The Order Anetshelloida (Mollusca, Rostroconchia): Origin, Patterns, Mechanisms
Anetshella golowkinskyi (Netchaev, 1894) and Oxyprora tschernyschewi (Licharew, 1931) are shown to be directly related despite their definitive shells being morphologically different. As there are no other known rostroconch species in the semi-isolated Kazanian basin, O. tschernyschewi is the only contender for an ancestral taxon. The material described in this paper shows that the morphologies of the juvenile specimens of O. tschernyschewi and the definitive shell of A. golowkinskyi are largely homologous. The aromorphic transition from the bivalvian shell of O. tschernyschewi, typical of rostroconchs, to the cap-like shell of A. golowkinskyi was on one hand fixation and subsequent development of juvenile characters, typical of the initial phase of the juvenile stage of the O. tschernyschewi shell, and on the other hand by positive allometry of the rostrum. The considerable restructuring of the body plan of Anetshella was accompanied by the appearance of morphological novelties uncharacteristic of other rostroconchs, pegma and two transverse septa, and evolutionary losses such as loss of a series of plesiomorphic characters.
Keywords:morphology evolution aromorphosis fetalization heterochronies allometry island faunas extreme specialization
I am sincerely grateful to T.B. Leonova (Paleontological Institute, Russian Academy of Sciences, Moscow, Russia) for critical reading of the manuscript and valuable remarks. I also thank O.P. Shilovsky (Kazan (Volga region) Federal University, Kazan, Russia) for help with the tomographic study of the fossil material.
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