Acritarcha Acritarchs

At one time, spiny vesicular microfossils were often termed hystrichosphaerids (a designation that alludes to their morphology) regardless of their biological affinities. In 1961, Evitt conclusively demonstrated that many hystrichosphaerids were the cysts of dinoflagellates. Consequently, the remaining "hystrichosphaerids" whose affinities remain uncertain or unknown are placed in the artificial group Acritarcha (Evitt, 1963 b,c). As a result the acritarchs represent a highly heterogeneous group of organic-walled vesicular microfossils (FIGS. 4.74-4.75) interpreted as (cysts of) protists of different biological affinities (Mendelson, 1993; Colbath and Grenfell, 1995; Strother, 1996; Montenari and Leppig, 2003), including the cysts of some naked dinoflagellates (Tappan, 1980). Other acritarchs, however, have been interpreted as multicellular (green) algae (Butterfield, 2004; Stanevich et al., 2007), algal (zygnematacean) spores

FIGURE 4.73 Haeringiella multifidiformis. Bar=1 cm. (From Krings and Butzmann, 2005.)

FIGURE 4.74 Dicrodiacrodium sp. (Ordovician). Bar = 10pm. (Courtesy M. Vecoli and T. Servais.)

FIGURE 4.73 Haeringiella multifidiformis. Bar=1 cm. (From Krings and Butzmann, 2005.)

FIGURE 4.74 Dicrodiacrodium sp. (Ordovician). Bar = 10pm. (Courtesy M. Vecoli and T. Servais.)

Ordovician Phytoplankton
FIGURE 4.75 Evittia sp. (Silurian). Bar = 10pm. (Courtesy M. Vecoli and T. Servais.)

(Grenfell, 1995), and fungi-like organisms (Butterfield, 2005); other possible origins outside of the phytoplankton have been reviewed by Colbath and Grenfell (1995).

Microfossils assignable to the Acritarcha are among the first eukaryotes preserved in the fossil record (Huntley et al., 2006); they first occur in the Paleoproterozoic (Vidal and Moczydlowska-Vidal, 1997; Huntley et al., 2006) and extend to the Holocene (Mendelson, 1987). Acritarchs dominate the microfossil record in Proterozoic and Cambrian rocks. Some of the earliest accounts come from the Paleoproterozoic of China (Z. Zhang, 1997; S. Sun and Zhu, 2000). Other early acritarchs include the forms described from the Mesoproterozoic Roper (Javaux et al., 2001) and Bangemall (Buick and Knoll, 1999) Groups in Australia, and the Billyakh Group in northeastern Siberia (Sergeev et al., 1995). Paleo- and Mesoproterozoic acritarch assemblages are characterized by rather simple forms, whereas more complex forms typify Neoproterozoic assemblages (Sergeev et al., 1995). Slightly younger Proterozoic acritarchs have been described from shales of the Meso-Neoproterozoic Ruyang Group in China (L. Yin, 1998) and the Neoproterozoic Doushantuo Formation in South China (C. Zhou et al., 2001; Xiao, 2004).

Acritarchs are variable in size (on average 5-200 pm) and shape (FIG. 4.78); the vesicle (body) of an acritarch ranges from oval to triangular in outline and may possess various forms of projections. They are classified in an artificial system of morphotaxa based on a complement of morphological characters, like those of spores (FIG. 4.76), including

FIGURE 4.76 Emphanisporites annulatus (Devonian). Bar = 10 pm. (Courtesy M. Vecoli andT. Servais.)

FIGURE 4.77 Stelliferidium sp. (Ordovician). Bar = 10pm. (Courtesy M. Vecoli and T. Servais.)

size and shape of the vesicle, number and form of projections, number of symmetry levels, and form of the exit rupture (Williams et al., 2000; Montenari and Leppig, 2003). Regardless of their natural affinities, the widespread occurrence and complex morphology of acritarchs, as well as their rapid rate of evolution, have made them extremely valuable in long-range correlation and biostratigraphic zonation

Group

Morphology

Type genera

Possible biological relationships

Acanthomorphitae (akantha = spine)

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