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Evolution of aplacophoran mollusks
Wanninger, Andreas Department of Integrative Zoology, Faculty of Life Sciences, University of Vienna, Austria.
- Analysis of shell plates
- Analysis of muscular architecture
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The phylum Mollusca is a highly species-rich and morphologically diverse animal grouping that includes wormlike groups (the aplacophorans), eight-shelled polyplacophorans (chitons), and the primarily single-shelled conchiferans [including monoplacophorans, scaphopods (tusk shells), bivalves (mussels and clams), gastropods (snails and slugs), and cephalopods (squids and octopuses)]. According to one prevailing traditional hypothesis, the shell-less aplacophorans (see illustration) constitute the earliest molluscan offshoot. As a consequence, the last common ancestor to all mollusks was likewise imagined as a simple “worm.” However, recent phylogenomic studies have argued against the “aplacophoran-first” hypothesis and instead suggest a basal split of Mollusca into two distinct groups: the univalved Conchifera and the Aculifera, which includes the aplacophorans and the polyplacophorans. Accordingly, three scenarios concerning the morphology of the last common molluscan ancestor appear principally possible: The “urmollusk” (last common ancestor to all mollusks) was (i) a mono-shelled conchiferan-like animal, (ii) a wormy shell-less aplacophoran-like mollusk, or (iii) an eight-shelled polyplacophoran-like creature.
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