The locomotion system of Mesozoic Coleoidea (Cephalopoda) and its phylogenetic significance

Abstract

A morphological comparison of shell-muscle contacts in coleoid cephalopods mainly from the Early Jurassic (Toarcian) Posidonia Shales of Holzmaden (Germany), the Middle Jurassic (Callovian) Oxford Clay of Christian Malford (UK), Late Jurassic (Kimmeridgian-Tithonian) plattenkalks of Solnhofen (Germany), and the Late Cretaceous (Cenomanian) of Hâdjoula and Hâkel (Lebanon) provides new and meaningful insights into their locomotion systems. The study shows that both pro-ostracum- and gladius-bearing coleoids are typified by a marginal mantle attachment and by distinctly separated fins, which usually insert (indirectly via the shell sac and basal fin cartilages) to posterior shell parts. While absent in gladius-bearing forms, mantle-locking cartilages might have existed already in pro-ostracum-bearing belemnoids. Similar to ectocochleate ancestors, funnel- and cephalic retractors are generally attached to the internal (ventral) shell surface. A comparison of Mesozoic and Recent gladius-bearing coleoids shows that the locomotion system (most significantly the dorsal mantle configuration, and the presence of nuchal- and funnel-locking cartilages) is fundamentally different. This does not support the concept of ‘fossil teuthids’, but suggests, owing to similarities with Recent Vampyroteuthis, placement of Mesozoic gladius-bearing coleoids within the Octobrachia (Octopoda + Vampyromorpha). Classification of Mesozoic gladius-bearing coleoids as octobrachians implies that: (1) unambiguous teuthids are still unknown in the fossil record and (2) the similarity between Recent and some fossil gladiuses represents a matter of homoplasy.