"TIL of Tullimonstrum Gregarium, aka the Tully Monster. An animal so strange, that despite having thousands of fossils, we don’t know whether it was a vertebrate or an invertebrate. To add to the strangeness, fossils of this creature have only been found in one fossil site in the world."
They stay on ice in the ocean for most of every year=marine. They retreat to mainland areas during the summers when the ice melts.