INTRODUCTION
The Acritarchs are the fossilized remains of unicellular protists that are preserved as organic-walled microfossils (OWMs). They are vesicular, always preserved as hollow balls, formed by a resistant wall, which may be variously ornamented with superficial sculptural elements or extensions of the wall itself, such as spines or muri. The original term, acritarch, is a combination of acri (unknown) and arche (origin) that was constructed by Evitt (1963) and others (Downie et al. 1963; Downie and Sarjeant 1963) to accommodate hystrichospheres that could not be transferred to the dinoflagellates. The intention of these authors was that, in the future, when the systematic affiliations of particular acritarchs became known, those taxa would be transferred (re-classified) as biological species. Acritarchs are most abundant in fine-grained, siliciclastic sedimentary rocks of Ordovician to Devonian age, where they are thought to largely represent the cysts of marine phytoplankton (Downie 1973; Martin 1993; Servais 1996). They are especially important for the study of Precambrian life in both marine (Timofeev 1966; Vidal and Knoll 1983; Jankauskas et al. 1989; Porter 2004; Huntley et al. 2006) and non-marine settings (Strother et al. 2011) as they represent the bulk of the fossil record prior to the occurrence of the Ediacaran biota beginning around 635 Ma.
EVOLUTION OF ACRITARCHS
The earliest acritarchs in the fossil record are from the 3.4 Ga Pilbara Block (NW Australia) (Sugitani et al. 2010) and the 3.2 Ga Moodies Group (South Africa) (Javaux et al. 2010). These Archaean granular organic spheroids, some of which may be quite large > 100 µm, are problematic systematically as they occur long before the origin of the eukaryotic cell around 1.9 to 1.8 Ga. Peng (2009) described ovoidal and spherical acritarchs from the ca. 1.7 Ga Chuanlinggou Fm in North China which they show convincingly to be the earliest fossil eukaryotes. But the fossil record of acritarchs begins in earnest, during the Mesoproterozoic, after 1600 Ma. where acritarchs like Valeria lophostriata and Shuiyousphaeridium macroreticulatum represent some of the earliest differentiated eukaryotes (Javaux and Knoll 2016). Precambrian acritarchs are fundamentally different from their Phanerozoic counterparts because assemblages of Precambrian acritarchs include both vegetative and encysted forms, all of which remain completely problematic, both in terms of their ecology and systematic position. The vast majority of Neoproterozoic acritarchs fall into the category of sphaeromorphs, simple smooth-walled vesicles often classified as Leiosphaeridia Eisenack, although Grey (2005) has recognized an assemblage of Large Ornamented Ediacaran Microfossils (LOEMs) as characteristic of the later Ediacaran.
Acritarchs are important elements of the Paleozoic marine ecosphere because they are the sole fossil proxy for primary producers in the ancient water column. Acritarch morphological diversity and species richness explodes beginning in the latest Cambrian through to the Siluro-Devonian, but they almost completely disappear from the fossil record in the Early Carboniferous (Tappan 1980). Acritarch species-richness appears to track the Great Ordovician Biological Diversification Event (GOBE) (Servais et al. 2010), which enhances support for their interpretation as largely marine phytoplankton. Their somewhat mysterious decline at the end of the Devonian has not been solved: Strother et al. (2008) discussed global pCO2 as a possible cause of phytoplankton extinction, whereas Martin & Servais (2020) consider global patterns of nutrient availability as controlling Phanerozoic phytoplankton diversity. In any case, acritarchs persist today in somewhat diminished numbers, but, except for the ongoing of the recognition of their inclusion in NPPs, their role in marine ecosystems was effectively replaced by the dinoflagellates beginning in the Triassic.
CLASSIFICATION OF ACRITARCHS
The informal group, Acritarcha Evitt 1963, was originally divided into the Subgroups: Acanthomorphitae, Polygonomorphitae, Prismatomorphitae, Oömorphitae, Netromorphitae, Dinetromorphitae, Stephanomorphitae, Pteromorphitae, Herkomorphitae, Platymorphitae, Sphaeromorphitae, and Disphaeromorphitae. Realistically, few authors use these categories as an aide to the characterization and classification of the acritarchs, instead, typically resorting to an alphabetical list of generic names in systematic treatments. The number of taxa transferred out of the acritarchs since 1963 is remarkably sparce, which is a testament to the difficulty in determining phylogenetic affinities of fossilized protists. Many palynologists have recognized that some acritarchs (e.g. Tasmanities, Halosphaera and Pterospermella) are Prasinophycean green (a+b) algae. Other forms of freshwater chlorophytes, including members of the Zygnematophyceae and Hydrodictyaceae have been acknowledged as such in recent publications (Mays et al. 2021). The distinctive acritarch Moyeria Thusu 1973, is now considered to belong to the Euglenophyceae, a photosynthetic protist group unrelated to marine phytoplankton (Gray and Boucot 1989; Strother et al. 2020).
RESOURCES FOR FURTHER STUDY
Acritarch taxonomy is not particularly difficult, but, as with many fossil groups, a thorough understanding of morphological variation at the population level is important for gaining an appreciation of how to describe new taxa. There are several reviews of acritarchs in the literature (Martin 1993; Strother 1996; Playford 2003), and even older ones (e.g. (Downie 1973) can be quite useful. These works all contain introductions to morphological terminology used to describe acritarchs, in addition to more information about the biostratigraphic applications of acritarchs. The index by Fensome et al. (1990) is the single most important reference for taxonomic work on acritarchs, but the online reference database, Acritax, based on the John Williams card index and the Jansonius & Hill (1976) card index are also essential resources for acritarch study.
Figure caption. Acritarchs. Scale bars =10µm. A, B. Precambrian acritarchs from the Nonesuch Fm (1.05 Ga) Michigan. A. Valeria lophostriata an early acritarch with striated wall structure (arrow). B. Leiosphaeridia, a basic sphaeromorph acritarch showing fungal (chytrid) damage (arrow). C-E, Lower Ordovician acritarchs from the Prague Basin. C. Arbusculidium filamentosum a diacrodioid (bipolar) form. D. Micrhystridium sp, a common acanthamorph acritarch E. Veryachium lairdi with a preformed opening (epityche). F. Hoegklintia a large polygonomorph with digitately branching tips, from the Silurian (Wenlock) of New York State.
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