COLD METHANE SEEPS AS AMMONITE HABITATS (Invited Presentation)

Ammonites occur in many habitats ranging from nearshore well-oxygenated water to offshore nearly dysoxic basins. One of the most unexpected habitats is cold methane seeps. These habitats are characterized by the emission of methane-rich fluids and host a wide variety of organisms including lucinid bivalves, crinoids, echinoids, and ammonites. In the last few years we have explored the cold methane seep deposits in the Upper Cretaceous (Campanian-Maastrichtian) Pierre Shale of the U.S. Western Interior. These seeps occurred at depths of less than 100 m and peppered the otherwise uniformly muddy sea floor. We argue that ammonites lived at the seeps and did not float into the sites after death. Many pieces of evidence support this interpretation. First, ammonites are abundant in the seep deposits and are rare or absent in the surrounding shale. Second, in those ammonites in which sexual dimorphs can be recognized, both dimorphs are present in the same seep deposit. Third, all stages of ontogeny are present, ranging from newly hatched specimens to adults. Fourth, the ammonites at the seeps bear lethal and sublethal injuries suggesting that the ammonites formed part of a complex, integrated ecosystem. Fifth, the seep deposits contain ammonite jaws and even associated hook-like structures, all of which are very delicate, and could not have been transported into the site. Sixth, the carbon isotopic composition of well-preserved ammonite shells is consistently lighter in specimens from seep deposits than in those from age-equivalent non-seep sites. Thus, although ammonites were mobile animals, once they stumbled onto a methane seep with its rich food resources, they may have stayed there the rest of their lives.