Distribution and variability of Nilssonia serotina Heer (Cycadales) in the Cretaceous and Paleogene floras of northern Pacific Region

Variability of Nilssonia serotinaleaves was studied by the material from the Sakhalin, from the Maastrichtian deposits of the Kakanaut Formation (Koryak Upland), the upper Albian-lower Turonian deposits of the Krivorechenskaya Formation (Anadyr River Basin) and the Santonian-upper Campanian deposits of the Chignik Formation (Alaska). Type material of Nilssonia serotina Heer from thе Santonian deposits of upper part of the Arkovo Formation of Sakhalin restudied for the first time. This collection is stored in Komarov Botanical Institute. A lectotype was selected, and an extended specimen diagnosis was defined. Leaves of N. serotina are varied from nearly entier with rare dissections, to those having numerous triangular or trapezoidal segments. Distribution of Nilssonia serotina from the Albian to the Paleoсеne was revealed. This species was distributed on the Sakhalin, Northeastern Russia, Western Canada, Alaska and North Dakota (USA).