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Importance for Acritarch Systematics SBORNÍK NÁRODNÍHO MUZEA V PRAZE ACTA MUSEI NATIONALIS PRAGAE Řada B – Přírodní vědy • sv. 64 • 2008 • čís. 2–4 • s. 97–107 Series B – Historia Naturalis • vol. 64 • 2008 • no. 2–4 • pp. 97–107 MORPHOLOGIC VARIABILITY IN LOWER PALAEOZOIC ACRITARCHS: IMPORTANCE FOR ACRITARCH SYSTEMATICS OLDŘICH FATKA Charles University, Institute of Geology and Palaeontology, Albertov 6, 128 43 Praha 2, Czech Republic; e-mail: [email protected] RAINER BROCKE Research Institute Senckenberg, Palynology and Microvertebrates of the Palaeozoic, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany; e-mail: [email protected] Fatka, O., Brocke, R. (2008): Morphologic Variability in Lower Palaeozoic Acritarchs: Importance for Acritarch Systematics. – Acta Mus. Nat. Pragae, Ser. B, Hist. Nat., 64(2–4): 97–107. Praha. ISSN 0036-5343. Abstract. Intraspecific variability of Lower Palaeozoic acritarchs has a basic impact on systematic classification on both species and genus levels. The description of new taxa (including taxonomic splitting) dominated the initial period of acritarch classification. A second period, indicated by the revision of morphological variability, started in the mid nineties of the last century. Major methods and results of these stu- dies are shortly summarized. n Lower Palaeozoic, acritarchs, systematics. Received November 24, 2008 Issued December 2008 Introduction phology. Strother (1996) pointed to the most specific fea- ture of this generally used classification: the basic taxonom- Evitt (1963) established the informal group Acritarcha ic unit is the genus and not the species. The artificial classi- incorporating all organic-walled microfossils (OWM) of fication is also represented by different subgroups as pro- unknown origin. The systematic classification of Acritarcha posed by Downie et al. (1963) and Deflandre and Deflandre is uncertain by its definition (Evitt 1963) and thus forms a (1964). Downie et al. (1963) established thirteen major “collection basket” of naturally not assignable organisms, morphological subgroups (Acanthomorphitae, Diacromor- belonging both to the animal and plant phyla. However, it is phitae, Dinethromorphitae, Disphaeromorphitae, Herko- generally accepted that a great proportion most probably morphitae, Nethromorphitae, Oomorphitae, Platymorphitae, represents various cysts of phytoplankton (Downie et al. Polygonomorphitae, Prismatomorphitae, Pteromorphitae, 1963; Tappan 1980; Martin 1993; Molyneux et al. 1996; Sphaeromorphitae, Stephanomorphitae). Deflandre and Servais et al. 1997; Strother 2008). Deflandre (1964) suggested to classify Acritarcha as ”Para- Despite their problematic origin combined with the ordo”, and consequently they designated Downie´s et al. comparatively short history of their studies and quite exten- (1963) ”subgroups” as ”Parafamiliae” and ”genera” as ”Pa- sive laboratory processing, acritarchs have been well ragenera”, respectively. Still before the definition of acri- employed for biostratigraphic purposes (Brocke et al. tarchs as widely used today, there were several proposals 1995). More recently, acritarchs are of increasing value for how to classify OWM obtained from Precambrian to Meso- palaeoecological application (e.g., Le Hérissé 1989). Simi- zoic sediments (e.g., Naumova 1949, 1950, Timofeev 1964). larly as in other incertae sedis groups, the classification of However, within the last fifty years this artificial classi- acritarchs is based on morphological features like shape and fication was repeatedly criticized by several acritarch work- symmetry of the cyst, number, length and distribution of ers and alternatively some of them proposed more natural processes, type of opening, etc. classification schemes. Mädler (1963, 1964) recommended Two major approaches are currently in use for the clas- a natural system indicated as “Algae incertae sedis” and sification of acritarchs: an artificial system of categorisation established the class Hystrichophyta for those taxa which and a naturally directed one. The artificial classification could be excluded from the acritarchs. In this sense, he pro- introduced by Downie et al. (1963) is purely phenetic, as posed the orders Tasmanales, Leiosphaeridiales and Hystri- they grouped the taxa by simple comparison of their mor- chosphaeridiales including several families. Eisenack (1969) 97 considered acritarchs (hystrichospheres) as a heterogenetic ply in alphabetical order but try to exclude those forms group but many of them he regarded as unicelullar algae which show clear affinities to different groups of algae. and thus applied the botanical nomenclature. He followed Mädler´s classification scheme in parts by using the term Methods of study Hystrichophyta. However, his concept for families