The Canning Basin, Western Australia The Ordovician period of the Paleozoic Era is an interval exhibiting increased faunal diversity and an abundance of marine life. One of the many significant fossil sites from this time is the shallow Canning Basin in northwestern Australia (180 miles south-east of the present day city of Derby). The land portion of the Canning Basin covers approximately 430,000 km2. During the early Ordovician period, the Australian continent was located at the equator. Although the environment today is desert, there is evidence suggesting that the basin had a deep water environment of high faunal assortment during the Ordovician period. In addition, the shallow portion of the basin endured turbulent tides. This was determined by observing erosional evidence and the poor preservation of gastropod shells found in the Canning Basin. Around seventy percent of the shells recovered from the basin are broken which suggests the occurrence of strong tides. Through the deposition and compression of sediments that eroded from the land, the formation of distinct strata containing marine microfossils occurred. Two major strata found in the northern Canning Basin are the Emanuel and Gap Creek Formations. The Emanuel formation, which lies directly below the Gap Creek formation consists of the earliest dated sediments in the Canning Basin. This formation (594 meters thick) is primarily composed of light grey limestone, green-grey shale (rock made of compressed mud), and a base of sandy dolomite. The Emanuel formation dates back to the Tremodocian (earliest Ordovician) and is characterized by the presence of many conodont microfossils. Two new species, Phragmodus polystrophos and Phragmodus flexuosus, were discovered in the Canning Basin. Conodonts, as part of larger animals, are composed of calcium phosphate and other organics. This collection of elements suggests that conodonts were part of the first vertebrates which were jawless, free-swimming fish. The tooth-like shape of conodonts (means suggested teeth) indicates their role as food gathering devices. Conodonts were widely distributed over several continents during the Ordovician period due to climate changes that occurred over time. Unfortunately, connections between conodonts found in the Emanuel formation and many others found in Europe and North America have yet to be determined. In addition to conodonts, brachiopods are also foundpreserved in the limestone and shale of the Emanuel formation. Brachiopods, sessile marine forms with bivalve shells composed of calcium phosphate, were dominant from the Cambrian period to the Permian period. However, during the Ordovician period, the brachiopod fauna changed greatly. This resulted in large differences between brachiopods from the early and late Ordovician. Such rapid evolution is important in biochronology between different periods because it allows researchers to formulate accurate correlations between different specimens in the fossil record. The age of the fossils found in the Emanuel formation were determined using potassium-argon and uranium radiometric dating. Above the strata containing Emanuel formation is the Gap Creek formation (192 meters thick) which consists of light brown dolomite interbedded with sandstone and shale. The Gap Creek formation has a break in its strata due to a relatively rapid rise in sea level. This break lead to the reappearance of carbonate and black shale deposits in the formation. Such sea level changes had a profound effect on fauna in the early Ordovician period. As a result, abrupt faunal turnovers occurred. Ordovician sea level changes can be traced in the Canning Basin strata with the appearance of condensed marine fossil sections. A great diversity of fossil gastropods has been uncovered in the Canning Basin. In the Emanuel formation alone, 130 specimens were uncovered which include six different species in five different genera. Present day gastropods, including slugs and snails, have the defining characteristic of torsion which results in the rotation of the visceral mass and the mantle on the foot. The gastropods that were found in the Emanuel formation were the species Peelerophon oehlerti, Bucania, Teiichispira kobayashi, Ecculiomphalus cf. abendanoni, Paraphistoma qualteriatum, Paraphistoma vaginati, Oriostoma? canningense, and Seelya emanuelensis. All of the samples have a stratigraphic range from the Tremodocian to the lower Argenian (late Ordovician) except the species Teiichispira kobayashi and Oriostoma? canningense which were found in the upper Argenian. Peelerophon oehlerti was found in the Emanuel formation by B.F. Glenister (University of Western Australia, 1957) and A.W. Lindner (1958). This species is represented by four specimens. The species Bucania was also uncovered in the Emanuel formation by B.F. Glenister (1957). Only a small, imperfect specimen of Bucania was gathered. Four incomplete shells and four opercula of the species Teiichispira kobayashi were discovered in the Gap Creek formation by W.S. Johnson (1987). Nine internal molds of the species Ecculiomphalus cf. abendanoni were found in the Emanuel formation by J. Pas (1991). Three internal molds of the species Paraphistoma qualteriatum and five specimens of Paraphistoma vaginati were also found in the same formation by Glenister (1957) and by D. Merrilees (1960). In addition, two specimens of the species Oriostoma? canningense were found in the Gap Creek formation by D. Merrilees (1960). Four specimens of Seelya emanuelensis were uncovered in the Emanuel formation by Glenister (1957) and Merrilees (1960). The last two species mentioned have only been found in the Canning Basin. The Emanuel formation and Gap Creek formation are only two Ordovician localities in which a diverse number of trilobite species have been found. However, an abundance of trilobite fossils were recovered in these two formations which may be associated with highly favorable Ordovician environmental conditions. Trilobites are an extinct form of arthropods that disappeared about 230 million years ago. They had hard outer skeletons, jointed legs, and oval segmented bodies. Trilobites are also the earliest animals known to have had eyes. The Emanuel and Gap Creek formations are important because of the high faunal diversity that their microfossils represent during the Ordovician period. Of special interest are the abundance of the aforementioned conodonts, brachiopods, gastropods, and trilobites which indicate the favorable marine conditions of the period. In addition, the fossils condensed in different marine strata aid in determining the many sea level changes which had a significant effect on the fauna of the Early Ordovician. The microfossils found in these two formations are highly interpretive and signify a time span consisting of an abundance of marine life and specifically, the first vertebrates in the fossil record as indicated by the conodonts. Such findings, when correlated with similar findings that took place in other Ordovician fossil localities, allow researchers to piece together the global extent of faunal change. Bibliography Bassett, M.G., 1976, The Ordovician System, Paleontological Association by the University of Wales Press & the National Museum of Wales, Cardiff. Brimmell K., Long J.A., McNamara K.J., 1991, Catalogue of Type Fossils in the Western Australian Museum, Western Australian Museum Publishing, Australia. Brunnschweiler, R. 1984. Ancient Australia: The Story of its Past Geography and Life. 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