ARTICULATED CAMBRIAN (GUZHANGIAN) TRILOBITES FROM THE WEEKS FORMATION, UTAH: REVISION AND REASSESSMENT OF THE SPECIES PROPOSED BY CHARLES WALCOTT

The Weeks Formation is the uppermost formation of the Guzhangian Stage (Miaolingian Series) within the northern House Range of Millard County, western Utah, and is primarily exposed at two localities, Notch Peak and North Canyon. The Weeks occurs only in the House Range, overlying the Wheeler and Marjum formations and underlying the Orr Formation of the Furongian Series. The Weeks Formation is a fossiliferous, silty limestone and the “Upper” Weeks is renowned for its exceptional preservation of soft-bodied marine fauna. The trilobite fauna here is commonly found as fully articulated specimens with many being the only articulated examples available for their genus. Despite their importance and having been widely cited in the literature, however, the Weeks Formation trilobites have not seen any revision since their initial description.

Charles Walcott was the first to discover and photographically describe the common trilobite species of the Weeks Formation in his original 1916 publication. While Walcott's illustrations were excellent for their time, new images are needed to properly interpret the species. While some subsequent work has been done on Weeks Formation trilobites, most of these later studies have focused primarily on the descriptions of new and rarer species. Here, the most commonly found and cited trilobite species from the Weeks Formation are revised based on modern photographs of the original historical types and newly collected material from North Canyon. These new data provide the basis for a reexamination of various sets of material from elsewhere in Laurentia that have been assigned to the Weeks Formation species, and inform new investigations of the phylogenetics of the groups involved. Species revised on the basis of new articulated exoskeletons include Cedaria minor (Walcott 1916), Dresbachia amata (Walcott 1916), Menomonia semele (Walcott 1916), and Weeksina unispina (Walcott 1916).