COLEOID CEPHALOPODS FROM THE LOWER CARBONIFEROUS OF NORTH AMERICA
Recognition of Paleozoic coleoids rests on the preservation of a mineralized structure called the rostrum that is deposited on the exterior of the outer shell of the phragmocone. The Arkansas and Utah specimens have preserved rostrums, but because of the preservation of the Bear Gulch material, rostrum recognition rests mostly on interpretation of shell impressions in the sediment. Three Bear Gulch specimens preserve a clear impression of a narrow, sharply pointed rostrum that covered the apical end of the phragmocone and are therefore these specimens are considered to be coleoid cephalopods. All have long body chambers. Based upon rostrum shape, phragmocone shape, septal spacing, and body chamber length, the Bear Gulch specimens are unlike the coleoid taxa described from Arkansas and Utah. The discovery of these coleoid cephalopods from the Mississippian of Montana is important evidence that this cephalopod order had already developed significant diversity by the end of the early Carboniferous and that the origins of the Order Coleoidea probable lies in the Devonian.