Fossil spiders (Araneae) from the Eocene Kishenehn Formation of Montana, USA

Abstract

The Kishenehn Formation contains a diverse assemblage of terrestrial arthropod fossils and represents a tropical ecosystem in North America during the Eocene (46 Ma). Most of the fossils are small insects, but spiders have also been recovered and recently made available for study. The fossil spiders are small and preserved as compressions in a thinly laminated oil shale, but abundant setae and other morphological features are preserved in relatively high detail. Here, the fossil spiders are described for the first time and include new species of orbweaving spiders from the family Araneidae and a ground-dwelling spider from the family Gnaphosidae. Most of the spiders in the assemblage resemble extant orbweaving spiders like Neoscona: Greenwaltarachne pamelae gen. & sp. nov. and Consteniusi leonae gen. et sp. nov.. A single gnaphosid, distinguished by widely separated cylindrical spinnerets, is likely a juvenile. A single male spider belonging to Araneomorphae is too poorly preserved to discern at family level. The fossils described here are the oldest fossil spiders recovered from Montana thus far.