RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6003, doi:10.2205/2005ES000183, 2005

Classification of Acritarchs. Prospects for Systematics

[65]  The distinctive features of the acritarch subgroups and genera [Downie et al., 1963; Evitt, 1963; Volkova, 1965] are the morphological ones. Only the diagnosis of the subgroup Implethomorphitae Jank. et Mikh., 1989 [Jankauskas et al., 1989] includes the property of physical resistance of cellular organelles to extreme conditions. Inadequate significance of this criterion in comparison with morphological features of the group implies its higher rank in relation to the latters. At the same time, the identity of taxonomic significance of morphological characters for both subgroups and the constituent genera is evident. This likely is the main reason why the supergeneric taxa of Acritarcha Evitt, 1963 [Downie et al., 1963] are unused in classification summaries [Butterfield et al., 1994; Hofmann and Schopf, 1983; Sergejev, 1992; Jankauskas et al., 1989]. Almost every subgroup is defined by a morphological feature of the constituent genera, part of which possesses characters of another subgroup. This chiefly concerns the forms of Implethomorphitae: most of them, according to morphological characters, can be referred to two or three subgroups.

[66]  Therefore, the modern system of acritarch subgroups, considering the diverse and convergent morphological features, does not permit to recognize the "biological sense" of most of the observed characters. In this system the members of different divisions and even kingdoms of organic world can be placed in a single subgroup of acritarchs. We propose a modified acritarch classification that on further transformation can present formal potentials for distinction of natural groups of ancient microorganisms. Within such groups presumably referred to the superior taxa of plant kingdom, one can actually distinguish more detailed taxonomic analogues. It should be noted that the establishment of natural taxonomic position for most of acritarchs is a multivariant, long-term process. A useful part of the process is the development of classification that takes into account the criteria acceptable for a transitional state. The environmental conditions and separation of acritarchs according to a visible evidence of physical and chemical resistance of envelopes, can be referred to such criteria for distinction of related groups. The acritarch classification can include units essentially corresponding to these requirements.

[67]  The proposed classification yields a marker of difference in physical and chemical resistance of forms, which is inadequate to usual morphological features differentiating the acritarch genera and subgroups. As is seen from the above-reported records, we state a higher rank of that marker defined by the lack of its combined character and as a reflection of a higher surviving of organisms or their reproductive organs. The hierarchy of the proposed classification includes a possibility of inputing ecological indicators that are an integral part of taxonomic studies. On paleontological research within any system, especially that of acritarchs, the inconclusiveness of results is evident. However, we consider the proposed variant as more promising for taxonomic investigations than the present one.

[68]  It is assumed that the construction of the acritarch group is governed by the International Code of Botanical Nomenclature, 1996 (hereinafter - Code). To judge the legitimacy of the proposed changes, the actual relationships of the acritarch system and the Code theses should be emphasized. Almost all the theses touch on the plant taxonomy with more or less established hierarchy of natural taxonomic categories. Among the formal fossil taxa only the generic ones are listed in the Chapter I clauses of the Code. Therefore, the Code theses almost cannot account for the situations with formal categories higher than the genus. As regards the acritarch taxonomic position, we can now state the presence in the group of both members of the plant kingdom (algae) and bacteria. On the other hand, in the absence of a "special code of paleobotanical nomenclature" the Code clauses must be, where possible, considered in the microfossil taxonomy. However, in the light of systematic sense of the Code clauses the latters do not prevent from experimental modifications of the acritarch system. Having regard to that situation we propose a variant of acritarch classification implying the different rank of supergeneric taxa.

[69]  The group Acritarcha Evitt, 1963 [Downie et al., 1963; Jankauskas et al., 1989] is subdivided into subgroups that differ in non-morphological criteria including the markers indicating a statistically significant possibility of biological differentiation of groups. Among them there can be vital activity conditions, degree of resistance to metamorphic processes, chemical composition, etc. Subgroups are subdivided into infragroups that differ in types of non-obvious affinity, namely, morphological, ecological, and probably some other. In so doing, a large part of the adopted morphological subgroups gains an infragroup rank. Names of the subgroups and infragroups are not typified and they themselves possess an experimental status permitting to vary or extend their diagnosis and range until elaboration of the regulated summary. Names of the new subgroups are given a little used ending "i". Type genera of subgroups and infragroups are not defined. Species and genera differ in morphological features. We suggest to divide the group Acritarcha Evitt, 1963 [Downie et al., 1963] into subgroups Implethomorphi (Jankauskas et Mikhailova, 1989) [Jankauskas et al., 1989], Stanevich comb. nov.; Oblidomorphi [Stanevich, 1997], Stanevich, nom. mut.; and Incertae sedis.

[70]  The subgroup Implethomorphi (Jank. et Mikh.) Stan. includes morphologically diverse acritarchs with the established capacity of retaining the envelope volume and features during diagenesis and metamorphism. It is desirable to include the records of biological and ecological interpretation in the generic diagnoses of both this subgroup and Oblidomorphi Stan.

[71]  The subgroup Oblidomorphi Stan. includes acritarchs, which organic-walled specimens occur only in a crumpled, leaflike form, i.e. which are not capable to retain the original volume at the very beginning of diagenesis. The subgroup is subdivided into infragroups differing in morphological features. The names, diagnoses, and description of the infragroups correspond to those previously accepted for subgroups within the group Acritarcha Evitt, 1963 [Downie et al., 1963; Jankauskas et al., 1989]. The range of infragroups (subgroups) decreases at the expense of genera possessing volume and permitting their reference to the subgroup Implethomorphi.

[72]  The subgroup Incertae sedis contains morphologically heterogeneous microfossils, which characters do not permit their assignment to the rest two subgroups of acritarchs.


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Citation: Stanevich, A. M., V. K. Nemerov, Yu. K. Sovetov, E. N. Chatta, A. M. Mazukabzov, V. I.  Perelyaev, and T. A. Kornilova (2005), Precambrian microfossil-characterized biotopes from the southern margin of the Siberian craton, Russ. J. Earth Sci., 7, ES6003, doi:10.2205/2005ES000183.

Copyright 2005 by the Russian Journal of Earth Sciences

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