Comparative light microscopy, scanning electron microscopy and transmission electron microscopy of selected organic walled microfossils

Abstract

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.

TEM evidence for eukaryotic diversity in mid‐Proterozoic oceans

“…Previous TEM studies of leiospheres, however, suggest that ultrastructure may provide a firmer basis for distinguishing different populations. Some leiospheres preserve a multilayered wall similar to chlorococcalean green algae (Arouri et al ., 1999), while others reveal a single‐layered, homogeneous wall (Peat, 1981; Arouri et al ., 2000; Talyzina & Moczydlowska, 2000), or a homogenous wall perforated by pore canals similar to some prasinophyte green algae (Loeblich, 1970). TEM of Roper leiosphaerids populations not only corroborates differences observed via light microscopy, but suggests that they represent distinct clades, implying a level of taxonomic diversity undetected by optical investigation.…”

“…1), northern Australia. The Roper Basin is well‐characterized in terms of sedimentary architecture (Abbott & Sweet, 2000) and is abundantly fossiliferous (Peat, 1981; Javaux et al ., 2001). Roper microfossil assemblages show an onshore‐offshore pattern of decreasing abundance, declining diversity, and changing dominance (Javaux et al ., 2001).…”

Wall ultrastructure of an Ediacaran acritarch from the Officer Basin, Australia

“…The earliest ultrastructural studies using TEM on Palaeozoic acritarchs (Wall 1962; Jux 1968, 1969a, b, 1971, 1977; Kjellström 1968; Martin & Kjellström 1973) and on Proterozoic acritarchs (Peat 1981) suggested that the genera Leiosphaeridia and Tasmanites are related to prasinophycean green algae. More recently, Talyzina & Moczydłowska (2000) revealed four distinct types of vesicle wall ultrastructure in some Cambrian species.…”

“…The earliest ultrastructural studies using TEM on Palaeozoic acritarchs (Wall 1962;Jux 1968Jux , 1969aJux , b, 1971Jux , 1977Kjellström 1968;Martin & Kjellström 1973) and on Proterozoic acritarchs (Peat 1981) (1) single-layered, electron-tenuous and fibrous ( Archaeodiscina umbonulata ), (2) electron-dense and homogeneous ( Globosphaeridium cerinum, Comasphaeridium brachyspinosum and Skiagia compressa ), (3) electron-dense and homogeneous but perforated by pore canals ( Tasmanites tenellus ), and (4) multilayered composite ultrastructure, including electrondense and tenuous homogeneous layers and laminated layer with a trilaminar sheath structure (TLS) ( Leiosphaeridia spp.). They concluded that the multilayered wall ultrastructure with diagnostic TLS of studied specimens of Leiosphaeridia spp.…”

Techniques for combining light microscopy and scanning electron microscopy: a survey of the literature

“…The encircling methods described can be improved when a smaller area is selected or even better when whole individual cells or pollen grains are marked (van Ewijk & Mulder, 1976;Rickberg et al, 1984;Peat, 1981).…”