Autotropic protists

(c) Thickened wall

Acritarchs

Acritarchs are hollow, organic-walled microfossils. They are believed to represent the cyst stages in the life cycles of planktic algae similar to a modern group, dinoflagellates, because both groups produce a characteristic molecule, dinosterane. They are one of the oldest groups of fossils and underwent a major radiation in the late Precambrian.

Morphology

Most acritarch vesicles range between 50 and 100 |lm in size and are usually preserved as compressed films in black shales. Acritarchs are generally spheroidal but their shape is very variable. Vesicle walls may be single or double layered and most central chambers have an opening considered to be for the release of the motile stage. Externally, acritarchs may be smooth or granulated and most have processes projecting from the vesicle surface. Some processes are branched or have elaborate distal structures, supported by stiff buttresses, while others are more simple and flexible (Figs 13.1 and 13.2). Acritarchs are classified on the basis of shape, wall structure and thickness, structure of opening, ornamentation, and form of the processes.

Paleoecology

Acritarchs mainly occur in marine sediments, often in association with other marine fossils. They have a worldwide distribution and occur in large numbers, consistent with being primary producers rather than consumers. Such evidence strongly suggests that they were members of the phytoplankton. Furthermore, their morphology shows adaptations consistent with a planktic mode oflife. In common with modern phytoplankton, nearshore acritarch assemblages tend to be of low diversity, often dominated by one species, whilst offshore assemblages are more diverse.

Evolutionary history

Acritarchs are amongst the oldest documented fossils. Known from chemical fossils from 2.7 billion years ago, they first became abundant 1 billion years ago and are arguably the most complex Precambrian microfossils. Most forms are large for single cells (between 50 and 100 |lm), and include species with a double-walled structure and ornate processes. Such diversity may represent an increase in marine productivity in the late Precambrian. A second radiation occurred in the early Cambrian. However, these forms are much smaller than those

Fig. 13.1 Acritarch morphology: (a) subcircular vesicle with broad-based processes that divide into two distally (diameter 30 |m), (b) subcircular vesicle with cristate (crested) surface sculpture (diameter 40 |m), and (c) circular vesicle with a thickened outer wall (diameter 25 |m).

of the Precambrian. Acritarchs continued to flourish through the Ordovician. They were affected by the end-Ordovician extinction event but recovered in the Silurian, where they may have reached their highest diversity. This level of diversity was maintained through the Devonian period until late Devonian times when there was a distinct diversity decrease. Acritarchs remained scarce for the rest of the Palaeozoic. A few specialized forms appeared in the Permian but dinoflagellate cysts, spores, and pollen are the dominant organic-walled microfossils of the Mesozoic and Cenozoic.

The first radiation of acritarchs, in late Proterozoic times, may represent an evolutionary phase of early experimentation of eukaryotic phytoplankton. This rapid rise in phytoplankton may also be linked to the establishment of sexual reproduction, if the acritarch cysts were part of a sexual life cycle. The second radiation in the early Cambrian corresponds with the major expansion of suspension feeders, emphasizing the important role of acritarchs in evolutionary history.

Acritarchs Morphology

Fig. 13.1 Acritarch morphology: (a) subcircular vesicle with broad-based processes that divide into two distally (diameter 30 |m), (b) subcircular vesicle with cristate (crested) surface sculpture (diameter 40 |m), and (c) circular vesicle with a thickened outer wall (diameter 25 |m).

(c) Thickened wall

Filamentous Acritarchs Neoproterozoic
Fig. 13.2 Early Neoproterozoic acritarch.

Dinoflagellates

Dinoflagellates are aquatic, unicellular organisms with organic-walled cysts. Usually referred to as algae, dinoflagellates have both plant-like and animal-like characteristics. During their life history about 10% of dinoflagellates develop resistant cysts that readily fossilize (Fig. 13.3). These cysts are first known from the Silurian and are important biostratigraphic indicators.

Morphology

The dinoflagellate life cycle has two stages: a motile stage, that rarely fossilizes, and a more durable benthic cyst stage. Cysts are formed from resistant organic material. Surface ornamentation may be smooth, granulated, or have raised crests and spines.

Ecology/paleoecology

Most living dinoflagellate species are photosynthetic and marine. They form an important part of the oceanic plankton and are one of the main primary producers in the open sea. As a group they are tolerant of a wide range of temperatures and salinities. Under some conditions shallow water dinoflagellate blooms, called red tides, can poison other marine groups and cause mass mortality.

Dinoflagellate paleoecology is difficult to determine as only a few living species produce cysts comparable with those found in the fossil record. Furthermore, cysts are easily transported by oceanic currents and fossil cysts may be found in areas not associated with the living species. However, some fossil forms are used as indicators of paleotemperature.

Coccolithophores

Coccolithophores are marine, unicellular, photosynthetic plankton of extremely small size (< 50 |lm, and therefore termed nan-noplankton). They secrete calcite platelets, coccoliths, that interlock to form a spherical shell, the coccosphere. Coccoliths have a distinctive circular or elliptical form. Similar-shaped calcite nannofossils form the Mesozoic and Cenozoic chalks.

