Glomalean Fungi from the Ordovician

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To eliminate possible problems of modern contamination, we removed 50 cm of the exposed rock before sampling. Samples were taken from the center of the outcrop, contained no weathered surfaces, and had not been penetrated by any modern plant. To eliminate ancient contamination, we used a rock saw to cut off the edges of the samples, which showed no inclusion or any evidence of past penetration. There were no obvious bedding planes in the rocks. If the rocks were contaminated some time after deposition, we would see other more common, similar-sized, resistant organic materials representative of later periods (e.g., vascular plant–type spores or pollen grains, root fragments, and diatoms). Whole samples of rock were subsequently submerged in two baths of concentrated HCl in new plastic containers. Sludge and acid from the first bath were discarded. The sediment from the second bath and occasionally remaining rock were examined under a dissecting microscope to separate the organic matter from the inorganic sediments. Specimens shown in Fig. 1, B and C and E to G, were deposited at the University of California Museum of Palaeontology, Berkeley (UCMP 151983 and 151984).

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In Fig. 1, C, F, and G were obtained with a Nikon compound microscope equipped with an Image Explorer system (Hitachi Genetic Systems, Alameda, CA) that was used to focus through the specimens and combine the images of the different focusing planes to a composite with a very large depth of field. Figure 1B was obtained with a Molecular Dynamics laser scanning confocal microscope (Molecular Dynamics, Sunnyvale, CA). The excitation wavelength was 530 nm, and the autofluorescence of the specimens above 600 nm was detected. This method allows similar optical sectioning as the method described above, but it also allows sectioning within optically dense areas of the specimen that are not transparent enough for light microscopy.

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We thank J. Gray for valuable suggestions. We also thank T. Bruns for suggestions and continuous support of D.R., J. Taylor and F. W. Byers for advice on the manuscript, S. Lee for introducing and giving access to the Image Explorer System, S. Ruzin and D. Schichnes for helping with the confocal laser scanning microscope, and F. Landis for establishing communication.