Article: Fortipecten takahashii, a reclining pectinid from the Pliocene of north Japan

The adaptive morphology and mode of life of a large bizarre pectinid, Fortipecten takahashii (Yokoyama) from the Pliocene of north Japan and Sakhalin, were examined mainly from the standpoint of relative growth. In spite of its similarity to some extant species of Patinopecten in the early growth stage, Gryphaea-like and unusually heavy valves are formed after the middle stage by a drastic change of growth pattern. In the later stage, unlike many swimming pectinids, the weight of the valves becomes positively allometric to the cube of shell size, indicating remarkable relative thickening of the shell. The decrease of umbonal angle, abrupt inward bending of the shell surface, and disappearance of anterodorsal and postero-dorsal gapes also suggest rapid loss of swimming ability in the middle stage. After it escaped from predators in the young stage, this pectinid probably abandoned a swimming strategy and became an immobile recliner on soft substrates. F. takahashii is thus regarded as an exceptional Cainozoic bivalve which succeeded, though only temporarily, in resurrecting Mesozoic-type reclining life habits in some inland seas of the north-western Pacific region.