Origin and evolution of the Class Rostroconchia

Abstract

The pseudobivalved Rostroconchia, first recognized as a separate class of molluscs in 1972, may be the only extinct molluscan class. Until recently, primitive rostroconchs (ribeirioids) were generally thought to be the carapace of crustacean arthropods and advanced rostroconchs (conocardioids) were considered to be unusual pelecypods. In fact, rostroconchs were a diverse class of molluscs (2 orders, 8 families, 31 genera, 400 + species known) that grew a bivalved adult shell from a univalved larval and juvenile shell. They were bilaterally symmetrical animals that probably had an anterior mouth, posterior anus, a pair of lateral gills, and a pelecypod-like foot. Most are believed to have been deposit feeders that used enlarged anterior mantle tissue to collect food, but some were clearly suspension feeders. They lived on top of the sea floor (rarely) or partly buried within it (commonly). Rostroconchs are known only from Palaeozoic rocks and range in age from earliest Cambrian to latest Permian (approximately 575-245 Ma ago). They evolved from untorted univalved molluscs (helcionellacean monoplacophorans) in the late Precambrian, remained an inconspicuous component of the biota through the Cambrian, and then radiated rapidly in the palaeotropical seas of the Early Ordovician. Relatively few genera survived the Ordovician, possibly because of competition by the Pelecypoda, but many species of these are found in younger Palaeozoic rocks. The last refuge of the class seems to have been the cool-temperature regions of the Permian Earth. By the middle Early Cambrian, the first pelecypod Fordilla had evolved from a primitive rostroconch. Rostroconchs were preadapted to exploit the pelecypod form, and the appearance of Fordilla may have been a relatively insignificant step. Somewhat later, probably in the Late Cambrian or Early Ordovician, the Scaphopoda were also derived from the Rostroconchia. This evolutionary event seems to have required the ventral fusion of an elongate rostroconch shell at the post-larval stage of development. Animals resembling primitive rostroconchs were required as theoretical links between monoplacophorans and pelecypods before the Class Rostroconchia was well studied. These hypothetical intermediates differ from the real thing in only one important respect; it was predicted that such forms would have many pedal muscle insertions on each valve, which they do not. The single anterior and posterior pedal muscle insertions of primitive rostroconchs therefore indicate a secondary simplification of the molluscan stock; this probably occurred after the anatomy of Neopilina was attained, but before the main radiation of the phylum.