THREE WINDOWS: LATE ORDOVICIAN LAGERSTäTTEN IN MANITOBA, CANADA

Ordovician lagerstätten are rare globally, but Manitoba has three sites with preserved soft tissue. The Cat Head and William Lake biotas occur in the Williston Basin, while the Airport Cove biota is in the Hudson Bay Basin. The Cat Head, of early Maysvillian age, is older than the others, which are Richmondian. All three occur in fine-grained dolostones. The unusual preservation may have resulted from primary dolomite deposition or early dolomitization in lagoon to tidal flat or supratidal environments, with magnesium-mediated microbial and abiotic interactions.

The Cat Head fossils, in the Cat Head Member, Red River Formation, include algae, sponges, conulariids, brachiopods, possible hydrozoans, nautiloids, trilobites, and dendroid graptolites. Some are remarkably preserved; the soft algae are among the best anywhere in the Early Paleozoic. Algae and some other fossils are preserved in part as carbon- or iron-rich films. The lithology and associated biota are consistent with deposition below storm wave base, though a restricted lagoon environment has also been suggested.

The William Lake and Airport Cove sites represent distinct depositional environments, but share biotic elements: eurypterids, xiphosurids, algae, and large problematic tubes. Together, these sites provide unique information about tropical shoreline communities prior to the Late Ordovician extinction.

The William Lake biota occurs in the Williams Member, Stony Mountain Formation. Thin-bedded dolomudstones, representing restricted low-energy conditions, contain common medusae (jellyfish). Medusae and articulated arthropods occur in homogeneous mud layers or lenses; both fossils and lithology suggest rapid deposition. Medusae are preserved as sparry dolomite slightly enriched in iron and silica, surrounded by degraded pyrite. Many of the eurypterids are preserved in a flexed lateral position, possibly due to lateral ecdysis.

Airport Cove fossils are in rocks tentatively assigned to the Churchill River Group. Laminated calcareous dolostones, deposited under more open circulation, contain scolecodonts and common noncalcified algae (seaweeds). Macrofossils are preserved with pyrite, as carbonate, and as organic matter. Eurypterid sclerites are commonly disarticulated, but organic cuticle retains exceptional detail.