Ancient Arachnids May Have Spun Silk Like Modern Spiders

Newswise — Geologists at Ohio State University have found evidence of silk spinning structures on the fossilized body of a long-extinct relative of modern spiders, one that lived 55 million years before the first dinosaurs.

The 300-million-year-old penny-sized creature, called Aphantomartus pustulatus, is a trigonotarbid -- part of an ancient group of arachnids that were among the first animals to colonize land. The burly eight-legged predator had a hard, segmented exoskeleton like a modern beetle, and scientists never suspected that it could spin a web -- until now.

The finding -- based on the remarkably well-preserved arrangement of tiny bumps called "microtubercle rows" along the creature's hindmost legs -- could shed light on the early evolution of arachnids, silk use, and life on land.

"If trigonotarbids did spin silk like a spider, they had the right structures in the right places to do it," said Cary R. Easterday, a master's degree student in geological sciences at Ohio State. "I suspect that other microtubercles located elsewhere on the body may have served as silk-producing structures, but I have no hard evidence yet."

Alternatively, the bumps could be cleaning structures, or specialized hairs that triggered fight-or-flight responses.

"I want to be clear that these 'microtubercle rows' are not conclusive evidence of silk-spinning in trigonotarbids," Easterday cautioned. "But the evidence is very intriguing."

Trigonotarbids, such as these ones preserved in a coalmine in eastern Ohio, coexisted with early spiders, Easterday explained. Early insects were probably their main food source.

Scavengers and bacteria normally destroy the thin, delicate exoskeletons of arachnids and insects when they die, but something about the chemistry of the fossil site preserved the fine details of these creatures, Easterday said.

One trigonotarbid fossil from the 7-11 Mine in Ohio was so detailed that when Easterday viewed it under a microscope, he found tiny bumps, or microtubercles, all over the body, including a row of bumps running down parts of both rear legs. These rows looked very much like a "calamistrum," a structure found in some modern spiders that is used to spin silk and is located near the same part of the body.

"The find suggests that silk-spinning has been around for a long time," said Conrad Labandeira, a paleontologist at the Smithsonian Institution in Washington, D.C., and an adjunct professor of geological sciences at Ohio State.

"Trigonotarbids were predators, so it's possible that they used silk to trap prey. Or they could have lined burrows with it, as a means of insulation or waterproofing, as modern spiders do," he added.

Though the site recently yielded the largest-ever fossil cockroach, Easterday called the trigonotarbid discovery the more important of the two.

"We all know that spiders and some insects, such as moths, produce silk, but to my knowledge, silk-spinning has never been demonstrated in an extinct fossil group. This would be a first," he said.

This evidence suggests that spiders and trigonotarbids may be more closely related than previously suspected. It could also suggest that silk-spinning evolved independently many times among arachnids, which means evidence for silk-spinning could one day be found in other extinct arachnids, too.

What was special about this area of Ohio that it preserved fine details? Easterday explained that environments with little or no oxygen, such as a lakebed, could do the job.

"Most likely, these arachnids died in or near an ancient lake and sank to the bottom where they were eventually buried," he said.

Still, William Ausich, a professor and former chair of geological sciences at Ohio State, was surprised that the site yielded complete trigonotarbid remains.

"Insects and arachnids are fairly delicate," Ausich said. "Often scientists find bits and pieces of insects, but not the whole thing."

Scientists believe the ancient climate of Ohio was changing around 300 million years ago, from wet tropical conditions to wet-dry seasonal conditions, which would have affected the lives of the plants and animals as well as how they were preserved.

Easterday obtained the trigonotarbid fossils in 1999 from Gregory McComas, an avid fossil collector and geology graduate of Youngstown State University in Ohio. McComas discovered the fossil site in 1979, and named it the "7-11 Mine" because it is located at the intersection of Ohio State Routes 7 and 11.

Easterday is currently completing his master's thesis, which details the trigonotarbids as well as other fossil plants and animals from the site.