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The Pterocephaliid biomere is a biostratigraphic unit of stage magnitude that includes the most consistently fossiliferous parts of the Upper Cambrian in the Great Basin of western conterminous United States. The thickness of strata containing faunas of this biomere generally exceeds 500 feet. This interval includes units consisting of fine-grained detrital material and of clean carbonate rocks; the carbonate rocks probably represent sediments that composed a broad carbonate bank, and the units of detrital material represent belts of predominantly land-derived detrital sediments that flanked the bank on the east and west. The ptychoparioid trilobite fauna of the Pterocephaliid biomere consists of 112 species representing 51 genera. The sequence of faunas within the Pterocephaliid biomere is divided into five zonal assemblages that can be traced throughout the Great Basin. The assemblages are, in ascending order, the A.phelaspis zone, D•icanthopyge zone, Prehousia zone, Dunderbergia zone, and Elvinia zone. The first three zones, which are valid throughout the Great Basin, may correlate with beds currently assigned only to the Aphelaspis zone in central Texas and the southern Appalachian region. The principal faunal elements of the Pterocephaliid biomere probably invaded the Great Basin region from oceanic regions to the west and annihilated nearly all previously existing faunas of the older Crepicephalid biomere. The invading faunas then evolved in place and were in turn annihilated by a new invading fauna, which terminated the Pterocephaliid biomere. Several possible evolutionary series are described, including one that extends through five species and two genera. Geographic distribution of contemporaneous ornamented species in several genera suggests some correlation between environment and ornamentation in some groups of trilobites. New taxa are Oligometopus contractus n. sp., Oheilocephalus brachyops n. sp., Oheilocephalus granulosus n. sp., Pseudokingstonia exotica n. gen., n. sp., Iddingsia intermedia n. sp., Apachia prima n. sp., Dellea? punctata n. sp., Dunderbergia calculosa n. sp., Dunderbergia brevispina n. sp., Elburgia intermedia n. sp., Irvingella transversa n. sp., H ardyoides mimicus n. sp., Aciculolenus peculiaris n. gen., n. sp., Xenocheilos granulosus n. sp., Terranovella brevis n. sp., Simulolenus quadrisulcatus n. gen., n. sp., Dicanthopyge convergens n. gen., n. sp., Dicanthopyge quadrata n. sp., Dicanthopyge reductus n. sp. Aphelaspis longispina n. sp. Olenaspella paucisegmenta n. sp., Litocephalus magnus n. sp., Stenambon megagranulus _n. gen., n. sp. Stenambon paucigranulus n. sp., Parahousia .~Jllbequalis n. sp., Prehousia diverta n. sp., Prehousia impolita n. sp., Preltousia indenta n. sp., Prehousia pr•ima n. sp., Tumicephalus depressus n. gen., n. sp., Oernuolimbus laevifrons n. sp., Oernuolimbus granulosus n. sp., Strigambitus bilobus n. gen., n. sp., Str-igambitus transversus n. sp., Strigambitus? blepharina n. sp., Pterocephalia? punctata n. sp., Erimanium? brachyamis n. sp., Eriman...

Carbon isotope data from Upper Cambrian sections in three Laurentian basins in northern Utah, central Iowa, and western Newfoundland record a large positive ␦ 13 C excursion (SPICE event) of up to ؉ 5‰. Peak ␦ 13 C ratios are well dated by trilobite collections to the middle of the Steptoean Stage (Dunderbergia Zone) and occur during maximum regression associated with formation of the Sauk II-Sauk III subsequence boundary on the North American craton. Maximum regression was marked by an influx of quartz sand into carbonate-platform settings in all three widely separated basins. In northern Utah, this quartz sand formed a thick sequence known as the Worm Creek Quartzite, which marks a conspicuous interruption of carbonate deposition during the Middle to Late Cambrian in the region. In western Newfoundland, the thickness of the quartz sand unit is much reduced but still marks a brief shutdown of the carbonate factory that is unique to the Cambrian shelf succession of the area. In the central Iowa area of the cratonic interior, an upward-shallowing carbonate succession culminates in cross-stratified trilobite grainstones at the peak of the SPICE in Dunderbergia Zone time, and the lowest point on the relative-sea-level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface.Although it is difficult to determine precisely the departure from baseline ␦ 13 C that marks the beginning of the SPICE excursion in the stratigraphic successions analyzed, our results are consistent with a rise and subsequent fall in ␦ 13 C tracking a major regressive-transgressive event recorded across northern Laurentia. The correlation of a major ␦ 13 C excursion with regression is similar to that described for the Late Ordovician, for which the pattern has been attributed to either increased carbonate relative to terrigenous weathering rates as ice sheets covered up organic-matter-containing silicates at high latitudes or high productivity and organic-carbon burial driven by oceanic overturn. The lack of known Steptoean-age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organiccarbon burial was the likely cause of the SPICE event. We suggest that increased weathering and erosion rates during relative sea-level fall (Sauk II-III) increased the burial fraction of organic carbon in an expanded region of fine-grained siliciclastic deposits in shelf and upper slope environments during the Steptoean.

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