Morphology, systematics and paleoecology of Shikamaia, aberrant Permian bivalves (Alatoconchidae: Ambonychioidea) from Japan

The extraordinarily large and aberrantly shaped shells of the Permian bivalve family Alatoconchi-dae (Ambonychioidea), thought to have a photosymbiotic mode of life like the modern fragine bivalve Corculum, are difficult to extract from their host limestone bodies; therefore, their morphologies have been reconstructed mostly based on broken shell pieces. The alatoconchid Shikamaia akasakaensis Ozaki, the type species of the genus from the middle Permian Akasaka Limestone of central Japan, was described based on only three fragmentary specimens; its shell characters were later reassessed from specimens from the Neo area, ca. 30 km NNE of the type locality. Because of the fragmentary nature of the type specimens of S. akasakaensis, this species is difficult to diagnose and cannot be compared taxonomically to the specimens from the Neo area or other known species. A shell reconstruction based on 19 specimens from the type locality shows that S. akasakaensis exhibits a very large, elongated elliptical shell form but clearly differs from the Neo specimens in its higher and longer dorsal crests and more dorsally reflected shell wings in the anterior portion, rendering the establishment of a new species (Shikamaia ozakii Asato and Kase sp. nov.) for the latter specimens. The outer shell consists of a very thin outermost prismatic layer that is underlain by thick layers of granular crystals in both species of Shikamaia, which suggests that the shell did not allow sufficient sunlight penetration to culture symbiotic microbes in the soft tissues of these animals. The "Corculum model" of photosymbiosis therefore is unlikely for the two species of Shikamaia. The discovery of the presence of a ventral gape, a previously unreported shell character, suggests extension of the soft body into the sediment through this opening during life. Three possible modes of life (fragine-like photosymbiosis, lucinid-like chemosymbiosis and normal suspension feeding) are discussed.