A Carboniferous Synziphosurine (Xiphosura) From the Bear Gulch Limestone, Montana, Usa

A new synziphosurine,Anderella parvagen. et sp. nov., extends the known range of this group from the Silurian to the Carboniferous and is the youngest known so far from the fossil record. Previously the youngest synziphosurine,Kasibelinurus, was from the Devonian of North America.Anderella parvahas a semi‐oval carapace with pointed genal regions, nine freely articulating opisthosomal segments and a long styliform tail spine. It is the third xiphosuran genus to be described from the Bear Gulch Limestone and its discovery highlights this deposit as containing an unusually diverse xiphosuran biota. It is also the only known locality where synziphosurines occur alongside the more derived xiphosurids. Xiphosurans reached their greatest diversity in the Carboniferous when the xiphosurids began to occupy brackish and freshwater habitats and became dominant over the synziphosurines. The discovery of the only known Carboniferous synziphosurine in marine sediments may indicate their inability to exploit these same environments.