Beak from the body chamber of an Early Carboniferous shelled longiconic coleoid cephalopod from Arkansas, USA

Abstract

Here we report the discovery of an Early Carboniferous (Late Visean) 3D cephalopod beak displaying significant similarity to the lower beak of Recent coleoids. It was uncovered in a fragmentarily preserved, longiconic shell from the Moorefield Formation in Arkansas, USA. This shell comprises a fractured 29-mm-long body chamber having a maximum diameter of ~14 mm and showing an indistinct pro-ostracum-like structure. The beak-bearing shell could easily have been mistaken for a bactritid or orthocerid if it were not for a coleoid-type, weakly mineralized, evidently organic-rich shell wall which shows a lamello-columnar ultrastructure of a bulk of shell wall thickness and plate ultrastructure of thin outer layer. The specimen is assigned to an as-yet unnamed shelled coleoid of a so far unknown high-level taxonomic group. A partially exposed, 4.0-mm-long portion of the beak is the lower beak in oblique view from its left side. It exhibits fractured anthracite-like black, apparently originally chitin material, helmet-like general shape, broad hood with narrow shallow median groove and small notch posteriorly, pronounced pointed, non-biomineralized upside belt rostrum, high shoulder and about a 90–100 degrees jaw angle. A broad hood and massive rostrum emphasize its similarity to the lower mandible of Recent Vampyroteuthis and signify that its unique, among living coleoids, structure has been existed for at least since Late Visean time (~333 my).