The Sundance Sea did not occur at a single time; geological evidence suggests that the Sea was actually a series of five successive marine transgressions—each separated by an erosional hiatus—which advanced and receded from the middle Jurassic onward.
In addition to fish, belemnites and to an extent ammonites have been found in sediments from the Sundance Sea.
[4] Pantosaurus, a 15–20 foot (4.5–6 m) long cryptoclidid plesiosaur, went after the easier-to-catch fish.
[5] The largest marine reptile in the Sundance Sea was Megalneusaurus, a 25-foot (8 m) long pliosaur similar to Liopleurodon.
[6] Its fossils have been found in Alaska and Wyoming, which were both covered by the Sundance Sea when it was alive.