An unusual strophomenide brachiopod association in the Late Ordovician William Lake Lagerstätte, Williams Member of the Stony Mountain Formation (latest Katian), Manitoba

The William Lake site within the Williams Member of the Stony Mountain Formation (Katian, Upper Ordovician) contains an unusual exceptionally preserved fossil fauna, including arthropods and cnidarian medusae. Several shells of the brachiopod that strongly resemble Holtedahlina paraprostrata Jin and Zhan, 2001 are described for the first time from this unit. Forty percent of the shells were found in an unusually close association with phosphatic tube-like fossils, perhaps indicating a symbiotic relationship, although there is not enough clear evidence to definitively determine the nature of this relationship. It was clearly advantageous to grow on or against skeletal material on a soft, muddy shallow marine seafloor, and both the brachiopod and the tube-like fossil may have benefited from the association at different times. Although other species of Holtedahlina have been found in a variety of depositional environments, Holtedahlina paraprostrata has thus far only been found in hypersaline or variable salinity settings, perhaps indicating an ability to tolerate unusual marine conditions, unlike most other rhynchonelliformean brachiopods that are typically stenohaline. Holtedahlina at the William Lake site, like other calcareous fossils, are rare and poorly preserved perhaps reflecting the unusual preservation at the site or representing a real ecological signal.