Morphology

Coccolithophores build a spherical skeleton from between 10 and 30 wheel-shaped platelets. These are very small, typically 8 |m in diameter. Coccolith shape and structure is variable. Typically, a coccolith has an oval, button-like form with a central cross bar and radially arranged elements (Fig. 13.4). Fossil platelets that are of a similar size but are pentagonal, rhombohedral, star-shaped, or horseshoe-shaped often occur

Motile stage: rarely fossilizes

Benthic cyst stage: fossilizes

Motile stage: rarely fossilizes

Benthic cyst stage: fossilizes

Fig. 13.3 Dinoflagellate motile and cyst stages (length c. 25 ^.m).

Elements

Fig. 13.4 Coccolithophore morphology: (a) coccolith, and (b) coccosphere and coccoliths (scale bar 1 ^.m).

Elements

Fig. 13.4 Coccolithophore morphology: (a) coccolith, and (b) coccosphere and coccoliths (scale bar 1 ^.m).

in association with elliptical coccoliths. These are sometimes referred to as nannoliths.

Ecology/evolutionary history

Living coccolithophores are restricted to the photic zone and prefer oceanic water of normal salinity between 35 and 38%o (parts per thousand). With dinoflagellates they are one of the main primary producers in the oceans. In common with other phytoplankton, coccolithophore diversity is highest in the tropics and lowest in the high latitudes. Although subject to taphonomic processes, particularly dissolution and longdistance transportation, coccoliths are useful paleoclimatic indicators. Some species are tolerant of a narrow temperature range and can be used to directly determine paleotemperature, whilst relative temperatures may be estimated using ratios of warm-loving and cold-tolerant forms.

Coccoliths are known from the Upper Triassic but are very rare. Their abundance and diversity gradually increased through the Jurassic and Cretaceous, with a major radiation occurring in late Cretaceous times. Chalk is almost entirely formed from calcareous nannofossils and was deposited across vast areas at this time. Few coccoliths survived the end-Cretaceous extinction event. However, they rediversified and achieved a diversity maximum in the Eocene. Diversity has since fluctuated and is currently at its lowest point since the Cretaceous. Coccoliths are important in Mesozoic to Recent biostratigraphy.

Diatoms

Diatoms are unicellular algae with a distinctive two-part, siliceous skeleton, or frustule. Diatoms are found in virtually all marine, brackish, and freshwater environments and are even common at polar latitudes. Using a sticky secretion, they can form colonies or attach to the substrate. In modern oceans they are important primary producers. Diatom oozes form sediments in fertile waters with high concentrations of silica, nitrate, and phosphorous. These oozes lithify to form diatomites that are mined commercially for use as filtering agents and abrasives.

(a) Pennate (b) Centrale

Fig. 13.5 Diatom morphology: (a) Rhaphoneis(length c. 80 ^.m), and (b) Coscinodiscus(diameter c. 60 ^.m).

Fig. 13.5 Diatom morphology: (a) Rhaphoneis(length c. 80 ^.m), and (b) Coscinodiscus(diameter c. 60 ^.m).

Morphology

The diatom frustule is formed from two overlapping, nested valves. Most frustules are between 10 and 100 |lm in diameter. Valve structure and surface texture form the basis for diatom classification. Elliptical, bilaterally symmetric diatoms with a linear structural center are called pennates. Circular forms with radial symmetry around a central point are called centrales (Fig. 13.5). Ornamentation, pore pattern, and the presence of specialized structures are used to identify diatoms to genera and species level.

Ecology/paleoecology

As diatoms are photosynthetic they are reliant on light and nutrients for growth and reproduction. They are limited to the photic zone and are most abundant in areas of oceanic up-welling, where nutrient-rich deeper water is brought up to the surface waters. Intense diatom blooms, lasting 2-3 weeks, occur in response to seasonal upwelling. Centrales are most common as marine plankton, whilst pennates are more common in benthic marine habitats or in freshwater environments.

Diatom distribution is influenced by temperature, salinity, nutrients, and pH. In the reconstruction of paleoenvironment ratios of cold water to warm water, species have been used to estimate paleotemperature, and interglacial/glacial stages have been identified for the Quaternary using diatoms as indicators of paleosalinity. Diatoms can also be used to interpret water conditions in ancient lakes. As indicators of pH and fertility, diatom assemblages are also important in the monitoring of acid rain and pollution.

Evolutionary history

The first true diatoms are recorded from the Jurassic. During the Cretaceous they underwent a major radiation, and they suffered much less than other microfossil groups at the end of the period with only 23% of the genera becoming extinct.

Diversity continued to increase throughout the Cenozoic, with fluctuations in abundance being related to changing patterns of oceanographic circulation. The Palaeocene saw the first major expansion of diatoms into freshwater habitats. Diatoms reached their peak in the Miocene, possibly associated with increased volcanic activity that provided the necessary silica.

Biogeography

Areas of permanent upwelling correspond with the distribution of sediments rich in diatoms that concentrate in three main belts in the modern ocean (Fig. 13.6). A southern belt relates to the circulation of the circum-Antarctic current, an equatorial belt corresponds to the equatorial upwelling zone, and there is a weaker band in the northern oceans.

Fig. 13.6 Oceanic areas of very high silica extraction by plankton (more than 250 g of silica per square meter per year) in near-surface ocean waters. As diatoms are the dominant siliceous plankton group in the oceans this map approximates to a record of diatom production.

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  • selam
    Which microfossils groups are most common in upwelling nutrient rich water?
    8 years ago

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