Fungal Spores

Although fungal spores are known throughout geologic time, it is only beginning in the Late Jurassic that they constitute a significant and important fraction of the palynomorphs recovered from most rocks (Elsik, 1976). Spores of obvious fungal origin, however, are known as early as the Late Silurian. There is an extensive terminology applied to fungal palynomorphs based on living fungi (Elsik et al., 1983). Spores are often named based on the concept of morpho-genera (Elsik, 1989), in which various morphologic features (e.g., size, shape, number of septations or pores, and type of ornament) form the basis of the generic concept (e.g., Fusiformisporites) . In addition to spores, fungal fragments recovered in macerations may include various forms of fructifications, hyphae, and mycelia. Funginite is the name applied to sedimentary organic matter (macerals) composed chiefly of fungal material (see Chapter 1).

The study of spores and other fungal remains represents an important, yet still largely untapped, paleomycological and paleobotanical data source. Although fungal spores have been utilized in some stratigraphic applications in Cenozoic rocks, especially around the Cretaceous-Paleogene boundary (Vajda and McLoughlin, 2004), perhaps their greatest untapped potential lies in the areas of biogeography, pale-oecology, and paleoclimatology (Sherwood-Pike, 1988). Jarzen and Elsik (1986) examined fungal palynomorphs from a modern, subtropical environment of open savanna and riparian forest in eastern Zambia. The results of that study provide a basis for quantifying the various fungal remains preserved in modern fluvial systems, with applications to similar environments in the fossil record. It also suggests another method for inferring paleoecological parameters when other microfossils and megafossils are lacking.

Fungal-Like Organisms

There are a number of organisms that historically were classified with the fungi, but as a result of subsequent research, and especially molecular systematic approaches to modern forms, have been removed from the Kingdom Fungi. Nevertheless, each of these organisms has a phylogenetic history, and some are preserved as fossils.

Peronosporomycetes (Oomycota)

The oomycetes, or water molds, are organisms that are now included within the Kingdom Straminipila; some authors refer to this group as the Peronosporomycetes. Most forms are filamentous, possess cell walls of cellulose rather than chitin, and lack cross walls, except where reproductive cells are produced. Asexual reproduction occurs by biflagellated zoospores. Sexual reproduction includes the production of sperm in antheridia which fuse with one to several eggs produced in an oogonium.

Although there are several pre-Cenozoic reports of per-onosporomycetes in association with fossil plants, many of these have been discounted because of the absence of a complete suite of diagnostic features (Johnson et al., 2002), whereas others regard at least some of these fossils as authentic (Blackwell and Powell, 2000). One interesting fossil that perhaps provides the best evidence of peronosporo-mycetes is Hassiella monospora (T. Taylor et al., 2006). This Rhynie chert organism consists of aseptate hyphae that randomly branch to form terminal oogonia, each approximately 30 pm in diameter. At the base of the oogonium is a funnel-shaped structure interpreted as the antheridium. Intracellular peronosporomycetes (Combresomyces cornifer) in lycopod periderm have also been described and illustrated from the upper Visean cherts of central France (Krings et al., 2007e; Dotzler et al., 2008). Another possible peronosporomycete is

Albugo Oogonia Image
FIGuRE 3.112 Albugo-like oogonia in Pennsylvanian seed tissues. Bar = 100 pm.

found in a seed-like structure from the Carboniferous (Stidd and Cosentino, 1975). Each spherical oogonium (100 pm in diameter) contains a single oosphere (FIG. 3.112); however, no well-defined antheridia are preserved. What makes this specimen so interesting and increases the chances that it represents a peronosporomycete is that the disruption of the tissues in the seed are identical to those found when the extant peronosporomycete Albugo infects flowering plants.

EubActenA ANd ArchAeA

Despite the microscopic size of the cells (only a few micrometers long), in terms of numbers of individuals and metabolic diversity, bacteria are the dominant organisms of the biosphere and, geologically, the oldest organisms on Earth (Chapter 2). They live in every possible habitat—soil, water, and even the deep subsurface of the continents and oceans; they can even be found in radioactive waste. They are critical in nutrient recycling (Nardi et al., 2002), including the fixation of nitrogen from the atmosphere into a form that is usable by plants. It has been estimated that there are from 40 million to 2 billion bacteria in a single gram of soil (Whitman et al., 1998).

Bacteria are classified today based on genetic sequence data. In older classification systems, five kingdoms of organisms were recognized: Protista, Monera (all prokaryotes), Fungi, Plantae, and Animalia. Although modern phyloge-netic work has long since shown that this system is out of date for the prokaryotes, it is still reproduced in some textbooks. The most widely used classification of organisms today includes three domains (a level above the level of

Kingdom): Eubacteria, Archaea, and Eucarya (Woese and Fox, 1977; Woese et al., 1990). Within the Domain Eubacteria are the Cyanobacteria (including the Chloroxybacteria), and other bacteria (e.g., purple, green sulfur, Gram-positive, spirochetes), with each group occupying the level of kingdom. All possess prokaryotic cells, which differ from the eukaryotic cells of plants, animals, and fungi. Prokaryotic cells are small (0.5-10 pm), and lack a nucleus or other membrane-bound organelles. Neither meio-sis nor mitosis occurs; prokaryotes reproduce asexually by means of binary fission. All the nutritional modes used by eukaryotic cells are also found among the prokaryotes, plus several that are unique to the group.

Despite the fact that bacteria are very small (0.5-5 pm) and delicate, there is an excellent fossil record of the group dating back to the Archean, with structurally preserved specimens reported as old as 3.7Ga. All morphologic forms (coccoid, bacilloid or rod-shaped, spiral, and filamentous) have been recognized in a variety of configurations that correspond to those of existing types. They occur as structurally preserved and compressed forms within most types of common mineral matrices and have also been found in fossil vertebrates, invertebrates, plant tissues, and coprolites (fossil fecal material).

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