Occurrences of chitinozoans in the Upper Ordovician Letná Formation of the Barrandian area (Czech Republic)
Abstract
One of the important outcrops in the Upper Ordovician Letná Formation - the Kazín section - has been examined for chitinozoans. All studied samples contained a highly diverse chitinozoan assemblage including specimens classified within more than thirty taxa.
A significant number of them has been previously unknown from the Prague Basin. Despite a comparatively poor to moderate preservation of chitinozoan vesicles, detailed observation based on SEM associated with morphometric measurement enables determination, up to the species level, in about one third of the studied specimens.
Belonechitina micracantha, Cyathochitina campanulaeformis, C. kuckersiana and Desmochitina minor are typical representatives of Ordovician Chitinozoans and represent the so called "long-ranging" taxa, but without any significant contribution to the age of the Letná Formation at the Kazín section. Linochitina pissotensis represents the index species of the upper Darriwilian, its LAD marks the upper boundary of the Middle Ordovician Series.
Only seven specimens of L. pissotensis have been reported from two lowermost samples at the Kazín section. The Darriwilian age is suppoerted also by Euconochitina vulgaris, a species that is restricted exclusively to the Middle Ordovician.
Twenty-seven specimens of E. vulgaris have been documented from four different samples in the upper part of the Kazín section. The third species, Conochitina chydaea, ranges from latest Middle to the Upper Ordovician.
This species is the most abundant in the Kazín section, with a total of fifty-nine specimens distributed in all samples. The abnormally high diversity and unexpected Middle Ordovician age apparently indicated by some of chitinozoans in samples from the Kazín section could be explained, most probably, by a re-deposition of typically Middle Ordovician chitinozoan species into the Upper Ordovician Letná Formation.
The exact time for sedimentation of the Letná Formation cannot be established due to the absence of stratigraphically important Upper Ordovician chitinozoan taxa.