title: A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors.
abstract: Aplacophorans have long been argued to be basal molluscs. We present a molecular phylogeny, including the aplacophorans Neomeniomorpha (Solenogastres) and Chaetodermomorpha (Caudofoveata), which recovered instead the clade Aculifera (Aplacophora + Polyplacophora). Our relaxed Bayesian molecular clock estimates an Early Ordovician appearance of the aculiferan crown group consistent with the presence of chiton-like molluscs with seven or eight dorsal shell plates by the Late Cambrian (approx. 501-490 Ma). Molecular, embryological and palaeontological data indicate that aplacophorans, as well as chitons, evolved from a paraphyletic assemblage of chiton-like ancestors. The recovery of cephalopods as a sister group to aculiferans suggests that the plesiomorphic condition in molluscs might be a morphology similar to that found in monoplacophorans.
authors: Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, CT 06520-8109, USA. jvinther@jsg.utexas.edu
authors: Jakob
authors: J
authors: Vinther
authors: Erik A
authors: EA
authors: Sperling
authors: Derek E G
authors: DE
authors: Briggs
authors: Kevin J
authors: KJ
authors: Peterson
mesh_terms: D000818:Animals; D001499:Bayes Theorem; D019143:Evolution, Molecular; D005580:Fossils; D049690:History, Ancient; D008957:Models, Genetic; D008974:Mollusca / Q000033:anatomy & histology / Q000145:classification* / Q000235:genetics*; D010163:Paleontology; D010802:Phylogeny