The Continuing Debate on Deep Molluscan Phylogeny: Evidence for Serialia (Mollusca, Monoplacophora + Polyplacophora)
Figure 1
Schematic trees of molluscan relationships. (a) showing traditional proposed subdivisions. (b) consensus tree of two recent molluscan phylogenies inferred from large-scale genomic data by Kocot et al. [5] and Smith et al. [8]. The traditional concepts of Aculifera and Conchifera are supported but with differing positions of scaphopods. Monoplacophora is missing in the data set of Kocot et al. [5] (dotted line reflects the position of Monoplacophora in Smith et al. [8]). (c) the preferred multilocus tree with morphological features indicated numerically on branches. Unfilled dots indicate maximum Bayesian node support, filled dots additional high (>75%) bootstrap support in ML analyses. The Ediacaran fossil genus Kimberella corresponds to the description of molluscan stem-group features (1–4, below); crown group taxa originating in the Cambrian and later are united by additional features. Black boxes indicate first appearance of features; grey boxes indicate significant adaptive change; unfilled boxes indicate trait reversals: (1) radula: bipartite in stem molluscs and paedomorphic aplacophorans; broadened, on cartilages and specialised in crown molluscs, stereoglossate-like in Serialia; lost in Bivalvia (and several gastropods); (2), foot with broad gliding sole: transformed into digging foot in variopods (and derived bivalves), narrowed and reduced in aplacophorans, and forming the funnel in cephalopods; (3) circumpedal mantle cavity, miniaturised and anteriorly dislocated in torted gastropods while placed posteriorly in vermiform molluscs; (4) separate mantle covered with cuticula (with calcareous spicules in chitons, aplacophorans, and probably Kimberella); (5) dorsal shell: duplicated/fragmented in bivalves and chitons, lost in aplacophorans (and members of most other classes); (6) head with paired appendages: multiplied into feeding tentacles in variopods; trait for head reduction in bivalves plus Serialia and aplacophorans; (7) pericardium: heart fused around intestine in Dorsoconcha; (8) paired ctenidia: expanded to serially repeating gills in Serialia (and nautiloid cephalopods) and reduced in Solenogastres and some gastropod lineages; (9) complex stomach with style (reduced in carnivorous subgroups and chitons; convergently (?) present in a caudofoveate family); (10) paired eightfold dorsoventral muscles; (11) (not shown) statocysts (lost convergently in chitons and aplacophorans); (12) (not shown) suprarectal visceral commissure (subrectal convergently in chitons and aplacophorans).
