THE WEEKS FORMATION LAGERSTATTE (HOUSE RANGE, UTAH): A UNIQUE INSIGHT INTO THE EVOLUTION OF SOFT-BODIED METAZOANS DURING THE LATE CAMBRIAN

Cambrian Lagerstätten offer extraordinary insight into the early evolution of metazoans and their organization into complex ecosystems. Recent discoveries in the Ordovician question the traditional view of an abrupt transition between the Cambrian and Paleozoic Evolutionary Faunas. This emerging picture of a more gradual evolution of metazoan communities during the early Paleozoic stresses the need for more data on Cambrian Series 3 to Furongian Series non-biomineralized animals. Here we present some preliminary results of our investigations of the Weeks Formation Konservat-Lagerstätte (Series 3: Guzhangian Stage, Cedaria Zone) of the House Range, Utah.

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.