Discussion of Phylogenetic Relationships
- Belemnoidea †
Extant coleoids form a monophyletic group, the Neocoleoidea, as evidenced by the uniform presence of suckers. Their sister group, the Belemnoidea, is characterized by the presence of hooks that are derived, perhaps, from muscular papillae, the homologues of trabeculae or cirri that lie lateral to the suckers in living coleoids (e.g. Engeser and Bandel, 1988, see also Haas, 1989).
The Decapodiformes is supported by only a single morphological character that has been demonstrated to be apomorphic (i.e. derived): the modification of the fourth pair of arms as tentacles (Young and Vecchione, 1996). The monophyly, however, is supported by molecular cladistic studies (Bonnaud et al. 1997; Carlini and Graves, 1999). The problem with morphology, however, is not due to a lack of shared character states among decapods but instead results from our inability to polarize these states. The outgroup on which most polarity decisions are based is the Nautiloidea. Unfortunately, this group is morphologically very different from coleoids and many characters are not applicable to both groups.
Young and Vecchione (1996, 2002) found five unambiguous character states that supported the monophyly of the Octopodiformes:
- Presence of an outer statocyst capsule.
- Superior buccal lobes adjacent (fused at edges) or fused to posterior buccal lobes of the brain.
- Arms II modified as filaments. (This assumes an "ordered" evolutionary relationship of the character states: arms II unmodified - arms II modified - arms II absent.)
- Primary gill lamellae possess a partial or complete median septum.
- Secondary gill lamellae possess a partial or complete median septum.
An evolutionary study by Hass (2002) also placed the Vampyromorpha and Octopoda together as a monophyletic group on the basis of a combination of characters four and five above (he independently recognized these characters but defined them slightly differently). Molecular studies support the monophyly of this group (Carlini and Graves, 1999) or give it equivocal support (Bonnaud et al., 1997).
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