Coccoliths: Tiny fossils with immense paleontological importance

Robert Sturm (Austria)

Microfossils, such as foraminifera, diatoms, ostracods and conodonts, are usually studied using a magnifying glass or under a stereo-microscope. However, nannofossils, such as coccoliths – with sizes measured in micrometres – are way beyond the resolving power of these optical tools.

In general, coccoliths are very regularly shaped, fine calcite platelets that are produced by unicellular, autotrophic (that is, capable of synthesizing their own food from inorganic substances), marine algae – the so-called Coccolithophorida (phylum: Haptophyta; class: Prymnesiophyceae). They are arranged in spheres (coccospheres) that completely enclose the organisms. After the death of the algae, the coccospheres are either preserved entirely or fall to pieces. Then, either way, they settle on the seafloor and can do so continuously over periods of millions of years.

Coccoliths are of immense value to the palaeontologist, because of their highly specific shapes. For this reason, they are frequently used for biostratigraphic investigations, where the ages of lithological strata are estimated by using the fossils found in them.

Palaeontology distinguishes between two types of coccoliths – the holococcoliths and the heterococcoliths. The first type is composed of calcite crystals of identical size, whereas the second consists of variably-sized calcite crystals. Extant Coccolithophorida preferentially produce heterococcoliths during the phase of their life cycle when they are immobile.

In this article, I will provide a brief overview of the biology of the Coccolithophorida, their biostratigraphic significance and the main steps that have to be undertaken for a successful preparation of sediment samples to look at coccoliths.

The biology and biostratigraphic significance of the Coccolithophorida

The morphology of recent Coccolithophorida differs from other planktonic algae in one essential way – besides the two regular flagella used for locomotion, they have a third, helically-shaped haptonema. The haptonema can be distinguished from the flagella by its internal structure and its basal connection to the cell. However, the function of the haptonema has not yet been clarified, because, unlike flagella, it is not discarded during the immobile phase of the organism.

The cell of the Coccolithophorida also contains a nucleus with a small nucleolus, as well as two chloroplasts that take up a considerable area. The chloroplasts may change their position within the cell to optimise photosynthesis. Mitochondria provide energy for the cell, vacuoles for the disposal of cellular waste and a method of for secreting coccolith-building substances.

In many species of Coccolithophorida, the outer cell membrane is covered by small organic platelets acting as a kind of template for the precipitation (that is, the crystallisation) of the coccoliths. The formation of these is both induced and controlled by light. This was impressively demonstrated for Emiliana huxleyi, whose production of coccoliths starts within the first 30 minutes of exposure to light, while the whole coccosphere is commonly synthesised within just 30 hours (Bown, 1998).

Coccolithophorida generally occur as haplonts (that is, with a single set of chromosomes) and as diplonts (double set of chromosomes), which both reproduce by mitotic cell division. The diploid, coccolith-producing cell phase exhibits an increased ability to reproduce asexually and this leads to a rapid increase in the population under optimum environmental conditions. The function of the coccoliths is to provide protection, buoyancy and be a light reflector protecting the cell from the sun’s radiation.

Although the use of fossil coccoliths for biostratigraphy was described by Sorby (1861) and Lohmann (1902) in the second half of the 19th century, the real palaeontological significance of these organisms was not really recognised before the 1960s, during a flowering of nannofossil research when significant contributions were made by the Deep Sea Drilling Project and the Ocean Drilling Program. Nowadays, coccoliths are preferential indicators to determine the age of post-Palaeozoic carbonate sediments.

The old calcareous nannofossils (the so-called nannoliths) date back to the late Triassic (about 220Ma) and can be found in the northern and southern limestone alps of Austria, in Timor, north-western Australia and Canada. However, the zenith of the Coccolithophorida radiation was during the Cretaceous (145 to 65Ma) with the occurrence of, for example, Watznaueria barnesae, Arkhangelkiella cymbiformis and Eiffellithus eximius and in the Tertiary (65 to 1.64Ma) with the appearance of the genera Discoaster, Chiasmolithus and Coccolithus.

Today, unicellular algae are believed to be among the most important phyto-planktonic organisms in the oceans.

