|North Pacific||AKSR-NASR||Anadyr-Koryak||Verkhoyansk-Chukotka||Okhotsk-Chukotka||N. Alaska||Yukon||Chignik|
|Phytostratigraphy||Images of fossils characteristic of the Type Taphoflora|
The Grebenka Phase of Floral Evolution
Samylina (1974), who originally distinguished the Grebenka phase “stratoflora”, considered it to be Turonian in age. Corresponding to this phase are the Grebenka flora of the Anadyr River middle reaches, which includes a series of taphofloras discovered on the right and left banks of this river, and the Middle Ginter taphoflora of the Ugol'naya Bay site. The taphoflora from the Krivorechenskaya Formation of the Yelisseev Locality in the Grebenka River middle reaches is the most diverse among them, being regarded as the type taphoflora of the phase (Shczepetov et al., 1992; Spicer et al., 2002, Herman, 2011a).
The boundary between the Early Ginter and Grebenka phases is substantiated by a succession of taphofloras in the Ginter Formation section at the Ugol'naya Bay site, where they are separated, however, by an interval of about 330 m thick lacking paleontological remains in the lower part and containing marine fossils of the middle Cenomanian Inoceramus pennatulus Zone in the upper part. Because taphofloras of the Grebenka phase from the Grebenka River basin correspond in age not only to the Cenomanian, but also to the late Albian (Spicer et al., 2002), the boundary between the two phases is most likely within the late Albian. The Grebenka phase is estimated to have been approximately 7 – 9 m.y. in duration.
The flora of the Grebenka phase is very diverse, consisting of at least 200 taxa. These taxa are listed in the panel to the right. Taphofloras typical of the phase are characterized by having angiosperms dominating the floral composition to the extent that they represent 35 – 40% of taxa. Ferns and conifers are also diverse comprising 20 – 25% of taxa, although fern impressions prevail in many local assemblages. Diverse and abundant Menispermites leaves belonging to at least eight species and platanoids represent a noticeable, though not dominant, component of the Grebenka flora. The platanoids, not being very diverse, largely belong to the genus Ettingshausenia. However one impression of the peltate Pseudoprotophyllum has been reliably identified (Yelisseev locality, collection of R.A. Spicer).
The presence of the genera Protophyllum and Credneria is doubtful. Angiosperms are fairly abundant and include entire-margined (Magnoliaephyllum, Myrtophyllum, Scheffleraephyllum, Dalbergites, etc.), lobed (Ettingshausenia, Cissites, Menispermites, Araliaephyllum, Dalembia) and compound (Scheffleraephyllum, Dalembia, Sorbites, etc.) leaves. Isolated specimens of Trochodendroides also occur.
Common among ferns are the genera Gleichenites, Birisia, Coniopteris, Hausmannia, Cladophlebis etc.; Birisia and Coniopteris remains are frequently very abundant in local assemblages. Caytonialeans (Sagenopteris) are relatively rare. Cycadophytes represented by leaves of Cycadites, Nilssonia (often forming accumulations of remains or ‘leaf mats’), Taeniopteris and by short shoots of Nilssoniocladus (Spicer and Herman, 1996) are common components in taphofloras of the Grebenka type. Ginkgoaleans include the genera Ginkgo, Baiera, and Sphenobaiera. The czekanowskialean genus Phoenicopsis is a very rare component. Conifers include the comparatively old, typically Early Cretaceous genera (Podozamites, Athrotaxopsis, Pagiophyllum, Pityophyllum) and remains of more advanced forms (Sequoia, Cupressinocladus, Cryptomeria); most common and frequently occurring species are Cephalotaxopsis intermedia, Araucarites anadyrensis, and Elatocladus smittiana associated with some remains with Sequoia aff. reichenbachii and Pagiophyllum triangulare.
Lebedev (1987) attributed a taphoflora from the Kondyreva River basin (right tributary of the Penzhina River) to the Grebenka phase of floral evolution. The Kondyreva River assemblage includes Onychiopsis psilotoides, Hausmannia sp., Cladophlebis (Birisia) oerstedtii, C. cf. frigida, Cladophlebis sp., Nilssonia alaskana, N. yukonensis, Cephalotaxopsis heterophylla, C. cf. intermedia, Araucarites anadyrensis, Menispermites sp., Celastrophyllum (?) sp., and Dicotyledones sp. (collection of Yu.M. Arsen'ev and E.A. Strizhko; determinations of E.L. Lebedev). The diversity of this flora is too low, however, to be confident that it is the Grebenka type and additional collecting is required before its affiliation can be confirmed.