DID ANCESTRAL DECABRACHIA POSSESS A PRO-OSTRACUM IN THEIR BODY PLAN?

Abstract

A pro-ostracum is commonly seen as the result of a ventrally opened body chamber. An outline is given of present knowledge about the morphology and occurrence of pro-ostraca in different coleoid groups. It turns out that most fossil Decabrachia lack a pro-ostracum and that the presumed pro-ostracum of Naefia and Groenlandibelus is more likely a dorsal growth allometry of a long and tubular body chamber and merely the result of an opened body chamber. The existence of a "true" pro-ostracum is therefore most probably restricted to phragmoteuthids, belemnitids and diplobelids. The sepiid clade demonstrates that reduction of the ventral phragmocone side can be alternatively achieved by vaulting the phragmocone and tilted septa. This, in turn, suggests that the loss of calcification, i.e. the development of a gladius might have occurred independently in Decabrachia and Vampyropoda. Since derivation of vampyropods from Phragmoteuthis-like belemnoids is widely accepted, a sister-group relationship between pro-ostracum-bearing Belemnoidea and Vampyropoda is plausible. As a result the taxon Neocoleoidea describes a paraphylum.