Contributions to Zoology, 67 (3) 155-185 (1998) SPB Academic Publishing bv, The Hague Palaeo- and archaeostomatopods (Hoplocarida, Crustacea) from the Bear Gulch Limestone, Mississippian (Namurian), of central Montana Ronald+A. Jenner, Cees+H.J. Hof& Frederick+R. Schram Institute for Systematics and Population Biology, University of Amsterdam, P.O. Box 94766. 1090 GT Amsterdam, The Netherlands Keywords: Malacostraca, Hoplocarida, Stomatopoda, phylogeny, Bear Gulch Abstract 1983). This fauna in fact has yielded the largest and most diverse collection of Carboniferous ver- The palaeostomatopod crustacean Bairdops beargulchensis tebrates in the world. Prominent among the Bear Schram & Horner, 1978 (Malacostraca, Hoplocarida) from the Gulch invertebrates are some ten species of mala- Mississippian Bear Gulch Limestoneis now seenas a taxonom- costracan crustaceans, five ofwhich are Hoplocar- ic composite that arose from the confusionof specimens of two ida. The of the fauna consists of extensive distinct rest an hoplocarid species. These species are herein described of as the palaeostomatopod Bairdops beargulchensis Schram & array fish, conodonts, molluscs (cephalopods, and Horner, 1978 and a new species ofarchaeostomatopod, Tyran- gastropods bivalves), brachiopods, annelids, xi- nophontes acanthocercus. Tyrannophontes acanthocercus is phosurans, sponges and various unidentified, enig- quite distinct from the Pennsylvanian archaeostomatopod T. & matic groups (e.g., see Melton, 1969; Schram theridion from the Essex fauna (Mazon Creek), with which it Factor & is sim- Horner, 1978; Feldmann, 1985; Conway was originally compared. Bairdops beargulchensis very ilar to the Mississippian palaeostomatopod, B. elegans, from Morris, 1990). the fauna. A The Bear Gulch Limestone beds the Scottish Glencartholm previously proposed syn- are part of of with is therefore re- onymy B. beargulchensis T. theridion Bear Gulch Member of the Heath Formation in the jected. A preliminary restudy of the archaeostomatopods T. Big Snowy Group of central and eastern Montana. theridion and Gorgonophontespeleron, and the palaeostomato- Other limestone beds and black shales comprise pods Perimecturus and P. reveals some rapax parki, newly the of the Bear Gulch Member recognized characters of those taxa and suggests some novel remaining part (Fac- interpretations of hoplocarid evolution. A cladistic phylo- tor & Feldmann, 1985). The Bear Gulch Limestone genetic analysis of the Hoplocarida including the Paleozoic crops out on Potter Creek Dome (Fig. 1), approxi- forms resolves the higher level relationships ofthe hoplocarids mately 50 km southeast of Lewistown in Fergus (palaeostomatopods, archaeostomatopods, aeschronectids, and Montana. The County, exact age and unipeltatans). stratigraphic relationships of the Bear Gulch Limestone have been the subject of some debate (see Williams, Introduction 1981 for a summary), but the fossils ofPotter Creek Dome strongly suggest a late Chesterian (upper- The Bear Gulch and most The Gulch palaeo- archaeostomatopods Mississippian) age (Fig. 2). Bear of extensive invertebrate fauna of are part an the Limestone was deposited in a low-energy, shallow, late Mississippian (Namurian) Bear Gulch Lime- tropical marine environment, and the excellent stone of central Montana.This Carboniferous Kon- preservation of the Bear Gulch fossils is perhaps servat-Lagerstdtte preserves a near-shore marine the result of high sedimentation rates combined & Factor & with low levels the sediment-water community (Schram Horner, 1978; oxygen at in- In Feldmann, 1985). quality of preservation and terface (Williams, 1983; Factor & Feldmann, species diversity, it is comparable to the Solnhofen 1985). Limestone of the Jurassic of Bavaria (Williams, The Bear Gulch fauna, the Middle Pennsylvani- Downloaded from Brill.com10/09/2021 11:38:13AM via free access 156 R.A. Jenner et al. - Fossil stomatopods from the Bear Gulch Limestone of a Carboniferous chronofauna and as such ex- hibit striking similarity in both ecologic structure and taxonomic composition (Schram, 1981). The pervasive ecologic and taxonomic stability of the near-shore crustacean communities at these three spatio-temporally distinct localities qualify them as the invertebrate analogue of a vertebrate chrono- fauna (Olson, 1966). Briggs & Clarkson (1990) subsequently indicated that ecologic stability, as represented by the trophic structure (based on crus- tacean feeding types; Schram, 1981) ofthe various localities could not be maintained unambiguously. The maintenance of taxonomic stability through time, however, remains essentially unquestioned. The faunas at the different localities share genera or even species. Crangopsis eskdalensis is part of the Bear Gulch and Glencartholm