Non-trilobite arthropods from the Silver Peak Range, Nevada

Journal of Paleontology, Jul 2003 by Waggoner, Ben

ABSTRACT-Two non-trilobite arthropods are described from the Emigrant Formation (Lower Cambrian-Lower Ordovician) in the Silver Peak Range, Esmeralda County, Nevada. A Middle or Upper Cambrian "arachnomorph" arthropod with a phosphatic exoskeleton has been noted in previous faunal lists, but has not been previously described. This fossil is here named Quasimodaspis brentsae gen. et sp. nov. Q. brentsae belongs in the Aglaspidida, a close outgroup to the true chelicerates; this is the second report of an aglaspidid from the Great Basin. Esmeraldacaris richardsonae gen. et sp. nov. is a newly discovered arthropod from the lower Ordovician, from beds transitional between the Emigrant Formation and the overlying Palmetto Formation. It is a survivor of an early arthropod lineage that does not belong in any extant taxon, but which may also include the Ordovician Corcorania and the Cambrian Mollisonia.

INTRODUCTION

IN 1960, DURING the geologic mapping of Esmeralda County, Nevada, J. P. Albers found some fossil arthropods in the Emigrant Formation (mapped as Middle to Upper Cambrian) in the type area of Emigrant Pass in the Silver Peak Range, between the towns of Dyer and Silverpeak. The fossils were placed in the United States Geological Survey collections and identified as resembling Malaria or Emeraldella, Middle Cambrian "trilobitoid" genera described from the Burgess Shale. Identified as such, they have been included in published faunal lists (Albers and Stewart, 1972; Robinson et al., 1976) but have not been described or figured. Fieldwork between 1999 and 2002 has yielded additional specimens of this arthropod. Although most are fragmentary, enough near-complete specimens have been found that the morphology is accurately known. Additional work in 1999 and 2000, in what has been mapped as the upper Emigrant Formation, has yielded a second non-trilobite arthropod; associated fossils indicate a Lower Ordovician age for this fossil. The purpose of this paper is to describe these fossils formally.

LOCALITY AND STRATIGRAPHY

All localities are in the northern part of the Silver Peak Range, within what has been mapped as the Emigrant Formation, in or near the type area at Emigrant Pass. In general, the Emigrant Formation represents a condensed outer shelf sequence of pelagic and hemipelagic sediments (Ketner, 1998; McCollum and Sundberg, 2000). The Emigrant Formation has Lower Cambrian Olenellus trilobites in the basal 1.5 meters; overlying trilobite assemblages range from early Middle Cambrian to late Upper Cambrian (Albers and Stewart, 1972; Beaver et al., 2000). Albers and Stewart divided the Emigrant Formation into three members, informally named (lowest to highest) the limestone and siltstone member, the shale member, and the limestone and chert member. However, extensive faulting in the Silver Peak Range has obscured the stratigraphy and made correlation difficult. A complete section of the Emigrant Formation does not exist in the type area, where the formation is both faulted intraformationally and in fault contact with the underlying Mule Spring Limestone (Lower Cambrian) and overlying Palmetto Formation (Ordovician) (Robinson et al., 1976). Adding to the difficulty is the fact that several lithologies are repeated throughout the formation.

The arthropod specimens from Emigrant Pass, described in this paper as Quasimodaspis brentsae gen. et sp. nov., come from a thickly bedded, clastic-rich dolomitic unit, ranging from dark red to pink, orange and yellow in color. The unit contains both siliceous and carbonate grains up to 2 mm in diameter, and is overlain by a thick sequence of partially metamorphosed, unfossiliferous "pencil" shales. Albers and Stewart (1972) recorded similar lithologies, exposed at several localities, within the limestone and chert member of the Emigrant Formation, which would place Quasimodaspis in the upper part of the formation. Identifiable fossils from the limestone and chert member are Upper Cambrian (Albers and Stewart, 1972), but the only other fossils directly associated with Quasimodaspis are poorly preserved inarticulate brachiopods (Fig. 2.7), which have not been identified. The arthropod fossils were originally identified as "questionable Middle Cambrian" on the basis of their resemblance to Burgess Shale taxa (Albers and Stewart, 1972). However, as will be shown in the systematic description, Quasimodaspis is not a Burgess Shale-type arthropod, but an aglaspidid. Most aglaspidids are Upper Cambrian in age, although a few are Middle Cambrian. The available evidence seems to favor an Upper Cambrian age for Quasimodaspis, but there is not enough evidence at this time to date Quasimodaspis precisely.

The other arthropod described here is Esmeraldacaris richardsonae gen. et sp. nov. This arthropod comes from a unit consisting of several hundred feet of fine-grained, platy, clastic-rich limestones interbedded with pale red to buff siltstones, referred to as the "platy limestone unit" for brevity. The "platy limestone" unit overlies, apparently conformably, a cliff-forming unit of interbedded limestone and chert with occasional flat-pebble conglomerates. The top of the platy limestone unit is not preserved; the unit is in fault contact with the overlying graptolite-bearing shales of the Palmetto Formation.