|North Pacific||AKSR-NASR||Anadyr-Koryak||Verkhoyansk-Chukotka||Okhotsk-Chukotka||N. Alaska||Yukon||Chignik|
|Phytostratigraphy||Images of fossils characteristic of the Type Taphoflora|
The Penzhina Phase of Floral Evolution
The phase was distinguished first by Herman (1988). It is characterized by taphofloras of the Konglomeratovyi Cape (southwestern Kamchatka) and Yelistratov Cape, by the Chineiveem taphoflora from left bank of the Anadyr River in its middle reaches, and by the Late Ginter flora described by Vakhrameev from the southern Bering Peninsula. The southern Bering Peninsula taphoflora consisting of six plant taxa only, and merely identified by Vakhrameev to generic level, can be attributed to the Penzhina phase in only a preliminary way. The relevant collection of fossil plants is unfortunately lost, but they are identical to taxa identified by him in the Valizhgen Formation of the Konglomeratovyi Cape (Grigor'ev et al., 1983; oral communication by Vakhrameev, 1985). The Penzhina taphoflora of the Konglomeratovyi Cape is the type for the Penzhina phase (Herman and Lebedev, 1991).
The boundary between the Grebenka and Penzhina phases is detectable based on successive taphofloras of two areas: the Ubienka–Chineiveem interfluve on the left bank of the Anadyr River in its middle reaches and the Bering Peninsula. In both areas, sections of plant-bearing deposits are not continuous. In the Ubienka–Chineiveem interfluve, where the section of the Krivorechenskaya Formation is younging in a northeasterly direction, the Grebenka flora is known from the Cenomanian – Turonian lower interval (southeastern outcrops), and the Chineiveem taphoflora of the Penzhina type is present in the late Turonian interval (northwestern outcrops). In the northern area of the Bering Peninsula (Ugol'naya Bay site), the Ginter Formation contains taphoflora of the Grebenka type in the middle part of the section (Cenomanian), while taphoflora of the Penzhina type is confined to the upper part of the formation (upper Turonian). Based on these observations, it is likely that the boundary between the floral phases under consideration is approximately at the level separating the lower and upper Turonian substages. The Penzhina phase of floral evolution is estimated to have been about 1.5 m.y. in duration.
Plant taxa characterizing the Penzhina floral phase are listed in the panel to the right. According to available data, the flora of the Penzhina phase is only half as diverse as the Grebenka flora. Angiosperm taxa make up a little more than a half of all plant remains. Dominant among them are species of the large-leaved platanoid genera Ettingshausenia, Arthollia, Paraprotophyllum, and presumably of Pseudoprotophyllum and Credneria. Despite size variability, leaves of these plants are large in general. The abundance of Menispermites leaves is somewhat lower in the Penzhina than in the Grebenka flora, although they are common components in taphofloras of the phase. Representatives of the genus Celastrophyllum that is characteristic of both phases can be abundant components of the Penzhina floral assemblages. The other common taxon is the genus Trochodendroides whose representatives play a significant role in the Penzhina taphofloras. Ferns are less diverse than in the Grebenka phase. Ginkgoaleans are represented solely by Ginkgo ex gr. adiantoides. The genus Sphenobaiera, caytonialeans, czekanowskialeans, and cycadophytes are unknown in the Penzhina flora. Cephalotaxopsis and Sequoia tenuifolia are most frequent taxa among abundant conifers.
The prevalence of large-leaved platanoids, the appearance of Protophyllocladus, Metasequoia, Paraprotophyllum, Viburniphyllum whymperi, "Zizyphus" smilacifolia, and a considerably larger proportion of Trochodendroides are the main features distinguishing the Penzhina from the Grebenka floral phase. In addition, all taphofloras of the Penzhina phase lack cycadophytes (except for Pterophyllum sp., a taxon of the impoverished taphoflora from the Ginter Formation, the southern Bering Peninsula, which is tentatively attributed to the Penzhina phase), Sphenobaiera and Phoenicopsis.