The late Pliensbachian (Early Jurassic) ammonoidAmaltheusin Japan: systematics and biostratigraphic and paleobiogeographic significance

Abstract

AbstractThe genusAmaltheus, one of the representative late Pliensbachian ammonoids, has biostratigraphic and paleobiogeographic significance in Japan. Four species,Amaltheus stokesi(Sowerby, 1818),A. margaritatusde Montfort, 1808,A. repressusDagis, 1976, andA. orientalisnew species, have been found in the Kuruma Group in central Japan;A. stokesiandA. margaritatusare also from the Toyora Group in southwest Japan. On the basis of taxonomic analysis of the genusAmaltheus, we distinguish two successive ammonoid biozones in the lower part of the Teradani Formation of the Kuruma Group: theAmaltheus stokesi–Amaltheus repressusand theAmaltheus margaritatusassemblage zones, in stratigraphic ascending order. This study also establishes the presence of theAmaltheus stokesiAssemblage Zone in the Higashinagano Formation of the Toyora Group. Thestokesi–repressusand thestokesiassemblage zones correspond biostratigraphically to theAmaltheus stokesiStandard Subzone of themargaritatusZone. ThemargaritatusAssemblage Zone is correlated with theAmaltheus subnodosusandAmaltheus gibbosusstandard subzones. The Japanese early–middle late Pliensbachian ammonoid faunas are composed almost entirely of pan-Boreal and Arctic species of the genusAmaltheus. This faunal composition has an affinity with that of the Northeast Russian region, and thus suggests a strong paleobiogeographic relationship between East Asian and Northeast Russian faunas throughout this time interval.UUID:http://zoobank.org/5F08121F-1DAF-4B24-BCBE-B08F7101CF29