faunas and Be- Fig. I. Map showing the extent ofthe Bear Gulch Beds at Potter lotelson magister is present in both the Bear Gulch Creek Dome, Montana (modified from Williams, 1983). and Mazon Creek faunas. As their a consequence of exceptional quality, an (Westphalian) Essex assemblage of the Mazon data obtained from the study of Kons ervat-Lager- Creek region of Illinois, and the Viséan (Middle stätten should be approached with special care. Mississippian) Glencartholm fauna from Scotland Diversity studies can be distorted by the use of data represent the three major Carboniferous Konser- from these kinds of faunas due to the large contrast vat-Lagerstdtten (see Allison & Briggs, 1991 for a in information content between the Lagerstdtten more complete list of Phanerozoic Konservat-La- and more typical fossil outcrops (Briggs & Clark- These well known for The of fossils gerstätten). Lagerstdtten are son, 1990). preservational quality their exceptional preservation, which is reflected from Lagerstdtten can also strongly influence tax- both in the exquisite quality of the fossils and their onomic practice when individual fossils with dif- remarkable quantity, both in absolute numbers of ferent preservational histories are to be compared. organisms and taxonomic diversity (Briggs & Problems arise when the taxa under comparison Clarkson, 1989, 1990). exhibit profound preservational differences. Per- One of the first endeavours to study evolution- sonal preferences for the splitting of taxa or lump- from ary patterns utilizing data Konservat-La- ing them into a single taxon easily divide workers in led to Schram’s ob- into of gerstdtten a systematic way, camps polarized interpretation. Similar servation of a Carboniferous near-shore faunal problems have arisen in the taxonomic history of continuum (Schram, 1979b). This Laurentian Car- the Bear Gulch tyrannophontids. boniferous faunal continuum denotes the similari- ty in ecological and taxonomic composition of the near-shore marine fauna from Europe to North Taxonomic and phylogenetic background of the America, extending from at least the Viséan through Hoplocarida the Westphalian. Briggs & Gall (1990) showed that in the concept of a continuum transitional environ- The classification of the hoplocarids has been a ments (near-shore to terrestrial) and the associated story of continuous change since the early 1960s fauna can be extended from the late Paleozoic into (Schram, 1986). Prior to 1962, only a limitedarray the Mesozoic. The crustacean communities of the of extant and fossil stomatopods occurred within Bear Gulch fauna, the Essex fauna of the Mazon the order Stomatopoda Latreille, 1817. However, Creek and the Scottish Glencartholm fauna are part at present the subclass Hoplocarida Caiman, 1904 Downloaded from Brill.com10/09/2021 11:38:13AM via free access Contributions to Zoology, 67 (3) - 1998 157 is divided into three separate orders: Stomatopoda Latreille, 1817, Aeschronectida Schram, 1969, and Palaeostomatopoda Brooks, 1962. The order Sto- matopoda is further subdivided into two suborders; Unipeltata Latreille, 1925 and Archaeostomatopo- dea Schram, 1969. The aeschronectids, palaeosto- matopods and archaeostomatopods are represent- ed exclusively by late Paleozoic forms. least 412 within Presently, at species five super- families and 109 genera constitute the extant uni- peltatan stomatopods (Manning, 1995). Their un- fossil record ambiguous goes back to the early Jurassic (Sinemurian). Except for the extinct Me- sozoic family Sculdidae Dames, 1886, all unipel- tatans can be accommodated within modern fami- lies. The taxonomy of the unipeltatan stomatopods has been in flux for at least the past thirty years. Much has been achieved since the Treatise on In- vertebrate Paleontology (Holthuis & Manning, recorded the 24 1969) genera of unipeltatan sto- matopods that were known at the time. Since then, substantial contributions to the description of ex- tant and extinct unipeltatans have been and are be- ing made. These developments, however, have not been paralleled by investigations into the phyloge- netic relationships within the stomatopods (Hof, in Fig. 2. Stratigraphic section of the mid-Carboniferousrocks of central Montana Potter Creek press). as exposed on Dome (modified after Williams, 1981). In 1962, H.K. Brooks erected the order Palaeo- to accommodate the Perimec- stomatopoda genera turus Peach, 1908 and Archaeocaris Meek, 1872. rus in his “Mysid-group” which, combined with his Brooks listed the following features as character- “Euphausid-group”, comprised the schizopods (La- istic of the Palaeostomatopoda; “Carapace shallow treille, 1817). Of his six initially described species of with articulated rostrum, four posterior thoracic Perimecturus, currently