Gastropod phylogenetic torsion – arising of a class

In the paper the author reconsiders the problem of phylogenetic torsion, which has for more than a hundred years been a subject of interest of numerous malacologists, and is still unclear. The author briefly describes the present state of knowledge of the fossil record and ontogenetic torsion observed in the Recent gastropods. He points out that these two sources of information are still insufficient, which leaves room for speculations based on functional and comparative morphology. He discusses in detail the main questions that usually are asked when considering the phylogenetic torsion. He stresses that several data and problems, like the musculature (muscle scars), geometry of ancestor’s shell, opercular profits, etc. are overestimated. He points out that the two phases of torsion must be considered separately. Finally, he presents a probable scenario of the process. The first, larval phase of torsion took place in an ancestor that fulfilled three (and not necessarily more) conditions: (a) a cyrtosomatid-type organization, (b) a benthonic and actively creeping adult, (c) a development with a free-swimming larva. These were necessary and sufficient to make the process both possible and inevitable, because of the larval problems with balance and with directional swimming, which had arisen when the foot/head development, caused by the active creeping of the adult, was so advanced that due to the growth of the ano-pedal flexure the larva became nearly semi-circular. Since any decrease in curvature would mechanically affect the mantle cavity of the veliger, the only way was to twist the visceral mass 90° in relation to the cephalopodium. Then, the second phase was simply a necessity for the settling larva to adapt itself to the benthic mode of life. After torsion was, the post-torsional gastropod proved better adapted and its range of possible adaptive radiation was much wider than that of its untorted ancestors whom it could later eliminate by competition, as the selective pressure grew. Key WordS: first phase of torsion, cyrtosomatid, free swimming larva, balance problems, ano-pedal flexure, veliger Folia Malacologica 5/1993 was originally published as No. 1462 of Scientific Bulletins of University of Mining and Metallurgy, Cracow. This digitalised version was prepared by the Association of Polish Malacologists and first published on-line on December 30th, 2016. http://dx.doi.org/10.12657/folmal.005.002 Folia Malacol. 5: 25–60