One of the largest shale formations in North America, the Pierre Shale, contains fossilized examples of entire ecosystems known as methane seeps. They formed yet another component of that broad Western Interior Seaway, and they probably functioned much like the ones in our modern oceans. Methane seeps as we know them today begin with methane and hydrogen sulfide gas bubbling up from underground. These chemicals attract gas-hungry bacteria that attract grazers, which then draw larger predators—octopuses today, ammonoids in the Cretaceous.
