The Ordovician period of the Paleozoic Era is an interval exhibiting increased animal 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, evidence suggests that the basin was a deep water marine environment of high faunal diversity during the Ordovician period.
In addition, the shallow portion of the basin endured turbulent tides, as evidenced 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 strata containing distinct 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, and a base of sandy dolomite. The age of the fossils found in the Emanuel formation were determined using potassium-argon and uranium radiometric dating. The 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 are part of larger animals, and 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 (which means "suggested teeth") probably means their role was one as food gathering devices. Conodonts were widely distributed over several continents during the Ordovician period; 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 found preserved in the limestone and shale of the Emanuel formation. Brachiopods, sessile marine forms with bivalve shells composed of calcium phosphate, were numerous and diverse animals 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.
Above the 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 (developmental twisting) which results in the rotation of the visceral mass and the mantle on the foot. Incomplete shells and opercula of Teiichispira kobayashi were discovered in the Gap Creek formation, as well as internal molds of Ecculiomphalus cf. abendanoni and Paraphistoma qualteriatum. In addition, two specimens of Oriostoma? canningense and four of Seelya emanuelensis were found in the Canning Basin, both of which have not been found anywhere else.
The Emanuel formation and Gap Creek formation are only two Ordovician localities in which a diverse number of trilobite species have been found. 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 abundance of trilobite fossils in these two formations may be associated with highly favorable Ordovician environmental conditions.
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 identifying and studying 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 with an abundance of marine life and the first vertebrates in the fossil record. 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.
