk-producing structures don’t turn up in the fossil record until about 290 million years ago, the age of the first known complete fossil spider. The most ancient preserved spider silk, found sealed in Lebanese amber, is even younger, just 130 to 120 million years old.
Easterday is filling in the evolution of silk-spinning through his studies of a group of spiderlike arachnids called trigonotarbids, which lived from 420 million to 280 million years ago. As part of his masters thesis work, he took a close look at the fourth, or last, pair of legs on a trigonotarbid known as Aphanotomartus pustulatus. The penny-size fossil had been unearthed more than a decade earlier from the Cemetery Hill coal deposit in eastern Ohio, but it had never been studied in detail before. (The same site also yielded the world’s largest known fossil cockroach, a 3.5-inch-long beastie called Xenoblatta scudderi, recently announced by Easterday.)
On the creature’s fourth pair of legs, Easterday observed a distinctive single row of microscopic bumps, or microtubercles. “Each bump represents the base of a hair,” he says. He noted that the location and arrangement of the bumps strongly resemble those of the calamistrum, a structure found on the fourth legs of some modern spiders. “The calamistrum is a row of strong hairs that is used to pull silk from a platelike organ called a cribellum. My evidence suggests that scientists have been looking for the wrong silk-producing structure in fossil arachnids. Rather than looking for spinnerets, the modified extremities on the abdomen from which silk is released in spiders, we should be looking for cribellumlike structures, which would look like a tiny dense patch of microtubercles. They would be difficult to find, but not impossible, so I’m encouraging other fossil arachnid workers to look.” In short, researchers may have been staring at the silk-producing organs on ancient arachnids without recognizing them.
Easterday admits the evidence that his creature was a silk-spinner is not conclusive. The microtubercles on the rear legs might have been cleaning structures used to groom the body, for example. Other small bumps also appear on the Aphanotomartus’s back and elsewhere on the body, but based on their distribution, Easterday guesses these other bumps probably are the bases of hairs that functioned as simple sensory organs. “Since most spiders have bad eyesight, they sense the world around themselves through vibrations in the air and on the ground via the hairs covering their bodies,” Easterday says.
The microtubercles look different, Easterday claims, which makes it likely they really were silk-spinning organs. “The location and arrangement of these microtubercle rows on the fourth walking legs suggest they had a more specialized function,” he says. “Silk-spinning appears to be an adaptation of terrestrialized—that is, land—arthropods. Modern spiders, pseudoscorpions, some insects, most centipedes, some millipedes, and spider mites produce silk. The evidence suggests that other extinct fossil terrestrial arthropods may have spun silk too.”