Transitional Spider Fossil

Via ScienceDaily:

...The research focuses on fossil animals called Attercopus fimbriunguis. While modern spiders make silk threads with modified appendages called spinnerets, the fossil animals wove broad sheets of silk from spigots on plates attached to the underside of their bodies. Unlike spiders, they had long tails.

The research findings by Paul Selden, the Gulf-Hedberg Distinguished Professor of Invertebrate Paleontology in the Department of Geology at KU, and William Shear, the Trinkle Professor of Biology at Hampden-Sydney College, were published this week in the Proceedings of the National Academy of Sciences.

Selden and Shear first discovered the fossils almost 20 years ago. At that time the specimens were thought to be the oldest spider fossils known, dating back to the Devonian Period, about 380 million years ago. Unearthed in upstate New York, the fossils were among the first animals to live on land in North America.

New finds near the same location, in Gilboa, N.Y., caused the paleontologists to reinterpret their original findings. The new fossils included silk-spinning organs, called spigots, arranged on the edges of broad plates making up the undersides of the animals. The researchers identified parts of a long, jointed tail not found in any previously known spider, but common among some of the spiders’ more primitive relatives.

“We think these ‘tailed spiders’ represent an entirely new kind of animal, not known before from living or fossil examples.” Shear said. “They were more primitive than spiders in many ways, and may be spider ancestors.” Besides having tails and spinning silk from broad plates, the animals also seem to lack poison glands.

Selden added, “This new information also allows us to reinterpret other fossils once thought to be spiders, and this evidence suggests these Uraraneida, or pre-spiders, existed for more than 100 million years, living alongside real spiders, which evolved later.”
The paleontologists think that Attercopus developed silk-spinning spigots in order to line burrows, make homing trails and possibly to subdue prey, but were not capable of making webs because of the limited mobility of the spigots. True spiders may have arisen when the genetic information for certain appendages was “turned back on” and the spigots moved onto them. The appendages became the modern spiders’ spinnerets, which can move freely and create patterned webs.