Abstract
The cephalopod arm armature is certainly one of the most important morphological innovations responsible for the evolutionary success of the Cephalopoda. New palaeontological discoveries in the recent past afford to review and reassess origin and homology of suckers, sucker rings, hooks, and cirri. Since a priori character state reconstructions are still ambiguous, we suggest and discuss three different evolutionary scenarios. Each of them is based on the following assumptions: (1) Neocoleoidea uniting extant Decabrachia and Octobrachia is monophyletic (= proostracum-bearing coleoids); (2) extinct Belemnitida and Diplobelida are stem decabrachians; (3) proostracum-less coleoids (Hematitida, Donovaniconida, Aulacoceratida) represent stem-neocoleoids; (4) Ammonoidea and Bactritoidea are stem coleoids. We consider a scenario where belemnoid hooks derived from primitive suckers as well-supported. Regarding belemnoid hooks and suckers as homologues implies that belemnoid, oegopsid, and probably ammonoid arm hooks arose through parallel evolution. Our conclusions challenge the widespread opinion, whereupon belemnoid hooks evolved de novo, and instead support earlier ideas formulated by Sigurd von Boletzky.
Highlights
The cephalopod arm armature is certainly one of the most important morphological innovations responsible for the evolutionary success of the Cephalopoda
The following scenarios are conceivable (Table 1, Figs. 4, 5, 6): Scenario 1: belemnoid hooks derived from primitive suckers This scenario suggesting that belemnoid hooks and suckers are homologous was assumed by Naef (1922, p. 165, 188), Jeletzky (1966, p. 138), Haas (1989, p. 182), and Boletzky (1999, p. 8, 2006, p. 35)
Scenario 2: belemnoid hooks did not derive from suckers (Fig. 6) Scenario 2 is identical to the evolutionary steps reconstructed in scenario 1B except that belemnoid hooks developed independently from the sucker complex
Summary
The Neocoleoidea represents a monophyletic group only when belemnitid and diplobelid belemnoids are included Such a topology unites all proostracum-bearing coleoids and simultaneously meets an important request from Sigurd von Boletzky, whereupon the differentiation of the decabrachian and octobrachian type of arm crown must have occurred independently at two speciation events In contrast to the sessile suckers of octobrachians (Fig. 1h–k), the decabrachian type of sucker (Fig. 1a–d) is stalked (pedunculated) and equipped with a sclerotised cylinder (sucker ring) and a muscular piston that fits into this cylinder (e.g., Haas, 1989; Nixon, 2011) Despite this and a number of other morphological differences such as the sucker symmetry (e.g., Nixon, 2011), the two sucker types share a sucker cup (acetabulum) and an attachment ring (infundibulum). Scale bar: 10 mm Evolution of the cephalopod arm armature: a review piston infundibulum acetabulum sucker ring (suckerin) peduncle a
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