On the position of Cambrian archaeobranchians in the system of the class Gastropoda

Resumo

Possible phylogenetic relationships of the Cambrian gastropod subclass Archaeobranchia with other high-rank taxa of the Golikov–Starobogatov’s gastropod macrosystem are reconstructed. Archaeobranchians can be direct or indirect ancestors to five extant subclasses, i.e., Cyclobranchia, Scutibranchia, Pectinibranchia, Divasibranchia, and Dextrobranchia. Three euthyneural subclasses (Sinistrobranchia, Opistobranchia, and Pulmonata) are later descendants of Divasibranchia. The model for transformation of the shell muscles arrangement reconstructed previously for the Cambrian cap-shaped mollusks, towards the muscle system of modern cyclobranchians is proposed. The evolution of Cyclobranchia through the miniaturization stage can explain reduction of the right portion of organs in the mantle complex.