Two simple techniques are described and illustrated. The first is for the study of one specimen by both light microscopy (LM) and scanning electron microscopy (SEM). The second is for the study of one selected specimen by LM, SEM and in ultrathin section by transmission electron microscopy (TEM). Although these techniques were developed for the comparative study of Precambrian organic walled microfossils (OWMs), they could be used for a wide range of other specimens.
I N T R O D U C T I O NMany organic-walled microfossils (OWMs) such as pollen grains and acritarchs (Downie et al., 1963; Diver & Peat, 1979) are so distinctive that they can be identified in rock fracture surfaces or strew mounts using SEM alone. However, when poorly preserved OWMs or simple OWMs such as cryptarchs (Diver & Peat, 1979) are being studied, the only way to be sure of recognizing them in the SEM is to pick out individuals and study them initially by LM and then by SEM so that they can be identified with some confidence.Three basic mEthods of specimen preparation are currently used for direct examination of OWMs by SEM. These are fractured surfaces, etched surfaces and strew mounts of rock digestions or macerates. Each of these methods can be useful in specific circumstances, but none of them is really useful for searching for OWMs in previously unexamined material.Fractures of solid samples tend to follow lines of weakness in the sample such as thin clay partings which can make it difficult to obtain a fracture surface in the desired direction. Fractured surfaces arc usually very uneven and show a great variety of surface textures. Using the SEM alone it is usually impossible to distinguish reliably between organic and inorganic structures and surfaces. Cryptarchs seem to be too intimately connected with the rock matrix to split out of it as mineralized skeletons do, so one is usually looking at partly exposed fossils or fragments.The same drawbacks are found with polished and etched surfaces, and here it can also be difficult to clean the etched surface before SEM examination. The etching process and reagents produce artefacts, and may also be a source of contamination (Peat, 1974). Fracture surfaces can be etched to expose microfossils, but they will then have an exaggerated topography and are impossible to clean. It can be useful to examine thin sections or chips of rock by LM, mark the positions of any microfossils, and then expose them for SEM by a judicious combination of grinding and etching. It is easy to produce sections of relatively large microfossils in this way, and microprobe analysis in the SEM can provide additional information (Muir et al., 1974; Zhang Zhongying, pers. comm.).In strew mounts it is usual to find that organic debris obscures the fossils to some extent, and the smaller and simpler the fossils the more difficult they will be to find among the rubbish.
Abstract. A critical re-appraisal of some reports of British Precambrian microfossils illustrates potential pitfalls in describing Precambrian microbiotas. The Ceinwen and Llanddwyn Spilitic Formation of the Gwna Group contains convincing filamentous microfossils, but a report of acanthomorph arcitarchs has not yet been confirmed. Although the Diabaig Formation of the Torridon Group is known to be abundantly fossiliferous, a report based on S.E.M. without thin section control is rejected. Chert pebbles from the Applecross Formation of the Torridon Group do not contain convincing microfossils. They are contaminated by fungi and contain abiogenic structures which resemble cells and tissues. Reports of diagnostic Middle Riphean microfossils from the Blackbrook Formation and the Maplewell Group of the Charnian and the Buxton Rock of the Longmyndian await confirmation.
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