HABITAT OF JUVENILE AMMONITES AT METHANE SEEPS IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

The upper Cretaceous Pierre Shale of the US Western Interior is peppered with cold methane seep deposits. We have studied these deposits from the upper Campanian Baculites compressus and Didymoceras cheyennense zones in Custer County, South Dakota. These deposits contain an abundance of ammonites. The large concentration of ammonites and their light carbon isotopic composition suggest that the animals were living at the seeps. We present additional evidence to support this hypothesis. We document the occurrence of juvenile ammonites belonging to Baculites and Hoploscaphites. Juvenile Baculites are more common than Hoploscaphites. Of the Baculites samples collected, the specimens range in length from a less than 1mm to approximately 2.5cm. The smallest specimens include an individual of a newly hatched Baculites with its ammonitella. All stages of ontogeny of Baculites and Hoploscaphites are represented in the sample. This evidence suggests that ammonites inhabited methane seep environments throughout their ontogeny, and played an integral role in these dynamic ecosystems.