Marine vertebrates of the Pembina member of the Pierre Shale (Campanian, upper cretaceous) of Manitoba and their significance to the biogeography of the Western Interior Seaway

Abstract

The Pembina Member of the Pierre Shale is a black, carbonaceous shale interbedded with seams of bentonite. Lateral continuity of bentonite beds demonstrates a correlation with the Sharon Springs Member of the Pierre Shale and the Baculites obtusus ammonite zone of late Early Campanian age. Lithologic and taphonomic studies suggest that the Pembina was deposited in quiet water, below wave base and that the sediment/water interface was probably anoxic. A large assemblage of marine vertebrate fossils has been collected from the Pembina. These have been salvaged from bentonite mines along the Manitoba Escarpment and are housed in the Morden and District Museum. High numerical abundance and disarticulation of specimens suggest that the Pembina vertebrate assemblage represents an attritional fauna, accumulated over long periods of slow sedimentation rates. No stratigraphic or geographic distributional patterns are evident within the assemblage, and it is treated as a single faunal aggregate. The Pembina vertebrate fauna is described and compared with other Early Campanian marine vertebrate faunas from the Western Interior Seaway. Mosasaurs and birds account for more than half the Pembina fauna, with Platecarpus and Hesperornis being the dominant genera. Diversity within the mosasaur component of the fauna is low, with 83% of the generically identified specimens being Platecarpus. The first North American record of Hainosaurus is reported in the Pembina, and a new species, Hainosaurus pembinensis, is represented. Plesiosaurs (primarily Trinacromerum) are common in the Pembina, and turtles and sharks are rare components of the fauna. Biogeographical hypotheses concerning the distribution of marine vertebrates in the Late Cretaceous Western Interior Seaway are tested by comparing Early Campanian vertebrate faunas spanning the seaway. The following five faunas are considered - the Anderson River, the Pembina, the Sharon Springs, the Niobrara and the Mooreville. The distributional patterns that emerge from these comparisons do not appear to be the result of temporal factors, as identical patterns are observed in a direct comparison of the contemporaneous Pembina and Sharon Springs faunas. These comparisons do not support the hypothesis that local environmental conditions (other than latitude) were of critical importance in determining tetrapod distributions throughout the seaway. The comparisons do support the hypothesis of separate northern and southern biotic subprovinces within the Western Interior Seaway, each with its own distinctive fauna. Latitudinal diversity gradients within marine tetrapod faunas are demonstrated both within the Western Interior Seaway as a whole, and within each biotic subprovince.