Morphology and epidermal characters of Ginkgo pilifera Samyl. leaves and distribution of this species in the Late Cretaceous of Northern Asia

In the result of investigation of new findings, the variability of leaf shape and epidermal characters of Ginkgo pilifera Samyl. were studied. Geographical and stratigraphic ranges of this species were analyzed on the base of new material and earlier published data. This species was described by Samylina (1967) from the upper part of the Timmerdyakh Formation exposed in the Lena-Vilyui depression in Eastern Siberia. After that many new occurrences of this species were reported. G. pilifera was widely distributed from late Albian to Maastrichtian in Asian part of the Siberian-Canadian paleogeographical region. The earliest occurrence was documented from the late Albian-lower Turonian deposits of the Krivorechenskaya Formation, Grebenka River basin, Northeastern Russia. Approximately in the same time (in the Cenomanian) remains of G. pilifera appeared in the lower part of the Timmerdyakh Formation in the Lena-Vilyui depression. In Western Siberia in the late Albian and Cenomanian other species of Ginkgo were found: G. sertensis N. Nosova et Golovn. and G. chlonoviae N. Nosova et Golovn. were described from the Kiya Formation and G. ragosinii N. Nosova et Golovn. and G. tjukansis Kiritch. were described from the Simonovo Formation. In the Turonian and Coniacian G. pilifera was discovered in the upper part of the Timmerdyakh Formation, in the Valizhgen Formation of Yelistratov peninsula in Penzhina Bay area, in volcanogenic deposits of the Kolyma River basin and in the Derevyannye Gory Formation of New Siberia Island. In the Santonian-Campanian this species was reported from the Arkagala Formation of the Kolyma River basin and from two localities in the Amur River basin: from upper part of the Kundur Formation in Russia and from the Taipinglinchang Formation in China. The last occurrence is the most southern finding of this species. In Maastrichtian G. pilifera penetrated in Western Siberia, that significantly increased its area in western direction. The most variable character in G. pilifera morphology is the leaf shape. Leaves change from entire to deep-dissected into several wide lobes with rounded or truncate apices. Dissected forms occured from late Albian to Coniacian and predominated in the Krivorechenskaya (late Albian-lower Turonian) and the Valizhgen (Turonian-Coniacian) Formations Northeastern Russia.During Santonian-Maastrichtian only entire forms were distributed. The variability of epidermal characters is insignificant. The most variable characters are degree of undulation of the anticlinal cell walls, degree of development of cuticular thickenings and number of stomata on adaxial epidermises and number of trichomes on of abaxial and adaxial epidermises. The specific diagnosis of this species was emended. Remains of G. pilifera occur in allochtonous taphocenosis together with other plants characteristic to riparian habitats. Probably, this species was confined to river valleys and was not connected with swamp environments.Paleoecological implication of great number of papillae and trichomes is steel unclear. Emended diagnosis. Leaves broadly fan-shaped, entire with undulate upper margin with several shallow incisions, or, more rarely, dissected into several wide lobes with rounded or truncate apices. Leaves amphistomatic. Anticlinal cell walls of adaxial epidermis sinuous with amplitude up to 2–3 μm; in costal zones sometimes slightly sinuous or straight. Periclinal walls with cuticular dome-shaped thickenings and rare trichomes. Number and development of cuticular thickenings vary in different areas of leaf lamina and in different leaves. Weak development of thickenings usually correlates with increased number of trichomes. Stomata at abaxial epidermis rare; the papillae of subsidiary cells do not cover the stoma completely. Anticlinal cell walls of abaxial epidermis slightly sinuous or straight. Periclinal walls usually with papillae (12-20 μm long), more rarely, smooth. Trichomes (up to 60–100 μm long) occur rather often. Big trichomes usually consist of 2-3 cells. Stomata at adaxial epidermis orientated irregularly, without formation of regular rows, surrounded by five to seven subsidiary cells. Each subsidiary cell bears a proximal papilla up to 15–25 μm long. These papillae completely cover the stoma.