Arcuatoceras, a New Genus of Nautiloid Cephalopods from the Early Carboniferous in the Midcontinent of North America

Abstract

A cyrtoconic nautiloid, Arcuatoceras saundersi gen. et sp. nov., from the upper shale member of the Imo Formation (late Chesterian, Early Carboniferous) in north central Arkansas, U.S.A., is described and tentatively assigned to the family Scyphoceratidae. One specimen of the new taxon retains the embryonic conch and the earliest growth stages of the post-hatching shell; this specimen is the first record of the embryonic shell of nautiloid cephalopods having cyrtoconic brevicones in the Carboniferous. The morphology of the shell ornament and the shape of the early shell suggest that the newly hatched animal was an active swimmer after hatching, whereas in later growth stages, the animal probably lived and crawled on the bottom as part of the nektonicbenthonic community. Additionally, some conchs of this new genus show the remains of color patterns preserved as dark gray markings on a lighter gray background. There is considerable variation in the color patterns. One small specimen has conspicuous longitudinal bands and spots on the venter and ventrolateral surface together with a faint chevron on the lateral and dorsolateral parts of the conch. Several larger specimens have only well developed chevrons on the venter, ventrolateral, lateral, and dorsolateral parts of their conchs. This color-pattern variation is interpreted as a change during ontogenetic development rather than as a systematic difference.