Silurian Encrinurinae (Trilobita) from the central United States

Abstract

Silurian (Llandovery–Ludlow) Encrinurinae from Wisconsin, Indiana, and Illinois include species ofBalizomaHolloway, 1980,CurriellaLamont, 1978,DistyraxLane, 1988,EncrinurusEmmrich, 1844,MackenziurusEdgecombe and Chatterton, 1990a, andNucleurusRamsköld, 1986.Mackenziurus lauriaen. sp. is described from Wenlock strata of the Racine and Sugar Run Dolomites; nine segments in the holaspid thorax are the fewest known for any encrinurid. A square rostral plate is diagnostic ofMackenziurus. New morphological data are provided by Illinois specimens ofBalizoma indianensis(Kindle and Breger, 1904) from the Wenlock–Ludlow Racine Dolomite, and ofCurriella tuberculifrons(Weller, 1907) from the Llandovery Elwood–Wilhelmi Formations.Nucleurusn. sp. is possibly from Llandovery strata in Wisconsin.Distyraxn. sp. occurs in the Brandon Bridge Member (late Llandovery–early Wenlock) of the Joliet Dolomite in Illinois. The type pygidia and possible topotype material ofEncrinurus nereusHall, 1867, confirm assignment to that genus as currently viewed; this reefal species is distinct from the closely related inter-reef speciesE. eganiMiller, 1880, and appears to be distinct from reefalE. reflexusRaymond, 1916. Lectotypes are selected for theseEncrinurusspecies.