COLEOID CEPHALOPODS FROM THE LOWER CARBONIFEROUS OF NORTH AMERICA

The oldest accepted record of North American coleoid cephalopods is from the Upper Paleozoic (Lower Carboniferous: Upper Mississippian - Chesterian). Until now only two locations (Arkansas and Utah) have yielded specimens belonging to three genera, two of which are represented by one or two specimens each. Recent evaluation of several cephalopod collections from the Bear Gulch Limestone in Montana, which is slightly younger in age than the Arkansas and Utah coleoid occurrences, has revealed this Lagerstätte contains three specimens that are new and undescribed coleoids. Evaluation of the cephalopods from the Bear Gulch material is difficult, since all the specimens are crushed and the aragonite shells have been mostly removed by diagenesis. Despite this, the remains of mandibles and crop and stomach contents are frequently preserved. Both ammonoids and nautiloids (coiled and orthoconic) are present in the fauna, but this is the first report of coleoids.

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.