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.     

Limpet home depressions in Cretaceous ammonites

Remarkable large pits were found on the shell surface of many ammonites from the Turonian-Maastrichtian sediments of Hokkaido, northern Japan, and Sakhalin, Russia. These pits are (1) round to elliptical in shape; (2) up to 20 mm (usually around 10 mm) in diameter; (3) shallow depressions to deep holes that almost penetrate the shell: (4) occasionally healed by a thin shell blister from inside the shell; (5) often overlapping one another; (6) several to more than 170 in number on one flank of the ammonites; (7) found on both flanks and predominantly on the body chamber and the final volution of the phragmocone; and (8) found only in the two families Pachydiscidae and Puzosiidae, predominantly of more than 300 mm in shell diameter. They can best be interpreted as the home depressions of patellogastropod limpets. The presence of pits on both flanks of the ammonites and those healed from inside the shell strongly suggest that the limpets were dwelling on mature swimming ammonites. Host specifity, their very small shell size compared with the host ammonites, and sparse Occurrence in sediments favor a mode of life as obligate pseudoplankton. We suggest that this remarkable limpet-ammonite association was well established in northwestern Pacific bioprovinces during the late Cretaceous. Taking this live association and the depth limit of algal growth as food for the limpets into consideration, the mature ammonites dwelled or periodically visited the upper layer of the euphotic zone, probably less than around 20 m in depth. *** Ammonite, limpet, home depression, life mode, pseudoplankton, Cretaceous.