Timing of Bellerophontoid (Gastropoda) Demise in the Early Triassic of South Primorye, Russian Far East

Abstract

Abstract. The stratigraphic distribution and modes of occurrence of Early Triassic Bellerophontoidea (Gastropoda) are studied at seven sections in South Primorye, Russian Far East, where depositional environments ranging from nonmarine, shoreface, to distal basin plain settings are recorded. Warthia zakharovi and Dicellonema abrekensis are abundant in Induan (Griesbachian and Dienerian) fine- to medium-grained, hummocky cross-stratified (HCS) sandstone beds occasionally intercalated with wavy-mudstone layers, whereas they are absent in coarser-grained cross-stratified successions. This observation suggests that bellerophontoids inhabited a lower shoreface environment above the storm wave base and possibly an inner shelf environment as well during this particular stage. Olenekian (Smithian and Spathian) bellerophontoids have not been found in the storm-induced sandstone beds, but W. zakharovi occurs in the lower Smithian sandstone beds of distal turbidites intercalated in the laminated mudstone. This mode of occurrence strongly suggests that W. zakharovi inhabited a deeper environment than lower shoreface, most probably an inner shelf environment, and after death, its shells were transported from their habitat to the basin-floor by sediment gravity flow. Bellerophontoids have not been found in middle Smithian and younger strata in South Primorye, and the timing of this disappearance is synchronous with other areas of the world. Bellerophontoids were distributed over wide-ranging areas from the equator to the high latitudes during Induan time, but they disappeared from the lower latitude areas and the shallower marine environments of middle latitude South Primorye during the early Smithian, before eventually becoming extinct during middle Smithian time. Such a step-by-step demise strongly implies that the severe global warming and related harmful events that occurred during the Smithian may have had a serious effect on bellerophontoids. The extinction of Bellerophontoidea before the beginning of the late Smithian suggests that the group may have been more sensitive to global warming and related harmful events than other organisms.