後期三畳紀二枚貝 Monotis の古生物学的意義 : その 1 研究史 Paleontological significance of late Triassic Bivalve Monotis : Part I A review
Of various, very thin-shelled Mesozoic extinct bivalves, which are noticeable for their peculiar morphology and unique mode of fossil occurrence, Late Triassic Monotis is suitable for evolutionary studies at the population level, because of its abundant occurrence in successive sequence. Furthermore, it is paleobiologically interesting animal, because it shows relatively rapid morphological change. In this part previous works are reviewed with some comments from the viewpoints of taxonomy, biostratigraphy, paleobiogeography and paleoecology. There are still many problems about the infrageneric classification and synonymic relation among numerous nominal species. Although Monotis is important for an index fossil of the upper Triassic, its exact range is still not confirmed by such other indices as ammonoids. Zonal divisions need to be reexamined, taking intrapopulational variation into account, by means of population samples. Monotis in Japan does not occur from comparatively off-shore sediments as Hallstadt limestone in Europe, but from near-shore silty to sandy sediments, in general. The distribution of Monotis is world-wide, but we recognize some major realms at the species or species-group level, roughly corresponding with Triassic faunal provinces. Some problems about its mode of life remain unsolved. Although there are still no positive evidences, it is suggested that Monotis was pseudoplanktonic or epibenthic bivalves attached to seaweeds on a sea floor, judging from its morphology and mode of occurrence.