Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, CT 06520-8109, USA.
Proc Biol Sci. 2012 Apr 7;279(1732):1259-68. doi: 10.1098/rspb.2011.1773. Epub 2011 Oct 5.
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.
一直以来，无板纲动物都被认为是最原始的软体动物。我们呈现了一个分子系统发育，包括无板纲动物 Neomeniomorpha（单板纲）和 Chaetodermomorpha（多板纲），该系统发育将 Aculifera（无板纲+多板纲）确立为一个单系群。我们的松弛贝叶斯分子钟估计，Aculifera 冠群出现在早奥陶世，与晚寒武世（约 5.01-4.90 亿年前）存在具有七个或八个背壳板的石鳖状软体动物一致。分子、胚胎和古生物学数据表明，无板纲动物以及石鳖，是从一个具有石鳖状祖先的并系类群中演化而来的。头足类动物与 aculiferans 作为姐妹群的恢复表明，软体动物的原始状态可能是类似于单板纲动物的形态。
