The results of this analysis support a monophyletic Aculifera and suggest that aplacophorans are derived from chiton-like ancestors, as shown by the placement of palaeoloricate taxa in the aplacophoran stem group. Although our analysis included only one conchiferan taxon to test the monophyly of total-group Aculifera, the polyplacophoran and aplacophoran clades are each well resolved. Taxa within the total-group Aplacophora show that many fossil ‘chitons’ probably did not have a foot.
The critical taxon ‘H.’ thraivensis
, the preserved specimen of which shows it had polyplacophoran-like valves but lacked a well-developed foot, is a sister to the remaining total-group aplacophorans; this transitional morphology links the dorsoventrally flattened chitons and the vermiform aplacophorans (). Septemchiton
also resolves in the aplacophoran stem group; other septemchitonids (e.g. Carnicoleus
) have valves that meet ventrally and cannot have possessed a functional foot. This observation lead Dzik (1994)
to suggest that septemchitonids may have been ancestral to aplacophorans, an inference that we consider prescient. Our analyses imply that all palaeoloricate polyplacophoran taxa within the total-group aplacophoran clade lacked a fully developed foot. This result has serious implications for the interpretation of any disarticulated polyplacophoran material, as the presence or absence of a foot is unlikely to be evident from shell valves alone.
The recovery of a monophyletic Neoloricata is in agreement with the non-cladistic taxonomic studies of Sirenko (1997
). Within this clade, the derived positions of the extant taxa with respect to the Palaeozoic fossils, and indeed the relative positions within the extant taxa, are compatible with neontological studies (e.g. Okusu et al. 2003
; Sirenko 2006
). We consider this a corroboration of the validity of our analysis and in particular the application of down-weighting multistate characters, since this topology is not resolved in unweighted analysis (see electronic supplementary material).
The results from our analyses differ fundamentally from the topology recovered in the only previous cladistic analysis including palaeoloricate fossils (Cherns 2004
). The earlier analysis was primarily concerned with the relationships among taxa known from disarticulated valves; it was by necessity based on fewer characters than the present study and did not include aplacophorans or problematic Cambrian taxa. Some of the results of Cherns' (2004)
analysis seem problematic (for instance, the placement of the multiplacophoran Strobilepis
as a sister group to the extant Chiton
); however, it does concur with our analysis at least in the recovery of a heloplacid clade (Acaenoplax
). The cladistic analysis of Conway Morris & Caron (2007)
is concerned with broader lophotrochozoan phylogeny; the molluscan portion of their tree, while poorly resolved, is in agreement with our findings.
The machaeridian genera in our analysis resolve close to the aplacophorans; they do not however unambiguously form a clade, possibly representing a grade of relatively derived members of the aplacophoran stem group. This suggested position remains speculative in the continued absence of fossilized machaeridian soft tissues.
Our analyses include only two representative living genera of aplacophorans and hence do not directly test the monophyly of living Aplacophora. In view of the disparity of total-group Aplacophora implied in our phylogeny, and especially by the newly identified ‘H.’ thraivensis, it seems plausible that the two aplacophoran groups Neomeniomorpha and Chaetodermomorpha had separate origins within this diverse assemblage.
The heterogeneous nature of the Palaeoloricata is evident in trees resulting from our analyses; as suggested by Vendrasco et al. (2004)
these fossils do not represent a natural group, but are stem forms to extant clades. Most palaeoloricate in our analysis resolve as stem-group aplacophorans, while the multiplacophorans and the allied Echinochiton
resolve as stem-group polyplacophorans. The Palaeoloricata is clearly non-monophyletic and we contend that the use of this taxon should be discontinued.
Our analyses do not test the monophyly of the Conchifera, as the only conchiferan included is the extant monoplacophoran Neopilina
. Partial sequence data for a monoplacophoran led Giribet et al. (2006)
to suggest a sister-group relationship linking Polyplacophora and Monoplacophora, ‘Serialia’. In their analysis, aplacophorans appeared more closely related to scaphopods and cephalopods, in a clade sister to the remaining Mollusca; Aplacophora, Testaria and the Conchifera are not recovered. These results, while intriguing, are not supported by morphological homologies (Nielsen et al. 2007
), run counter to all morphologically based phylogenetic analyses, ours included, and are not congruent with other molecular phylogenies (e.g. Rosenberg et al. 1997
; Passamaneck et al. 2004
). A resolution of the discrepancies between molecular and morphological signals is clearly still required, but is beyond the scope of this paper.
The topologies of all of our recovered trees support the aculiferan rather than the testarian model of molluscan evolution. This debate is at the core of molluscan phylogeny: what are the synapomorphies of the Mollusca, and what did the most recent common ancestor of the Mollusca look like? Our results imply a character set for this animal that includes seven- to eightfold serial repetition, the presence of valves and a foot, and a creeping rather than worm-like mode of life. The consideration of informative fossil taxa in phylogenetic analyses provides insights into the disparity and evolutionary history of groups that cannot be obtained from solely neontological studies.