Aplacophora as Progenetic Aculiferans and the Coelomate Origin of Mollusks as the Sister Taxon of Sipuncula

Abstract

Evidence is presented in support of the following phylogenetic hypotheses: (1) Sipuncula are the sister taxon of Mollusca; (2) the two aplacophoran taxa, Neomeniomorpha (= neomenioids) and Chaetodermomorpha (= chaetoderms), are monophyletic with a common neomenioid-like ancestor, and of the two taxa, Chaetodermomorpha are more derived; (3) Aplacophora and Polyplacophora are sister taxa and form a clade, Aculifera; (4) Aculifera are the sister group of the remaining extant mollusks, Conchifera; and (5) Aplacophora are progenetic Aculifera. The evidence is based on homologies of early and late embryological development, adult morphologies, and molecular analyses. Embryological development in sipunculans and mollusks shows a close relationship between them, and embryological development of the shell separates Aculifera and Conchifera. Adult morphologies indicate: (1) monophyly of Aplacophora; (2) sister-group relationship between Aplacophora and Polyplacophora; (3) a molluscan plesiomorphy of nonsegmented serial replication of organs; and (4) progenesis in Aplacophora. Molecular evidence supports the embryological and morphological relationships between Sipuncula and Mollusca. Mollusca are thus hypothesized to be coelomate Eutrochozoa, which share an ancestor that probably had serial replication of organs. Differences in size and structure of the coelom among Eutrochozoa are hypothesized to have been brought about by changes in the timing and the process of cavitation of the mesodermal bands that arise from cell 4d. Through the process of progenesis Aplacophora retained an ovoid embryological shape and several internal structures that, although they appear to be in a primitive state, are actually secondarily derived as is quadrant D specification during early cleavage.

Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons

“…Interestingly, this clade, which we name ''Serialia,'' contains the two classes whose members present a variable number of serially repeated gills and eight sets of dorsoventral pedal retractor muscles. This result clearly contrasts with previous cladistic hypotheses suggesting that Monoplacophora constitute the sister group to the remainder of the conchiferans (4,5,14), those molluscs with a true shell unlike that of chitons or the vermiform aplacophorans, although it finds no clear support for the exact position of Serialia. To our knowledge, this is also the first published analysis that demonstrates monophyly of the phylum Mollusca using a range of appropriate outgroups, but we caution the reader to consider that jackknife support for molluscan monophyly is low.…”

“…Recent attempts to resolve their relationships by using morphological data found limitations in character homology definitions and polarization because of uncertainty regarding the molluscan sister group (4,5,14). Molecular attempts have not been conclusive, but they have aided to refute the ''Diasoma'' hypothesis (a clade uniting bivalves and scaphopods).…”

A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors

“…This supports the hypothesis that these forms represent stem aculiferans [19], and that aplacophorans lost the shell plates and reduced the ventral foot region secondarily from such an ancestor (figure 4b). Thus, both the molecular data (presented herein) and the fossil record [19,22] support the hypothesis of aplacophorans as derived chiton-like molluscs [4]. However, while the empirical WAG þ G 4 model supports Aplacophora as a monophyletic group, aplacophorans are recovered as a paraphyletic grade leading to chitons with very low support under the overall best-fitting model (CAT þ GTR þ G 4 ), which collapses to a polytomy when only the slowly evolving sites are analysed.…”

A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale

“…However affinities of Halkiera still remain highly contentious. Neomeniomorpha have several morphologies homologous to those in Polyplacophora 28 . Some of these are plesiomorphies of the Mollusca (Fig.…”