Acritarch Morphology


Figure 9.10 Some radiolarian morphotypes: Lenosphaera (x100), Actinomma (x240), Alievium (x180), Anthocyrtidium (x250), Calocyclas (x150) and Peripyramis (x150).

was a monument to our ignorance. Although many more taxa have been described since, and their value in biostratigraphic correlation has been proved, uncertainty still surrounds the origin and affinities of the group. Similarity, however, with the cyst stages of modern prasinophytes and dinoflagellates suggests a relationship to primitive green algae. However, because they are very useful in hydrocarbon exploration, perhaps the minor issue of their identity can be left for future generations!

Morphology and classification

The composition and broad morphology of the acritarchs suggest similarities with the dinocysts; like the dinocysts, acritarchs are also often found in clusters. The group probably had a similar life cycle to that of the dinoflagellates, single-celled protists that mainly live in the marine plankton today. Acritarchs seem to show encystment structures, or cysts - protective devices similar to those of modern dinoflagellates, in which the organism can survive drying out or lack of food for long periods. When conditions return to normal, usually when the cyst is covered with water again, the organism "escapes" by bursting through the watertight skin of the cyst, and resumes feeding and reproducing. A number of escape structures have been described including median splits, pylomes and cryptopylomes, that would have allowed material to seep out.

Acritarchs consist of vesicles composed of various polymers combined to form sporopol-lenin (Fig. 9.12). They range in shape from spherical to cubic and in size from usually 50 to 100 ^m, although some specimens from the Triassic and Jurassic are as small as 1520 ^m. Many lose these morphological details when preserved as flattened films in black shales. There is a huge variety of basic shapes (Fig. 9.13). Acritarchs can have single- or double-layered walls; the wall structure is often useful taxonomically. The central cavity or chamber can be closed or open externally through a pore or slit called the pylome. The opening or epityche presumably allowed the escape of the motile stage and may be modified with a hinged flap.

On the outside the acritarch may be smooth or, for example, have granulate or microgranulate ornament. Moreover, the vesicle may be modified by various extensions or processes projecting outwards from the vesicle wall. If an acritarch has a set of similar processes, they are termed homomorphic, and if it has a variety of different projections it is heteromorphic.

Over 1000 genera of acritarchs are known, defined mainly on shape characteristics (Box 9.7). All acritarchs were aquatic with the vast majority found in marine environments. The classification of the group is based on the wall structure, the shape of the body vesicle, pylome type and the nature of the extensions and processes.

Box 9.6 Ernst Haeckel, art and the radiolarians

The link between art and paleontology has always been strong, with many images finding their inspiration in the beauty of the fossil form. Ernst Haeckel (1834-1919), the German evolutionary biologist, responsible for such terms as "Darwinism" and "ecology", the phrase "ontogeny recapitulates phylogeny" and the first detailed tree of life (see p. 128) was also an accomplished artist; he believed in the esthetic dimension of morphology (Fig. 9.11). His giant opus Art Forms in Nature (1899-1904) is considered to be one of the most elegant, artistic works of the 19th century, his illustrations being a paleontological precursor to the Art Nouveau movement. His style is nowhere better presented than in his monograph on the Radiolaria (Haeckel 1862). Unfortunately his attempts to associate science with art may have damaged his career, but current interest in the tree of life has generated a Haeckel renaissance. His illustrations are even available now as an attractive screensaver!

You can see these beautiful images at

Haeckel Shape

I.TlnlasMsphaeia bifidca.QE. SZyiostepliiuius Wilileri.HtL 5-{i llii'lvtiflm Messuuirasis, ffij I Petalospyris anrtaoifeHM "SjftTidnWtns QiaacikHlil JII flotnncatu^> linjtBtlinliimia.IB.lll.i Spanoosfhapra Maudes EUJ 14.1' S^idiscus IfcdileininimiB:

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Figure 9.11 Haeckel's radiolarians: plate 12 from Die Radiolarien (Rhizopoda Radiaria) by Ernst Haeckel (1862).

Box 9.7 Classification of the main organic-walled groups: acritarch form classification

The classification of acritarchs is based entirely on morphology and as such is merely a set of shape and ornament categories with no phylogenetic status. The names of the main groups thus are also used as morphological terms to define the variation in shape (see Fig. 9.12). Clearly such a classification is rife with convergent morphotypes that may never be properly classified. Recent studies, however, suggest that understanding the mode of encystment may be a step towards the development of a more phylogenetic classification.