differs from the one Mädler had established (1964). Cramer (1970) Research in OWM since the early 1930ies has given rise and Cramer and Díez (1979) did not adopt Downie´s to two major methods of optical microscopy: scheme as well, they introduced three, more widely defined – study of isolated specimens from organic residues informal units instead (Sphaeomorphitae, Acanthomorphi- received after laboratory treatment (mechanical and tae, and one undefined unit for the remaining taxa) and rec- chemical maceration) ommended a simple alphabetical listing of taxa. Tappan – thin sections of preferable fine grained sediments like (1980) did not follow the non-Linnean classification but she shale, siltstone and marl simply listed the earlier established subgroups, and effec- This classical approach has been supplemented by the tively transferred some acritarchs (e.g., Tasmanites, application of scanning electron microscopy (SEM) starting Leiosphaeridia, Cymatiosphaera) to the group of Green in the early 1960. More methods have followed, employed Algae (Prasinophyta, Chlorophyta and Euglenophyta). for special application, like TEM (Jux 1971, 1977), infrared Today, the majority of acritarch workers follows Cra- microscopy (e.g., Brocke and Wilde 2001), confocal mic- mer´s (1970) pragmatic approach to classify acritarchs sim- roscopy and Raman microscpopy (e.g., Kudryavtsev et al. Text-fig. 1 Ranges of the revised taxa from Cambrian to Devonian; also indicated in table 1. 98 2001). The observation of objects scratched from the sur- 5. general morphological trends in stratigraphical succes- face of metamorphosed rocks was introduced by Pacltová sion associated with transgression; e.g., Fatka and (1986), similarly as the scanning electron microscopy of Brocke (2008) fresh rock surface (Pacltová 1977). SEM of polished, slight- 6. orientation of specimens within a cluster; e.g., Vangues- ly etched rock surface applied for sedimentological study of taine in Streel et al. (1988). Silurian micritic limestones from the island of Gotland revealed Examples of simple biometrical analyses are shown in three-dimensionally preserved OWM, including acritarchs figures 2 and 3. Several measured morphological parame- and prasinophytes (Munnecke and Servais 1996). ters are indicated for Leiosphaeridia (Text-fig. 2A), Navi- Attempts to implement a biological approach in the fusa (Text-fig. 2B) and Eliasum (Text-fig. 2C), and scatter classification of OWM have lead to the separation of pra- diagrams of the central body length (CB ) against central sinophyte phycomata from the so-called Hystrichospheres L body width (CB ) for specimens of Leiosphaeridia spp. (Mädler 1963). However, not till the substantial summary W of plant protists by Tappan (1980) the prasinophytes (e.g. (Text-fig. 2D) and Navifusa bacilla (Text-fig. 2E). Contin- the families Cymatiosphaeracea, Leiosphaeridacea, Pteros- uous variability in samples KL-45, Da-10 and DA-15 doc- permatacea, Tasmanitacea, Pterosphaeridiacea and Pteros- uments the presence of one single taxon, while several dis- permellacea of green algae) have achieved sustained effect crete clouds in sample KL-51 gives evidence for the pres- to exclude them from the artificial group Acritarcha. ence of more taxa. Colbath and Grenfell (1995) recognized three, probably In the text-figure 3, two height histograms for a popula- evolutionary clades based on wall architecture, namely tion of Eliasum show the number of specimens attributed to six classes which are based on the central body width (fig. (1) the Cambrian–Permian Baltisphaeridium clade, 3A), and the number of thickenings on the central body (2) the Middle-Upper Ordovician Peteinosphaeridium cla- (Text-fig. 3B). de, and The scatter diagram of measurements of central body (3) the Lower Silurian-Lower Devonian Cymbosphaeridi- length (CB ) against central body width (CB ) for speci- um clade. L W mens of Eliasum show a continuous morphological vari- In this context, Colbath and Grenfell (1995) also empha- ability (Text-fig. 3C) and changing number of thickenings sized the resemblance of dinocysts and members of the on the central body of Eliasum when plotted in stratigraph- Cymbosphaeridium clade. ic order (Text-fig. 3D). Until the end of 1980ies acritarch studies were mainly In addition to the acritarchs, a comparable morphologi- focused on their biostratigraphic application and thus cal variability has also been documented from other OWM, numerous new taxa have been established. Unfortunately, e.g. from the genera Chuaria WALCOTT 1899 (see Steiner the majority of these new genera and species are based on a 1997), Marpolia WALCOTT 1919 and Siphonophycus quite restricted number of specimens studied without taking SCHOPF 1968 emend. KNOLL, SWETT et MARK 1991 (see into account
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