A new biota including lightly sclerotized and soft-bodied organisms occurs in finely laminated argillaceous dolomites of late Llandoverian age in Waukesha County, Wisconsin. This discovery fills a gap between well known Cambrian and Devonian Konservat Lagerstatten. The biota is dominated by arthropods. A dalmanitid is the most numerous of 13 genera of trilobites; the crustaceans include phyllocarids and ostracods; the chelicerates are represented by the earliest well preserved xiphosure and the fauna includes a possible marine uniramian. The earliest representative of the enigmatic class Thylacocephala, and at least three arthropods of uncertain affinity are also present. There are at least four worm taxa including a possible leech and a papillate annelid. The locality has also yielded a conodont animal, Panderodus. Graptolites and conulariids are common, but echinoderms, brachiopods, bryozoans, corals and molluscs are extremely rare or absent. The unusual composition and exceptional preservation of this assemblage indicates that the biota lived and died in environments rarely represented in the Silurian fossil record.