Group Acritarcha

Downie (1973, 1974), Fensome et al. (1990) and Doming (in Benton 1993, pp. 33-34) have reviewed the classification of the acritarchs. Informal groupings have been established on overall morphology, wall structure and type of excystment opening. None of the published schemes reflects biological affinity or evolution and some workers prefer to list taxa alphabetically. Biometrical studies (e.g. Servais et al. 1996) and the chemistry of the vesicle (e.g. Colbath & Grenfell 1995) may offer the potential for a more natural classification to be developed. Alete spores can be distinguished from acritarchs with difficulty based on their thicker walls and colour variation within an individual specimen. Most acritarchs fall into three morphological groups, each of which includes one or more acritarch subgroups.

1 Acritarchs lacking processes or crests

Subgroup Sphaeromorphitae (Precamb.-Rec.; Fig. 9.2) This includes acritarchs with a spherical or ellipsoidal vesicle which may be variously ornamented. The often thin, simple, imperforate wall may develop an irregular or cyclopyle opening. Many of the large Neoprotoerozoic acritarchs, such as Chuaria (U. Precamb.), may belong here, although this genus is exceptionally large (<5 mm diameter) and may cu .S

o CL

till III

a.

1

1

1

c c 0) CL

1 Miss.

J

\

j

L

I Dev. I

I

|

1

w

■g O

1

1 Cam.l

1

1

1

o

Fig. 9.2 Generalized ranges of major acritarch groups. Arrows indicate groups with a post-Permian record, width of line approximates to number of species. (After Mendelson in Lipps 1993.)

represent the carbonaceous impression of a nosto-cales cyanophyte alga (Martin 1993). Leiosphaeridia (U. Precamb., Palaeozoic, Fig. 9.1a) may have had green algal affinities.

2 Acritarchs with crests but lacking processes

Subgroup Herkomorphitae (Camb.-Rec.; Fig. 9.2) These have spherical to subpolygonal acritarchs in which the vesicle is divided into polygonal fields by crests, for example Cymatiosphaera (Camb.-Rec., Fig. 9.1i). The

Tasmanites

Fig. 9.3 Photomicrographs of selected acritarchs and a prasinophyte. Scale bars = 20 |m. (a) SEM image of Diexallophasis sp. (b) Leiosphaeridium sp., from the Upper Proterozoic. (c) Leiofusa sp., from the Whitcliffe Formation, Ludlow. (d) Tasmanites pradus, a prasinophyte. (e) Pterospermella sp., Silurian. ((a) Reproduced from Lipps 1993, figure 6.12, D2); (d) reproduced from Traverse 1988, figure 6.9l; both with permission.)

Fig. 9.3 Photomicrographs of selected acritarchs and a prasinophyte. Scale bars = 20 |m. (a) SEM image of Diexallophasis sp. (b) Leiosphaeridium sp., from the Upper Proterozoic. (c) Leiofusa sp., from the Whitcliffe Formation, Ludlow. (d) Tasmanites pradus, a prasinophyte. (e) Pterospermella sp., Silurian. ((a) Reproduced from Lipps 1993, figure 6.12, D2); (d) reproduced from Traverse 1988, figure 6.9l; both with permission.)

wall is perforate and without known excystment openings. The vesicle, originally spherical or polygonal, is divided into fields by crests. Members of this subgroup are now considered along with the Prasinophyta as green algae. In Cymatiogalea (M. Camb.-Tremadoc, Fig. 9.1j) the vesicle is divided into polygonal fields by crests, somewhat resembling a proximate dinoflagel-late cyst, but it has a large cyclopyle opening. Some species of Cymatiogalea bear processes and these may belong in the Acanthomorphitae.

spherical central body, over 20 |m in diameter, with simple hollow or solid processes with closed tips. Micrhystridium (L. Camb.-Rec., Fig. 9.1g) has a spherical central body <20 |im in diameter with simple processes. Visbysphaera (L. Sil.-L. Dev., Fig. 9.1d) is spherical, characterized by a double-layered wall and a lateral rupture and bears processes that are produced from the outer wall.

Subgroup Diacromorphitae (Camb.-Dev.; Fig. 9.2) This subgroup comprises acritarchs with a spherical or elliptical vesicle in which the equatorial zone is smooth and the polar areas are ornamented. The simple wall tends to split up into angular plates when damaged. The openings are of varying kinds but the vesicles are typically elongate with the sculpture concentrated at one or both poles. Acanthodiacrodium (M. Camb.-M. Ord., Fig. 9.1c) has small processes at both poles and an equatorial constriction.

Subgroup Netromorphitae (Precamb.-?Triassic; Fig. 9.2) Long elongate, fusiform acritarchs in which one or both poles may be extended as processes, with a median-or lateral-split or a C-shaped epityche opening, for example Leiofusa (U. Camb.-U. Carb., Figs 9.1l, 9.3c). Deunffia (Sil., Fig. 9.1m) bears a single process whilst Domasia has three processes (Sil., Fig. 9.1n).

Subgroup Oomorphitae (Camb.) This subgroup has an egg-shaped, polarized, vesicle, generally smooth at one end and ornamented at the other. Ooidium (Camb., Fig. 9.1o) is ovate with a granular sculpture at one pole and a spongy sculpture at the other, with fine striae between.

Acritarchs

Subgroup Pteromorphitae (Ord.-Rec.; Fig. 9.2) These are similar to the herkomorph acritarchs in overall shape, but are distinguished by possessing an equatorial flange (ala), for example Pterospermella (Figs 9.1k, 9.3e).

Was this article helpful?

0 0
Freehand Sketching An Introduction

Freehand Sketching An Introduction

Learn to sketch by working through these quick, simple lessons. This Learn to Sketch course will help you learn to draw what you see and develop your skills.

Get My Free Ebook


Post a comment