Palynomorphs Pollen grain Pollen has an outer sheath, called a
sporopollenin, which affords it some resistance to the rigours of the fossilisation process that destroy weaker objects. It is produced in huge quantities. There is an extensive fossil record of pollen grains, often disassociated from their parent plant. The discipline of
palynology is devoted to the study of pollen, which can be used both for biostratigraphy and to gain information about the abundance and variety of plants alive — which can itself yield important information about paleoclimates. Also, pollen analysis has been widely used for reconstructing past changes in vegetation and their associated drivers. Pollen is first found in the
fossil record in the late
Devonian period, but at that time it is indistinguishable from spores.
Bacterial spores are not part of a sexual cycle but are resistant structures used for survival under unfavourable conditions.
Fungal spores Chitinozoa chitinozoan from the
Burgsvik beds showing its flask shape
Chitinozoa are a
taxon of
flask-shaped,
organic walled
marine microfossils produced by an as-yet-unknown organism. Common from the
Ordovician to
Devonian periods (i.e. the mid-Paleozoic), the millimetre-scale organisms are abundant in almost all types of
marine sediment across the globe. This wide distribution, and their rapid pace of evolution, makes them valuable
biostratigraphic markers. Their bizarre form has made
classification and ecological reconstruction difficult. Since their discovery in 1931, suggestions of
protist,
plant, and
fungal affinities have all been entertained. The organisms have been better understood as improvements in microscopy facilitated the study of their fine structure, and it has been suggested that they represent either the
eggs or juvenile stage of a marine animal. However, recent research has suggested that they represent the
test of a group of protists with uncertain affinities. The ecology of chitinozoa is also open to speculation; some may have floated in the water column, where others may have attached themselves to other organisms. Most species were particular about their living conditions, and tend to be most common in specific paleoenvironments. Their abundance also varied with the seasons.
Acritarchs Acritarchs, Greek for
confused origins, are organic-walled microfossils, known from about to the present. Acritarchs are not a specific biological taxon, but rather a group with uncertain or unknown affinities. Most commonly they are composed of thermally altered acid insoluble carbon compounds (
kerogen). While the
classification of acritarchs into
form genera is entirely artificial, it is not without merit, as the form taxa show traits similar to those of genuine
taxa — for example the '
explosion' in the
Cambrian and the
mass extinction at the
end of the
Permian. Acritarch diversity reflects major ecological events such as the appearance of predation and the
Cambrian explosion. Precambrian marine diversity was dominated by acritarchs. They underwent a boom around , increasing in abundance, diversity, size, complexity of shape, and especially size and number of spines. Their increasingly spiny forms in the last 1 billion years may indicate an increased need for defence against predation. Acritarchs may include the remains of a wide range of quite different kinds of organisms—ranging from the egg cases of small
metazoans to resting cysts of many kinds of
chlorophyta (green algae). It is likely that most acritarch species from the
Paleozoic represent various stages of the life cycle of algae that were ancestral to the
dinoflagellates. The nature of the organisms associated with older acritarchs is generally not well understood, though many are probably related to unicellular marine
algae. In theory, when the biological source (taxon) of an acritarch does become known, that particular microfossil is removed from the acritarchs and classified with its proper group. Acritarchs were most likely
eukaryotes. While archaea, bacteria and cyanobacteria (
prokaryotes) usually produce simple fossils of a very small size, eukaryotic unicellular fossils are usually larger and more complex, with external morphological projections and ornamentation such as spines and hairs that only eukaryotes can produce; as most acritarchs have external projections (e.g., hair, spines, thick cell membranes, etc.), they are predominantly eukaryotes, although simple eukaryote acritarchs also exist. Acritarchs are found in sedimentary rocks from the present back into the
Archean. They are typically isolated from siliciclastic sedimentary rocks using
hydrofluoric acid but are occasionally extracted from carbonate-rich rocks. They are excellent candidates for index fossils used for dating rock formations in the
Paleozoic Era and when other fossils are not available. Because most acritarchs are thought to be marine (pre-Triassic), they are also useful for palaeoenvironmental interpretation. The Archean and earliest
Proterozoic microfossils termed "acritarchs" may actually be prokaryotes. The earliest eukaryotic acritarchs known (as of 2020) are from between 1950 and 2150 million years ago. Recent application of
atomic force microscopy,
confocal microscopy,
Raman spectroscopy, and other analytic techniques to the study of the ultrastructure, life history, and systematic affinities of mineralized, but originally organic-walled microfossils, have shown some acritarchs are fossilized
microalgae. In the end, it may well be, as Moczydłowska et al. suggested in 2011, that many acritarchs will, in fact, turn out to be algae. cell morphologies
Archean cells Cells can be preserved in the
rock record because their cell walls are made of proteins which convert to the organic material
kerogen as the cell breaks down after death. Kerogen is
insoluble in mineral
acids,
bases, and
organic solvents. Over time, it is mineralised into
graphite or graphite-like
carbon, or degrades into oil and gas hydrocarbons. There are three main types of cell morphologies. Though there is no established range of sizes for each type, spheroid microfossils can be as small as about 8
micrometres, filamentous microfossils have diameters typically less than 5 micrometres and have a length that can range from tens of micrometres to 100 micrometres, and spindle-like microfossils can be as long as 50 micrometres. ==Mineralised==