• Spherical forms lacking processes but with ornamented walls. These morphs are often variably ornamented

• Precambrian (Animikean) to Recent


• Subpolygonal or spherical with polygonal ornament defined by crests

• Cambrian (Lower) to Recent


• Forms equipped with an equatorial flange

• Ordovician (Caradoc) to Recent


• Spherical forms lacking an inner body and crests, with simple or branching processes Polygonomorphs


• Elongate, commonly fusiform morphs with poles variably developed as processes or spines Diacromorphs

• Spherical to ellipsoidal, with ornament restricted to around the poles Prismatomorphs

• Polygonal or prismatic, with edges commonly extended as flanges Oomorphs

• Egg-shaped forms, one end smooth and the other highly ornamented







Figure 9.13 Some acritarch morphotypes: Multiplicisphaeridium (x800), Baiomeniscus (x200), Leiofusa (x400) and Villosacapsula (x400).

Evolution and geological history

Acritarchs had a wide geographic range, apparently mainly controlled by latitude; the entire group ranged from the poles through the tropics. The wide distribution of the group is similar to that of the dinoflagellates and strongly suggests that acritarchs were also members of the phytoplankton. Biogeographic provinces have been established for the Ordo-vician, Silurian and Devonian periods and have helped reconstruct ancient climate belts and oceanic currents. Acritarchs have also been of considerable value in regional correla tions, particularly during the Ordovician and Silurian.

The acritarchs are some of the oldest documented fossils with a history of over 3000 myr, although the group was not common until some 1 Ga, when the first major diversification of the group, predating the Ediacara biota (p. 242), was marked by large sphero-morphs, acanthomorphs and polygono-morphs. During the important Early Cambrian radiation of the group, spinose morphs such as Baltisphaeridium and Micrhystridium, together with the crested Cymatiosphaera, appeared. Significantly these armored vesicles evolved during the expansion of marine predators: Was this a form of arms race or merely a coincidence? By the Late Cambrian to Early Ordovician, acritarch palynofacies (pollen and spore assemblages) were dominated by three main groupings: the Acanthodiacro-dium, Cymatiogalea and Leiofusa groups (Box 9.8). The acritarchs declined during the Devonian, and are rare in Carboniferous-Triassic rocks. Nevertheless the group staged a weak recovery during the Jurassic and continued through the Cretaceous and Tertiary.


The dinoflagellates, or "whirling whips", comprise a group of microscopic algae with organic-walled cysts. The life history of these organisms thus oscillates between a motile (swimming) and a cyst (resting) stage; the cysts usually range in size from 40 to 150 ^m. The motile phase is either flexible and unar-mored, or rigid and armored with a network of plates, the theca; the arrangement of the

Box 9.8 Acritarchs and the food chain

Groups such as the acritarchs formed a prominent base to the relatively short, suspension-feeding Early Paleozoic food chains, yet it is virtually impossible to quantify the abundance of microfossils in sediments because many factors such as cyst production, hydrodynamic sorting and taphonomy come into play. Unfortunately, diversity cannot be used as a proxy for abundance, so there is no direct evidence in the fossil record of just how densely packed the water column was with phyto-plankton, say during the Ordovician. However, it may be possible to speculate that primary production increased rapidly during the Ordovician: This period was marked by the appearance and radiation of the graptolites, phyllocarids, some groups of echinoderms and the radiolarians. Huge bursts in diversity are seen among the brachiopods, mollusks and trilobites, while there was increasing complexity in benthic and reef communities. Yet little is known about the cause of this phenomenal diversification. Marco Vecoli and his colleagues (2005) have now suggested that these massive metazoan radiations probably signal a cryptic explosion in primary production in the world's oceans (Servais et al. 2008) that may have been one of the main triggers for the great Ordovician biodiversification (see p. 253). The diversity curves of these protistan groups appear to match perfectly those of the metazoans (Fig. 9.14).

Acritarchs Morphology

0 50 100 100 40 80 10 20 50 100 150 20 40 20 40

species/genera genera species genera species species genera

Figure 9.14 Acritarch and invertebrate diversity through Ordovician Period. (Courtesy of Thomas Servais.)

0 50 100 100 40 80 10 20 50 100 150 20 40 20 40

species/genera genera species genera species species genera

Figure 9.14 Acritarch and invertebrate diversity through Ordovician Period. (Courtesy of Thomas Servais.)

apex or anterior end apex or anterior end

Calpionellids Structure
antapex or posterior end ventral surface dorsal surface

theca cyst

Figure 9.15 Descriptive morphology of (a) a dinoflagellate, and (b) a dinoflagellate theca (left), unpeeled (middle) to reveal the corresponding cyst (right).

thecal plates comprises the dinoflagellate tabulation.

