The basic form of the spider must be a good one, because it has remained nearly unchanged since Pennsylvanian time, about 300 million years. There are plenty of variations on the theme, of course, and today more than 40,000 species have been described.
Spiders are arthropods, a group whose name means jointed leg and which includes insects, scorpions, centipedes, and trilobites as well as spiders.
The first true spiders probably evolved from Devonian spider-like arachnids called Attercopus, found in the Devonian Gilboa Fossil Forest of New York which is dated to about 380 million years ago. That ancestor could make silk, but recent work by Paul Selden and his colleagues suggests that it didn’t have spinnerets and did not weave webs. This means that as far as we know, the first true spiders developed by the Pennsylvanian Period, 300 million years ago or so. You may recall that there is a pretty dramatic gap in the fossil record of insects during Mississippian time, 320 to 360 million years ago, and that includes spiders as well. We don’t have much evidence for spider evolution during that time.
But there is abundant evidence for spiders in Pennsylvanian coal beds and the concretions like those we talked about at Mazon Creek, Illinois, on July 11. Many Pennsylvanian spiders, at least at a glance, are practically indistinguishable from modern varieties.
—Richard I. Gibson
Papers by Paul Selden
Pennsylvanian spider drawing by James Dwight Dana (public domain). The original specimen is about an inch across.