Fossil evidence for the origin of spider spinnerets

Abstract

Silk production from opisthosomal glands is a defining characteristic of spiders (Araneae). Silk emerges from spigots (modified setae) borne on spinnerets (modified appendages). Spigots from Attercopus fimbriunguis (Shear, Selden & Rolfe, 1987^1^), from Middle Devonian (386 Ma) strata of Gilboa, New York were described in 1989^2^ as evidence for the oldest spider and the first use of silk by animals. Slightly younger (374 Ma) material from South Mountain, New York, conspecific with A. fimbriunguis , includes spigots and other evidence which elucidate the evolution of early Araneae and the origin of spider silk. No known Attercopus spigots, including the original specimen^2^, occur on true spinnerets but are arranged along the edges of plates. Spinnerets originated from biramous appendages of opisthosomal somites 4 and 5; while present in Limulus , no other arachnids have opisthosomal appendage homologues on these segments. The spigot arrangement in Attercopus shows a primitive state prior to the re-expression of the dormant genetic mechanism which gave rise to spinnerets in later spiders. The inability of Attercopus precisely to control silk weaving suggests its original use as a wrapping, lining or homing material.