Morphology and classification

The plates of a dinoflagellate theca are arranged from the apex to antapex as follows: apical, precingular, cingular, postcingular and antapical; the first two are part of the eipi-theca and the last two, the hypotheca (Fig. 9.15). There are a number of other plates with further specialized terms and together the plates are commonly labeled and numbered in sequence. The motile phase is rarely fossilized. In contrast, the cysts are chemically resistant and relatively common. The morphology of a motile dinoflagellate is crudely similar to its theca and comparable structures in the motile form are prefixed by the term "para".

Cysts have a paratabulation that is useful taxonomically (Box 9.9): for example, the cysts of peridiniaceans have seven precingular and five postcingular paraplates, whereas the gonyaulacaceans have six precingular and six postcingular paraplates.

Dinoflagellates are abundant and diverse members of the living and more recent fossil phytoplankton (Fig. 9.16), forming an important part of the base of the food chain of the oceans; they may in fact be second only in abundance to diatoms as primary producers. However, dinoflagellate blooms or red tides, when there is huge population explosion, can lead to asphyxiation of other marine groups. Mass mortalities of Cretaceous bivalves in Denmark and of Oligocene fishes in Romania have been blamed on fossil red tides.

There are three main cyst types. The proximate cyst is developed directly against the theca itself and has a similar configuration. A

Box 9.9 Classification of the main organic-walled groups: dinoflagellate form classification


Most dinoflagellates belong to this class, which includes fossil representatives. They are free-living cells with a large nucleus and numerous chromosomes; some are parasites and symbionts.


• Cretaceous to Recent


• Cretaceous


• Triassic to Recent






• Jurassic to Recent Order PERIDINIALES

• Triassic to Recent


• Triassic to Recent


Parasites on or in copepods and other animals. Order BLASTODINIALES


Very large, naked cells lacking chloroplasts. Order NOCTILUCALES


Symbionts or endoparasites lacking chloroplasts Order SYNDINIALES


Figure 9.16 A prasinophyte (a) and some dinoflagellate taxa (b-h): (a) Tasmanites (Jurassic), (b) Cribroperidinium (Cretaceous), (c) Spiniferites (Cretaceous), (d) Deflandrea (Eocene), (e) Wetzeliella (Eocene), (f) Lejeunecysta (Eocene), (g) Homotryblium (Eocene), and (h) Muderongia (Cretaceous). Magnification x250 (a, d, e), x425 (b, c, f, g, h). (Courtesy of Jim Smith.)

Figure 9.16 A prasinophyte (a) and some dinoflagellate taxa (b-h): (a) Tasmanites (Jurassic), (b) Cribroperidinium (Cretaceous), (c) Spiniferites (Cretaceous), (d) Deflandrea (Eocene), (e) Wetzeliella (Eocene), (f) Lejeunecysta (Eocene), (g) Homotryblium (Eocene), and (h) Muderongia (Cretaceous). Magnification x250 (a, d, e), x425 (b, c, f, g, h). (Courtesy of Jim Smith.)

chorate cyst is smaller than the theca and the cysts are contained within the theca, interconnected by various appendages and spines, which are related to the external tabulation of the theca. In cavate morphs there is a gap between the cyst and the theca at the two poles.

Evolution and geological history

Dinoflagellate biomarkers have been identified in Upper Proterozoic and Cambrian rocks. Moreover the Late Precambrian and Paleozoic diversifications of the acritarchs may mark an early phase in dinoflagellate radiation, involving non-tabulate forms. To date, however, the oldest dinoflagellate cyst is probably Arpylorus from the Ludlow (Upper Silurian) rocks of Tunisia; the cyst has feeble paratabulation and a precingular archeopyle. Oddly, there is a long gap after this record until the Early Triassic, when Sahulidinium appears off northwest Australia. Some authors have suggested that a number of Paleozoic acritarch taxa may in fact be dinoflagellates. Multiplated forms such as Rhaetogonyaulax and Suessia appearing in the Late Triassic characterize dinocyst floras ranging from Australia to Europe. Nannoceratopsis cysts with characteristic archeopyles and tabulation are common in Early Jurassic floras, while Ceratium-like forms appeared first during the Late Jurassic and diversified in the Cretaceous. Many precise zonation schemes for Mesozoic and Cenozoic strata are based on dinocyst distributions. However, during the Eocene the global biodiversity of the group began a steady decline.


The Ciliophora today consist of some 8000 species of single-celled organisms that swim by beating their cilia, minute hair-like organs. Two fossil groups, the calpionellids and tin-tinnids, may belong here. Calpionellids are a s x







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