The biostratigraphic importance of conchostracans in the continental Triassic of the northern hemisphere
Published:January 01, 2010
Heinz W. Kozur, Robert E. Weems, 2010. "The biostratigraphic importance of conchostracans in the continental Triassic of the northern hemisphere", The Triassic Timescale, Spencer G. Lucas
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Conchostracans or clam shrimp (order Conchostraca Sars) are arthropods with a carapace consisting of two chitinous lateral valves. Triassic conchostracans range in size from 2 to 12.5 mm long and are common in deposits that formed in fresh water lakes, isolated ponds and brackish areas. Their dessication- and freeze-resistant eggs can be dispersed by wind over long distances. Therefore many conchostracan species are distributed throughout the entire northern hemisphere. In the Late Permian to Middle Triassic interval, several of these forms are also found in Gondwana. Many wide-ranging conchostracan species have short stratigraphic ranges, making them excellent guide forms for subdivision of Triassic time and for long-range correlations. The stratigraphic resolution that can be achieved with conchostracan zones is often as high as for ammonoid and conodont zones found in pelagic marine deposits. This makes conchostracans the most useful group available for biostratigraphic subdivision and correlation in continental lake deposits. Upper Triassic Gondwanan conchostracan faunas are different from conchostracan faunas of the northern hemisphere. In the Norian, some slight provincialism can be observed even within the northern hemisphere. For example, the Sevatian Redondestheria seems to be restricted to North America and Acadiestheriella n. gen. so far has been found only in the Sevatian deposits from the Fundy Basin of southeastern Canada. Here we establish a conchostracan zonation for the Changhsingian (Late Permian) to Hettangian (Early Jurassic) of the northern hemisphere that, for the most part, is very well correlated with the marine scale. This zonation is especially robust for the Changhsingian to early Anisian, late Ladinian to Cordevolian and Rhaetian to Hettangian intervals. For most of the Middle and Upper Triassic, this zonation is still preliminary. Five new genera, six new species and a new subspecies of conchostracans are described that are stratigraphically important.
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The Triassic Timescale
The Mesozoic Era begins with the approximately 50-million-year-long Triassic Period, a major juncture in Earth history when the vast Pangaean supercontinent completed its assembly and began its fragmentation, and the global biota diversified and modernized after the end-Permian mass extinction, the most extensive biotic decimation of the Phanerozoic. The temporal ordering of geological and biotic events during Triassic time thus is critical to the interpretation of some unique and pivotal events in Earth history. This temporal ordering is mostly based on the Triassic timescale, which has been developed and refined for nearly two centuries. This book reviews the state of the art of the Triassic timescale and includes comprehensive analyses of Triassic radio-isotopic ages, magnetostratigraphy, isotope-based and cyclostratigraphic correlations and timescale -relevant marine and non-marine biostratigraphy.