Fossils found in a South Dakota cliff have revealed that ancient, free-swimming shelled creatures called ammonites were permanent residents around a methane seep at the bottom of the inland sea that divided the North American continent more than 65 million years ago.
Methane and other chemicals, such as hydrogen sulfide, leak from the ocean floor at so-called cold seeps, providing the basis for a food chain and an oasis for life on the seafloor.
Researchers had assumed that ammonites, extinct relatives of squid and nautiluses, simply passed through these communities at the bottom of the ancient sea. However, an examination of the fossils found around a 74-million-year-old seep indicated that the animals spent their entire lives there. In addition to analyzing the chemistry of the shells, the researchers found fossils of both adult and juvenile ammonites.
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