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Cover Download Save contents. Title Page, Copyright pp. Another group the Suborder Involutina have a two chambered test composed of aragonite. The Robertinina also have a test composed of aragonite and the Suborder Carterina is believed to secrete spicules of calcite which are then weakly cemented together to form the test.
The morphology of foraminifera tests varies enormously, but in terms of classification two features are important. Chamber arrangement and aperture style, with many subtle variations around a few basic themes. These basic themes are illustrated in the following two diagrams but it should be remembered that these are only the more common forms and many variations are recognised.
As previously mentioned, foraminifera have been utilised for biostratigraphy for many years, and they have also proven invaluable in palaeoenvironmental reconstructions most recently for palaeoceanographical and palaeoclimatological purposes.
For example palaeobathymetry, where assemblage composition is used and palaeotemperature where isotope analysis of foraminifera tests is a standard procedure. In terms of biostratigraphy, foraminifera have become extremely useful, different forms have shown evolutionary bursts at different periods and generally if one form is not available to be utilised for biostratigraphy another is.
For example preservation of calcareous walled foraminifera is dependent on the depth of the water column and Carbonate Compensation Depth the depth below which dissolution of calcium carbonate exceeds the rate of its deposition , if calcareous walled foraminifera are therefore not preserved agglutinated forms may be.
The oldest rocks for which foraminifera have been biostratigraphically useful are Upper Carboniferous to Permian strata, which have been zoned using the larger benthic fusulinids. Planktic foraminifera have become increasingly important biostratigraphic tools, especially as petroleum exploration has extended to offshore environments of increasing depths. The first and last occurrence of distinctive "marker species" from the Cretaceous to Recent particularly during the Upper Cretaceous has allowed the development of a well established fine scale biozonation.
Benthic foraminifera have been used for palaeobathymetry since the 's and modern studies utilise a variety of techniques to reconstruct palaeodepths. For studies of relatively recent deposits simple comparison to the known depth distribution of modern extant species is used.
For older material changes in species diversity, planktic to benthic ratios, shell-type ratios and test morpholgy have all been utilised. Variations in the water temperature inferred from oxygen isotopes from the test calcite can be used to reconstruct palaeoceanographic conditions by careful comparison of changes in oxygen isotope levels as seen in benthic forms for bottom waters and planktic forms for mid to upper waters.
This type of study has allowed the reconstruction of oceanic conditions during the Eocene-Oligocene, the Miocene and the Quaternary. Benthic foraminifera have been divided into morphogroups based on the test shape and these groups used to infer palaeo-habitats and substrates; infaunal species tending to be elongate and streamlined in order to burrow into the substrate and epifaunal species tending to be more globular with one relatively flatter side in order to facilitate movement on top of the substrate.
It should be remembered, however, that a large variety of morphologies and possible habitats have been recognised making such generalisations of only limited use. Studies of modern foraminifera have recognised correlations between test wall type for instance porcelaneous, hyaline, agglutinated , palaeodepths and salinity by plotting them onto triangular diagrams. Studies of living foraminifera, in controlled laboratory environments, have provided limited information regarding trophic strategies but much has been inferred by relating test morphology to habitat.
Foraminifera utilise a huge variety of feeding mechanisms, as evidenced by the great variety of test morphologies that they exhibit.
From the variety of trophic habits and test morphologies a few generalisations may be made. It is believed that most of the animal phyla in existence today had their origins during this time, often referred to as the Cambrian explosion. Echinoderms, mollusks, worms, arthropods, and chordates arose during this period.
One of the most dominant species during the Cambrian period was the trilobite, an arthropod that was among the first animals to exhibit a sense of vision.
Trilobites : These fossils a—d belong to trilobites, extinct arthropods that appeared in the early Cambrian period million years ago and disappeared from the fossil record during a mass extinction at the end of the Permian period about million years ago. The causes of the Cambrian explosion are still debated. There are many theories that attempt to answer this question. Environmental changes may have created a more suitable environment for animal life. Examples of these changes include rising atmospheric oxygen levels and large increases in oceanic calcium concentrations that preceded the Cambrian period.
Some scientists believe that an expansive, continental shelf with numerous shallow lagoons or pools provided the necessary living space for larger numbers of different types of animals to co-exist.
There is also support for theories that argue that ecological relationships between species, such as changes in the food web, competition for food and space, and predator-prey relationships, were primed to promote a sudden, massive coevolution of species.
Yet other theories claim genetic and developmental reasons for the Cambrian explosion. The morphological flexibility and complexity of animal development afforded by the evolution of Hox control genes may have provided the necessary opportunities for increases in possible animal morphologies at the time of the Cambrian period. Theories that attempt to explain why the Cambrian explosion happened must be able to provide valid reasons for the massive animal diversification, as well as explain why it happened when it did.
There is evidence that both supports and refutes each of the theories described above. The answer may very well be a combination of these and other theories. Unresolved questions about the animal diversification that took place during the Cambrian period remain. For example, we do not understand how the evolution of so many species occurred in such a short period of time. Furthermore, the vast diversification of animal species that appears to have begun during the Cambrian period continued well into the following Ordovician period.
Despite some of these arguments, most scientists agree that the Cambrian period marked a time of impressively-rapid animal evolution and diversification that is unmatched elsewhere during history. The post-Cambrian era was characterized by animal evolution and diversity where mass extinctions were followed by adaptive radiations. The periods that followed the Cambrian during the Paleozoic Era were marked by further animal evolution and the emergence of many new orders, families, and species.
As animal phyla continued to diversify, new species adapted to new ecological niches. This phenomenon has been invaluable in scientific and ecological research, and allows researchers to characterize the biology of ecosystems that existed millions of years ago. The amber that contained the termite used in this study came from a mine first excavated in in the Hukawng Valley in Myanmar, in a formation that was between 97 and million years old. Materials provided by Oregon State University.
Note: Content may be edited for style and length. Science News. Description of an early Cretaceous termite Isoptera: Kalotermitidae and its associated intestinal protozoa, with comments on their co-evolution.
ScienceDaily, 15 May Oregon State University. Retrieved November 11, from www.
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