Early and Mid Cambrian trilobites from the outer‐shelf deposits of Nevada and California, USA

Abstract

ABSTRACT. A latest Early Cambrian and earliest Mid Cambrian polymeroid trilobite fauna, consisting of 16 genera and 25 species, is reported from the peritidal deposits of the Mule Spring Limestone and outer-shelf deposits of the Emigrant and Monola formations of Nevada and California. The Mid Cambrian fauna includes a new genus, Tonopahella, and four new species, T. gold®eldensis, Oryctocephalus americanus, Onchocephalites claytonensis, and Syspacephalus variosus. The peri-Gondwana species Oryctocephalus orientalis, Oryctocephalites runcinatus, and Paraantagmus latus are recorded in Laurentia for the ®rst time.KEY WORDS: Lower/Middle Cambrian Boundary, international correlation, trilobites. L I T T L E is known about the early Mid Cambrian polymeroid trilobites in the outer-shelf environments of Laurentia. Taxonomic work on these assemblages is limited to Rasetti's (1951Rasetti's ( , 1957 McCollum and Sundberg (1999). This paper expands upon these previous papers by reporting a moderately diverse polymeroid trilobite fauna from two sections in the basal shale member of the Emigrant Formation at Split Mountain and Gold®eld Hills, Esmeralda County, Nevada, and a third section in the basal Monola Formation in the northern Saline Range, Death Valley National Park, Inyo County, California (Text-®g. 1). This fauna, consisting of 11 genera and 18 species, spans three biozones, the Eokochaspis nodosa, Amecephalus arrojosensis, and Oryctocephalus indicus. This paper also reports on the Lower Cambrian trilobites from the uppermost Mule Spring Limestone and basal metre of the Emigrant Formation at the Split Mountain section. This limited fauna consists of seven olenelloid and ptychopariid species.The occurrence of both endemic and pandemic species allows a direct correlation of early Mid Cambrian faunas of Laurentia with the peri-Gondwana region. The basal 20 m of the Emigrant Formation consists of a mudrock facies containing a few thin, argillaceous limestone beds, in a well-exposed, accessible, and continuously fossiliferous section located 1 km north-east of Split Mountain, in Clayton Ridge, Esmeralda County, Nevada (McCollum and Sundberg 1999). This section occurs in a mining area, where trenching has exposed the non-resistant mudstone facies, and although the fossils are compacted, they are well preserved. The section is on land managed by the Bureau of Land Management, and no permits are required for collecting. (2000) at this locality, occurs within a mudstone, beginning at a 15-cm-thick, silty limestone, and ending at a 15-cm-thick, concretionary, bioclastic limestone, in an interval from 9´85 to 15 m above the base of the Emigrant Formation. A 3-m-thick interval of dark, laminated clayshales occurs above the 15-m concretionary horizon, and includes two oryctocephalid species in the lower half, which are assigned to the Oryctocephalus indicus Biozone. The Gold®eld Hills section (Text-®g. 4) is also on land administered by the Bureau of Land Management, and is accessible by vehicle. This section was originally de...

Cambrian Stratigraphy of San José de Gracia, Sonora, Mexico: El Gavilán Formation, a new lithostratigraphic unit of middle Cambrian open shelf environment

“…El Gavilán Formation has a thickness of 233 m. This unit has abundant trilobites, inarticulate and articulate brachiopods, sponge spicules, chancellorids, and hyolithids, among others. (Palmer and Halley, 1979) in California, and with the Emigrant Formation located in Groom Range, Nevada (Sundberg and McCollum, 2003). Both localities have the Mexicella mexicana Zone and they were deposited in an open shelf environment.…”

The Henson Gletscher Formation, North Greenland, and its bearing on the global Cambrian Series 2–Series 3 boundary

“…Nevertheless, Palmer (1964) discussed the possible presence of a pygidium which he described as "genus and species undetermined 4". This would place the species into close proximity with Poliella denticulata Rasetti, 1951 at the same time as lending it and its accompanying species an aspect of the upper Plagiura-Poliella faunas in the traditional sense (see McCollum 2003 andSundberg 2007 for biostratigraphic details). With regard to the Greenland record, Blaker & Peel (1997) already noted that a possible associated pygidium, which they described as "corynexochid genus and species undetermined A", was found in the same sample from the Freuchen Land section as the Stephenaspis?…”

“…A novel approach for the definition of a GSSP for the base of Series 3 and Stage 5 concerned a species referred to Oryctocephalus indicus. Although a key species, it had not played a major role in Lower-Middle Cambrian boundary discussions prior to its proposal as a suitable marker for the base of a Middle Cambrian series (Yuan et al 1997, Sundberg et al 1999, Peng et al 2000, McCollum & Sundberg 2002, Sundberg & McCollum 2003, Sundberg 2009.…”

Small shelly fossils from the Montezuman–Delamaran of the Great Basin in Nevada and California

“…These highly fossiliferous, mixed carbonate-siliciclastic successions have been extensively investigated for their trilobite and brachiopod faunas (e.g., Rowell 1966Rowell , 1977Rowell , 1980Rowell and Henderson, 1978;Palmer and Halley, 1979;Sundberg and McCollum, 1997, 2003aHollingsworth 2005Hollingsworth , 2011aStreng and Holmer, 2006;Sundberg, 2011;Webster, 2011a). However, other faunal elements, especially small shelly fossils (SSFs), are generally unconsidered.…”