FUNCTIONAL MORPHOLOGY OF THE EOCENE COLEOID BELOSEPIA

Belosepiids are Eocene coleoids related to sepiids (cuttlefish), with an endoskeleton similar to that of sepiids but developed in distinctive ways. A hallmark of belosepiids is the prong: a curved postero-dorsal elongation of the endoskeleton that is comparable to but an exaggerated version of the spine of sepiids. Dorsal elongation of the prong contrasts with antero-posterior elongation of most coleoid endoskeletons. This novelty was grounds for suggesting an unusual body form or life habits for belosepiids. However, most belosepiid specimens are very incomplete and show only the prong and adjacent areas. Better specimens show the endoskeleton has a large thin proostracum in front of the phragmocone that is comparable to the proostracum of the sepiid cuttlebone, but much exaggerated. The belosepiid proostracum is also similar to phragmoteuthid and some Mesozoic teuthid proostraca. This large proostracum indicates a belosepiid animal of much larger size than previous reconstructions. The smooth surface of the prong contrasts with the irregular rough surface of skeleton enclosing the phragmocone. In cross section, the prong shows annulations and regular growth increments. A few specimens show evidence of repair to breaks to the end of the prong. The prong may have been covered with mantle tissue only part of the time, being exposed at other times. A vertical fissure divides the prong into two lateral halves that are weakly joined, indicating the halves were secreted by separate lobes of mantle tissue in a manner similar to secretion of secondary shell on the outer surface of cypraeid gastropod shells.

The belosepiid prong at the posterior end of the animal is a novel feature for coleoids. It is composed of solid carbonate, a high mass feature poorly positioned for use as a ballast mechanism for swimming activity. Belosepiid fossils are generally found in deposits of nearshore sandy or muddy sediment and were probably largely benthic dwellers. The prong may have aided in digging into sediment, a life habit of some modern sepiids. The prong, being largely inert, could not have been sensory in itself but might have had sensory tissue attached to it. Being part of the endoskeleton, the prong was probably not useful for defense. Breakage patterns on some of the belosepiid fossils suggest that they were eaten, perhaps by fish.