Palaeo- and archaeostomatopods (Hoplocarida, Crustacea) from the Bear Gulch Limestone, Mississippian (Namurian), of central Montana

Abstract

The palaeostomatopod crustacean Bairdops beargulchensis Schram & Horner, 1978 (Malacostraca, Hoplocarida) from the Mississippian Bear Gulch Limestone is now seen as a taxonomic composite that arose from the confusion of specimens of two distinct hoplocarid species. These species are herein described as the palaeostomatopod Bairdops beargulchensis Schram & Horner, 1978 and a new species of archaeostomatopod, Tyrannophontes acanthocercus. Tyrannophontes acanthocercus is quite distinct from the Pennsylvanian archaeostomatopod T. theridion from the Essex fauna (Mazon Creek), with which it was originally compared. Bairdops beargulchensis is very similar to the Mississippian palaeostomatopod, B. elegans, from the Scottish Glencartholm fauna. A previously proposed synonymy of B. beargulchensis with T. theridion is therefore rejected. A preliminary restudy of the archaeostomatopods T. theridion and Gorgonophontes peleron, and the palaeostomatopods Perimecturus rapax and P. parki, reveals some newly recognized characters of those taxa and suggests some novel interpretations of hoplocarid evolution. A cladistic phylogenetic analysis of the Hoplocarida including the Paleozoic forms resolves the higher level relationships of the hoplocarids (palaeostomatopods, archaeostomatopods, aeschronectids, and unipeltatans).