Orthoceres was one of the first members of the class Cephalopoda to appear in the fossil record. Cephalopods are fast moving predatory marine invertebrates. Living members of this group include squid, octopi and nautiloids. Orthoceres occupied an elongated straight cone shaped shell. This extinct group of ‘uncurled’ cephalopods first appeared during the Ordovician Period. A group of these evolved into a semi-coiled form which is generally believed to be an older (earlier) form which eventually gave rise to the curled ammonites (also extinct) and the nautiloids which still inhabit the oceans of the world today.