Ammonites of Tethyan origin from the Ryazanian of the Russian platform: Genus <Emphasis Type="Italic">Riasanites</Emphasis> spath

The genus Riasanites, represented in Central Russia by two successive dimorphic species, is revised. R. swistowianus is found in the basal beds of the rjasanensis Zone. Its descendant R. rjasanensis is also found in this zone, but upwards in the section, including the beds with Surites spasskensis and Externiceras solowaticum. The representatives of Riasanites from the Crimea and Northern Caucasus are assigned to two species, R. crassicostatus and R. maikopensis, respectively. It is suggested that Riasanites evolved from Sub-Mediterranean Himalayatidae, which migrated from the Western Tethys via the Polish Passage into the Central Russian Basin, and from there to Mangyshlak, the Northern Caucasus, and the Crimea.