THE WEEKS FORMATION LAGERSTATTE (HOUSE RANGE, UTAH): A UNIQUE INSIGHT INTO THE EVOLUTION OF SOFT-BODIED METAZOANS DURING THE LATE CAMBRIAN
The Weeks Formation comprises a 300-m-thick sequence of thin-bedded lime mudstones, wackestones, and grainstones with variable amounts of shale. It represents an open-shelf marine environment and a late stage in the filling of the House Range Embayment. Two types of exceptional preservation of fossils are recognized in the Weeks Formation: 1, pyritization of major morphological details with subsequent coatings of chlorite and oxidization to iron oxides; and 2, phosphatization of digestive tracts of arthropods.
The Weeks Formation yields taxa characteristic of Cambrian Epochs 2-3, but also includes a mix of taxa representative of older and younger Konservat-Lagerstätten. The biota can be considered as a transitional one that lends support to the hypothesis of more gradual evolution of metazoans during the early Paleozoic. The Weeks Formation has yielded a rich trilobite assemblage, abundant inarticulate brachiopods and sponge spicules, and rare hyolithids and echinoderms. The upper Weeks has also produced non-biomineralized and weakly biomineralized organisms. This exceptionally preserved biota is dominated by arthropods and worms (paleoscolescids, priapulids), but it also includes sponges, a possible comb jelly, and other organisms. The arthropod fauna is rich in aglaspidids, a group best known from Guzhangian-Furongian deposits. These are associated with animals characteristic of Cambrian Series 2-3 (Anomalocaris, leanchoiliids) and Ordovician (Tremaglaspis) biotas. The importance of the non-trilobite arthropod fauna is also highlighted by the presence of several species of uncertain affinities.