The Endocerida and Actinocerida were important
contributors to early Paleozoic, mainly Ordovician, marine faunas. Both cephalopod orders are well represented by straight (orthoconic) genera, some which produced large shells, some in excess of 3 or 4 meters. What distinguished them, above all else, is their siphuncle. The picture at the right shows a segment of an endocerid siphuncle on top, with nearly straight edge and sectioned endocones and of an actinocerid below showing expanded segments along the margin and parietal deposits within.
The picture is a composit of two specimens housed at the New Mexico Museum of Natural History and Science in Albuqerque, New Mexico.
