UPPER ORDOVICIAN EURYPTERIDS FROM THE WILLIAM LAKE LAGERSTÄTTE, MANITOBA, CANADA – PHYLOGENETIC AND PALEOBIOLOGICAL IMPLICATIONS

Ordovician marginal marine assemblages are variable and may contain unique endemic fossils. Upper Ordovician ( Katian) dolomudstones of the William Lake Lagerstätte, central Manitoba, are yielding exceptionally preserved remains of a wide range of soft-bodied and weakly sclerotized organisms associated with a sparse shelly fauna. These sediments record a shallowing-upward depositional cycle under increasingly restricted and very low-energy conditions within a marginal marine environment. The fauna includes a variety of non-biomineralized arthropods, among them many specimens of two new eurypterid taxa. Occurrences of pre-Silurian eurypterids are extremely limited, with only a dozen species described worldwide. The early evolutionary history of eurypterids is not well established, and any new material from this interval is potentially significant for clarifying basal phylogeny. Preliminary analysis of the available William Lake material reveals one species that does not easily fit into any of the established clades within the Eurypterida. This species has a mixture of features typical of a number of different superfamilies, as well as unique characters not seen anywhere else in the Order. The second species belongs to the Adelophthalmoidea, being the first member of this group prior to the Silurian.The poorly known record of Ordovician eurypterids may be obscuring a more complicated early evolutionary history than previously inferred.