UPPER MARJUMAN (UPPER CAMBRIAN) SHALLOW WATER TRILOBITE FAUNAS, GREAT BASIN, WESTERN USA

Although the Great Basin is known for rich and stratigraphically important Lower Paleozoic trilobite faunas, large portions of the record remain essentially unstudied and poorly known. Older Marjuman faunas of the Bolaspidella Zone have been well described, but the interval including the Cedaria and Crepicephalus zones has received little attention and is virtually undocumented. Similarly, faunal dynamics at the younger terminal Steptoean and terminal Sunwaptan mass extinctions have received quantitative attention, but those across the terminal Marjuman extinction event (which occurs at the top of the Crepicephalus Zone) have seen little modern investigation.

Extensive calcareous trilobite collections were made from sections through the Big Horse Member, Orr Formation, northern House Range, Utah, members B and C of the Emigrant Springs Formation, Schell Creek Range, Nevada, and the Highland Peak Formation, Highland Range, Nevada. All are from shallow water settings, with the dominant lithologies including lime mudstones, trilobite grain- and packstones, intrarudites, oolites, oncolites, and calcisiltites. Thrombolitic buildups are developed at several horizons in the House Range and Schell Creek Range.

The trilobite faunas are of moderate diversity (5-12 species), and are dominated by Blountia, Crepicephalus, Coosina, and Tricrepicephalus, accompanied by a variety of rarer taxa. Species turnover is rapid through the sections and despite the generally shallow water setting, several distinct generic associations are apparent. Assemblages occurring with buildups tend to be strongly dominated by single effaced species of Blountia. Some horizons show clear evidence of taphonomic sorting, including a preponderance of convex pygidia.

Phylogenetic work in progress suggests that the extent of the terminal Marjuman extinction has been overestimated. Taxa such as Marjumioidea (which includes Crepicephalus, Coosina, and Tricrepicephalus) became extinct in name only. Further, the depauperate post-extinction fauna dominated by "aphelaspidines" is strikingly similar to that which characterizes the interval following the younger terminal Steptoean extinction. In both cases, stage level turnover and family replacement has been greatly exaggerated by taxonomy based on stratigraphic occurrence.