Mode of life of the Silurian uncoiled gastropod Semitubina sakoi n. sp. from Japan

Semitubina sakoi n. sp. from the late Silurian of Japan represents the second species of this genus and also the first record of a Silurian gastropod in Japan. The gastropod shells occur in a thin mudstone bed and were found to be encrusted exclusively by corallites of ? Favosites sp. These corallites reveal that encrustation proceeded as the gastropod shells grew. The ecological relationship between the two organisms is considered to be symbiotic. This mode of life allowed the coral to live on a muddy substrate because clear sea water passed over the colony as the gastropod moved along. The gastropod benefited from this relationship by being protected from shell-boring or shell-crushing predators by the encrusting corallite. In Semitubina sakoi the body whorl is separated from the penultimate one by a considerable gap in a later growth stage and S. sakoi has been cited as one of the uncoiled gastropods. The uncoiling of this gastropod results primarily from rapidly increasing whorl translation rate in the latest growth stage. Taking the symbiotic relationship with ? Favosites sp. into consideration, a deposit feeding or benthic scavenging mode of life is suggested for this gastropod.     

Autecology of Labrocuspis, a Middle Devonian omphalotrochid gastropod

Labrocuspis kobayashii , a Middle Devonian euomphalacean omphalotrochid gastropod from Japan has a wide callus pad on its base similar to that seen in the living trochid Umbonium. L. kobayashii could support the shell over the cephalopedal mass in a similar fashion to that seen in the Umbonium species. This reconstruction indicates that the labral projection in the outer lip was above the head, and the opisthocyrt basal lip and the sinus in the outer lip were loci of inhalant and exhalant currents, respectively. Therefore, the animal is assumed to have had only one ctenidium (the left). The presence of a wide callus pad is indicative of an infaunal life habit for the Umbonium species, Labrocuspis and some Paleozoic gastropods; clamping of the shell against the foot is also suggested for some omphalotrochid gastropods. These features indicate an additional adaptive strategy employed by euomphalaceans.