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The AAPG/Datapages Combined Publications Database
CSPG Special Publications
Abstract
Boundaries and Correlations
Stratigraphic significance of the stems of Crinoids in solving the Siluro-Devonian boundary problem
Abstract
The existing method of study of crinoid stems, isolated from the calyx, permits their systematization according to artificial classifications only. It is possible, however, to characterize local stratigraphic subdivisions with the help of established assemblages and to correlate these within the individual structure-facies zones.
The application of morphofunctional analysis to the study of crinoid stems permits the recognition of some evolutionary trends in their development, traceable within the entire Palaeozoic sequence, and to distinguish mutual phylogenetic relations between the newly established taxonomical categories. This in turn renders crinoid columns useful for biostratigraphic purposes.
The Late Silurian Epoch is marked by the appearance of crinoids the stems of which are characterized by the development of new adaptation trends, such as the broadening of ligament areas of the joints, which had a narrow axial canal. The most important adaptations are those characterized by the following features: 1) the development of the differentiated, lobed ligament areas: Decarcrinus (col.), Kuzbassocrinus (col.), Podoliocrinus (col.), Anthinocrinus (col.), Facetocrinus (col.); and 2) the development of a ligament area which is separated from the cavity of the axial canal by a sort of regular cylinder Hexacrinites (?), Asperocrinus (col.), Nimiocrinus (col.).
The genera listed above are of great value in drawing the boundary between the Silurian and Devonian. They made their appearance at the end of the Late Ludlow Epoch, and reach the peak of their development in the Early Devonian.
In central Kazakhstan, Hexacrinites, Decarcrinus, Kuzbassocrinus and Podoliocrinus first appear in the Karaespin unit; in the Pribalkhash unit they assume a massive abundance. The boundary between the Devonian and Silurian systems could accordingly be drawn as low as the first appearance of the above mentioned genera. In this case, the Karaespin unit within the local Silurian-Devonian stratigraphic succession of central Kazakhstan will be included in the Devonian system.
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