How to prepare and view samples containing coccoliths

The diminutive size of nannofossils makes them mostly insensitive to any types of mechanical influence on the host sediment. Furthermore, the size of the fossils also guarantees a high number of fine-looking objects within very small volumes of sediment. Their drawbacks include the dissolution of fragments that sink below the so-called carbonate compensation depth (about 3,500m) and their possible integration into sediments of different ages.

To prepare samples of sediments, small amounts of the fine-grained material should be transferred onto a glass slide (18 x 18mm), where a suspension of the sample is produced by using distilled water. The material can then be evenly distributed over the glass with the help of a toothpick. During the next step, the suspension is dried on a hot-plate and mounted on a slide (26 x 76mm), commonly using Canadian balsam (refraction index: 1.55). Instead of this so-called ‘smear slide method’, the sediment can also be brought to suspension, before it is transferred onto the slide. This alternative method has the advantage that, before the suspension is pipetted onto the slide, it can be cleaned by the insertion of several steps involving centrifuges.

The investigation of specimens is best done by using a light-microscope, which should be equipped with a set of polarisation filters to observe the respective fossils in the bright and dark field. If available, the nannofossils should also be studied using a phase-contrast or interference-contrast microscope, because these optical devices provide more detailed insights into the microscopic world of the coccoliths.

Where can we find coccoliths?

In general, coccoliths can be found in all kinds of calcareous marine sediments. As a result, index fossils belonging to the group of the Coccolithophorida have been recorded from all over the world. Important places of discovery include the Mediterranean, Barbados and the so-called Molasse Zone, north of the central-European Alps. However, I also want to introduce three more sample localities that are near my home village and can be assigned to the alpine foreland, north of Salzburg.

These three localities – Mattsee, Nussdorf am Haunsberg and Oichtental – either belong to the Molasse Zone or to the marginal parts of the Flysch Zone. While the Molasse Zone is mainly characterised by gravels, sands and loams, the Flysch Zone commonly consists of marls and sandstones. The Mattsee is a lake, whose bottom substrate includes several strata of Tertiary sediments. These sediments can also be found near the village Nussdorf and in the Oichtental, a lovely valley about 20km north of Salzburg.

The most remarkable samples of coccoliths from these three localities are displayed in the light-microscopic image accompanying this article. The so-called pentaliths (small pentagonal calcite platelets) belong to the species Braarudosphaera bigelowii. They are very common in all the samples I have taken from these sites. Other important coccoliths can be attributed to the genera Discoaster and Chiasmolithus and, of the discoasterids, Discoaster lodoensis (an index fossil of the early Eocene) is also worth mentioning.

The coccoliths of this species have a star-like shape. Other species, which can be found in the samples from the Mattsee, are Discoaster multiradiatus and Discoaster mirus, both of which are conspicuous due to their highly symmetric, coccolith morphology. However, the most eye-catching coccolith can be assigned to the species Chiasmolithus grandis, whose calcite platelets reach a diameter of up to 10 micrometres. Chiasmolithus has similar value as an index fossil for the late Palaeocene and Eocene as Discoaster lodoensis. In the sample from Nussdorf am Haunsberg, discoasterids are absent, such that a microscopic image usually only displays the species Watznaueria barnesae, Micula staurophora, Arkhangelskiella sp., Eiffelithus turriseiffeli, and Eiffelithus eximius.

In my opinion, the world of nannofossils (including coccospheres and coccoliths) is an extremely interesting and very substantial field for the palaeontologist, as well as for the hobby microscopist. However, it has to be kept in mind that the successful investigation of these objects can only be carried out with the help of expensive, professional, microscopic equipment, otherwise the identification of single species is impossible. However, a remarkable advantage of the study of nannofossils is the simple acquisition of small amounts of sample material, is sufficient to obtain a marvellous insight into the diversity of locally occurring species.

References

Bown, P. (Hrsg.): Calcareous Nannofossil Biostratigraphy. Chapman and Hall, London (1998).

Lohmann, H.: Die Coccolithophoridae. Eine Monographie der Coccolithen bildenden Flagellaten, zugleich ein Beitrag zur Kenntnis des Mittelmeerauftriebs. Archiv für Protistenkunde 1, 89–165 (1902).

Sorby, H. C.: On the Organic Origin of the So-Called ‘Crystalloids’ of the Chalk. Annals and Magazine of Natural History 8, 193–200 (1861).