Boreskov

PP‐34EARLY STAGES OF MOLLUSCAN EVOLUTIONParkhaev P.Yu.Borissiak Paleontological Institute of the Russian Academy of Sciences, Moscow, Russia,pparkh@paleo.ruThe phylum Mollusca is one of the largest metazoan group, its taxonomical diversity issmaller only than that of the arthropods, and consists of approximately 130 000 nominalrecent species and 70 000 nominal extinct species (Haszprunar et al., 2008). According to therecent estimations (Haszprunar et al., 2008; Ponder, Lindberg, 2008), the total biodiversity ofmodern malacofauna approaches the number of 200 000 species, so that about 70 000unnamed recent forms await its formal description.In addition to the high taxonomic diversity, molluscs are characterized by a number ofdifferent bauplans. The molluscs dwell in almost all types of habitats, occupying variableecological niches in all range of marine and freshwater basins, and on the land. Suchdiversity and obvious ecological success were achieved during the long evolution of thephylum, lasting at least during the entire Phanerozoic.The earliest finds of undoubted molluscs come from the terminal Precambrian(uppermost Nemakin‐Daldynian) – basal Cambrian (lowermost Tommotian stage) strata(Parkhaev, 2005, 2008). The Vendian soft‐bodied animals (Kimberella, etc.), recentlydeclared as molluscan ancestors (Fedonkin, Waggoner, 1997), have principle ethologic andstructural differences (Parkhaev, 2008), hence possibly represent other animals phyla butnot molluscs.Studies of ancient molluscs bring us closer to the problem of Mollusca origin, however,the pulling down of the molluscan stem to the Precambrian‐Cambrian boundary, i.e. to themoment of appearance of skeletal fossils in the geological history, leaves us the very subtlehope, that the problem of molluscs origin can be solved on the base of paleontological findsof “transitional forms”, linking molluscs with their supposed ancestors, turbellarianflatworms (Salvini‐Plawen, 1980), or annelids (Minichev, Starobogatov, 1975; Ivanov, 1990).Nevertheless, the study of Cambrian molluscs undoubtedly reveals interesting data forgeneral and evolutionary malacology, shedding the light on the earliest diversification andevolution of major branches of the phylum. As a result, we can affirm the followingpeculiarities of the earliest molluscan evolution:1) The oldest representatives of phylum appear just below the Precambrian‐Cambrianboundary. The classes Monoplacophora, Polyplacophora, Gastropoda, and Bivalvia have194