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Paleofloristic and Phytostratigraphic Terminology
In this website we have adopted the paleofloristic and phytostratigraphic terminology that has been developed by Russian researchers, in large part because it is defined rigorously. To understand the approach it is useful to examine its evolution.
Samylina (1974) (p. 7) used the concept of 'phases of floral evolution' in her work on the Lower Cretaceous phytostratigraphy of Northeastern Russia and introduced the term ‘stratoflora’, which is defined as "… general taxonomic composition of plant remains from concurrent deposits of a limited though considerable area that went through its own geological development and evolution history of organic world".
Deposits with floral assemblages of each stratoflora have been acknowledged as horizons (Resolutions of the Second…, 1978). Subsequently Kirichkova (1985) (p.6) detailed this approach by means of distinguishing floral and phytostratigraphic units of lower rank (phytostratigraphic assemblages and beds with flora, respectively), which had been used for subdivision and correlation of Jurassic and Lower Cretaceous deposits in the Lena River basin. As she noted, "boundaries between regional subdivisions with particular stratofloras corresponding to phases in palaeoflora (sic) evolution appear to be isochronous and comparable in certain cases … with chronostratigraphic boundaries of the International stratigraphic scale".
In this work, the term "stratoflora" is not used. It is useful but not etymologically correct: it means a fossil flora from a concrete stratigraphic subdivision (horizon), whereas the flora determines that subdivision, not vice versa. Nevertheless, the terms "stratoflora" and "flora of a [particular] phase" used in this work are equivalent.
The recognition of phases in the evolution of a fossil flora is a classification procedure in essence, and the objects of classification are taphofloras. The taphofloras (or floral assemblages) represent all the plant fossils from one or several burials of plant remains at close stratigraphic levels, which characterize vegetation of a particular time span in a given and restricted area.
Plants whose remains are detected in a taphoflora can be representatives of immiscible plant communities, which coexisted, however, within a limited territory during a short period of geological history. It is important to stress that one taphoflora should include representatives of plant communities whose remains occur in different sedimentary facies to characterize regional vegetation. A single taphoflorule (assemblage) is far more prone to taphonomic distortion than a combination of several taphoflorules that make up a taphoflora. A taphoflorule might also represent a specialized local community and not the regional vegetation.
Quantitative proportions of taxa in a taphoflora supplement its purely floristic characterization (as a list of taxa) with additional diagnostic paleophytocoecological parameters.
Taphofloras of one type, i.e., those with essentially similar characteristics, represent one phase in evolution of regional or subregional flora. All taphofloras of such a phase, named flora of a [particular] phase, have a particular set of characteristics in common (combination of taxa, qualitative and quantitative relations between plant groups). In addition to these features in common, each taphoflora of the phase has its own specific features reflecting geographic, ecologic, and age uniqueness.
A taphoflora of indefinite (uncertain) type is a term for a fossil flora that cannot be clearly attributed to a particular phase of floral evolution because of a low taxonomic diversity and/or ambiguous composition. In fact, it simply means that some fossil plants have been found in a certain part of a section.
The phytostratigraphy of plant-bearing deposits and recognition of regional and subregional phytostratigraphic horizons is here based on periodization of floral evolution: deposits containing taphofloras of one regional or subregional phase in the development of a flora represents one horizon. The term horizon means all the concurrent formations, their parts and auxiliary lithostratigraphic subdivisions, provided that paleontological characterization is the main criterion determining an horizon (Vakhrameev, 1982; Stratigraphic Code…, 2006). Hence, an horizon bears a correlative function within its geographic range.
The lower boundary of a phytostratigraphic horizon corresponds to the base of the formation, sequence or member, where taphofloras of the identified regional or subregional phase in floral evolution is found at its lowest stratigraphic level. The base of the subdivision with the lowermost occurrence level of taphofloras characterizing the next phase is simultaneously the upper boundary of the underlying horizon.
Because recognition of phytostratigraphic horizons is based on periodization of paleofloral evolution, they characterize predominantly non-marine plant-bearing deposits. The horizons can be regarded as regional or subregional stratigraphic subdivisions, because they fit criteria determining such subdivisions. They:
Beds with flora represent auxiliary biostratigraphic units containing certain assemblages of plant remains (Stratigraphic Code…, 2006). Their correlation potential is usually lower than that of the phytostratigraphic horizons.
In order to maintain a stable nomenclature of floral evolutionary phases (and corresponding phytostratigraphic horizons), they are named after a type taphoflora (similar to biological classification where the nomenclatural type specimens similarly maintain stability of taxa names). In other words, the type taphoflora is the carrier of the name (but not necessarily of all the characteristic features) of the floral evolutionary phase, although data on composition and age of type taphofloras are naturally of prime significance, and are a focus of this work.
The stratotype of a phytostratigraphic horizon corresponds respectively to that part of the section, which bears the type taphoflora (Stratigraphic Code…, 2006). The priority principle is adopted to maintain stability of nomenclature concerning phytostratigraphic horizons and phases in floral evolution.
Strictly speaking, the approach of recognizing regional and subregional phytostratigraphic horizons based on periodization of ancient floral evolution is applicable only to the Russian part of the North Pacific due to the regulations of the Russian Stratigraphic Code (Stratigraphic Code…, 2006). However, for consistency and to permit correlation across the Bering Straits, this approach is also used for Alaska.
In the context of Alaska Smiley (1966; 1969a; 1969b; 1972a; 1972b) used the term florule to refer to individual plant fossil assemblages at a given site. It is not clear if this included a combination of several burial layers from several different sediment types of it it referred only to one burial layer. Based on its useage it is likely it refers to collections made from a single geographic location and as such may include material recovered from several different sedimentary facies and burial layers.