Seven species of oryctocephalids occur in the Lower to Middle Cambrian boundary strata of the southern Great Basin. These include Oryctocephalites palmeri n. sp. and an Oryctocephalinae species from the uppermost Lower Cambrian (upper Olenellus Biozone); Oryctocephalus indicus (Reed, 1910), Microryctocara nevadensis n. gen. and n. sp., and Oryctocephalites rasettii n. sp. near the base of the Middle Cambrian (lower Plagiura Biozone); and Oryctocephalus primus Walcott, 1886, and Oryctocephalus nyensis Palmer, 1979, from slightly higher strata (upper Plagiura Biozone). Oryctocephalus and Oryctocephalites are emended based on a cladistic analysis of Oryctocephalinae Beecher, 1897.

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...

Twelve ptychopariid species assigned to five genera occur in the Lower-Middle Cambrian boundary interval, Pioche Shale, eastern Nevada. New taxa include one genus, Eokochaspis, and seven species, Eokochaspis nodosa, E. delamarensis, E. metalaspis, E. longspina, Kochina? walcotti, Mexicella antelopea, and M. robusta. Two new biozones, the Eokochaspis nodosa and overlying Amecephalus arrojosensis Biozones, are proposed between the Lower Cambrian Olenellus Biozone and the Middle Cambrian Plagiura-Poliella Biozone. In addition, the three remaining informal shale members of the Pioche Shale are formally named.

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