Pre-Late Wisconsinan Pleistocene biota from southeastern Michigan, U.S.A.

Fossiliferous sediments underlie the Late Wisconsinan Mill Creek and Fisher Road tills at the Mill Creek site northwest of Port Huron, Michigan. Fragmentary remains of 5 fish and 11 mammal taxa occur with 39 taxa of molluscs, 6 of ostracodes, and a pine-spruce pollen assemblage. While the fish are wide-ranging, the mammals include distinctly northern elements such as Dicrostonyx sp., Lemmus sp., Mictomys borealis, and Microtus xanthognathus. The molluscs include a mixture of cold (Vertigo alpestris oughtoni and V. modesta) and warm (Lioplax sulculosa) taxa. None of the vertebrates have been found at Michigan sub-till sites before, and Lioplax is new to Michigan, fossil or living. Vertigo hannai is known only as a Pleistocene fossil. sent the age of the biota is indeterminate. The stratigraphy indicates a Middle Wisconsinan or greater age. A finite 14C age of 48.3 ± 0.8 ka (QL-1215) was later supplemented by a TL date of 57 ± 9 ka, but perhaps as old as 300 ka. Two clusters of amino acid allo-isoleucine/isoleucine values obtained from six genera of molluscs are interpreted to represent the co-occurrence of early Sangamonian (AIle/Ile values ranging from approximately 0.15 to 0.25) and Illinoian (AIle/Ile values ranging from approximately 0.25 to 0.35) mollusc fossils at the Mill Creek site. These results highlight the need for more reliable Quaternary dating methods.