The δ 13 C carb records from two geographically close sections of the shallow-water Kozel Limestone Member (late Wenlock Motol Formation; Perunica microplate) significantly differ in the magnitude of the Homerian carbon isotope excursion as well as in their dolomite content. The present paper tests a hypothesis as to whether a difference of about 2‰ in the magnitude of the δ 13 C carb anomaly is caused by the different content of dolomite, which could be enriched in both 13 C and 18 O relative to coexisting calcite, as has been suggested by experimental data. The new data obtained reveal that the δ 18 O composition of calcite and dolomite was probably controlled by the pore-fluid composition during limestone diagenesis and that both carbonates seem to be close to equilibrium in oxygen isotope composition. On the other hand, the δ 13 C values of dolomite are similar to those of calcite, and thus the carbon isotope composition of both carbonates was probably determined by the precursor carbonate composition. Moreover, the values of δ 13 C dolomite /δ 13 C calcite ratios as well as their variability suggest that both calcite and dolomite did not reach equilibrium in their carbon isotope composition. Whole-rock, mineralogical, and C and O isotope data clearly show that dolomite is not the cause for the differences in magnitudes of the δ 13 C records observed between dolomite-bearing and dolomite-lacking shallow-water limestone successions. The question as to whether the observed differences in the δ 13 C records of the studied sections across the Homerian carbon isotope excursion were controlled by the dependence of seawater composition on water depth and (or) proximity to shoreline or if the δ 13 C values were later affected by secondary processes during limestone diagenesis is still unsolved.
A significant number of small-sized gastropods are described from Emsian (late Early Devonian) strata on the south flank of Limestone Mountain, Medfra B-4 quadrangle, west-central Alaska, providing the first detailed taxonomic inventory of Emsian gastropods from the Farewell terrane of southwestern and west-central Alaska. The fauna is distinctly of Old World Realm character, and contains not a single species in common with Emsian faunas of nearby nonaccreted rocks of western Canada and east-central Alaska (“Western Canada Province” of Blodgett et al., 2001a). The genera Balbinipleura Bandel and Frýda, 1996 and Nanochilina Frýda, 1998, as well as the subgenus Palaeozygopleura (Rhenozyga) Frýda, 2000, are reported for the first time in the Devonian of the Western Hemisphere. The gastropod fauna includes members of three (i.e., Archaeogastropoda, Caenogastropoda, and Heterobranchia) of the five modern gastropod subclasses, illustrating that these gastropod lineages were separated from each other since the Early Devonian. New taxa include the new genera Arctozone, Farewellia , and Medfrazyga , represented by the new species Arctozone cooki, Farewellia heidelbergerae , and Medfrazyga clauticae. Further new species include Quadricarina (Quadricarina?) noklebergi, Balbinipleura krawczynskii, Decorospira lepaini, Decorospira? minutula, Palaeozygopleura (Rhenozyga) reifenstuhli , and Nanochilina gubanovi. In addition, the following previously described gastropods are also discussed and illustrated: Alaskiella medfraensis Frýda and Blodgett, 1998; Alaskacirrus bandeli Frýda and Blodgett, 1998; and Kuskokwimia moorei Frýda and Blodgett, 2001a.
The Late Ordovician tropidodiscid gastropod Alaskadiscus , known only in two areas in Alaska, was named by Rohr, Frýda, and Blodgett (2003). Peter Wagner and Stephen K. Donovan have pointed out to us that the name was previously used for a Cambrian trilobite, Alaskadiscus Zhang, 1980. We propose the new name Alaskodiscus with the same type species, A. donensis Rohr, Frýda and Blodgett, 2003. We feel the similarity in names is not likely to cause undue confusion since the taxa are widely separated by phyla and age.
Review of all species of the family Palaeozygopleuridae Horný, 1955 (Gastropoda) known from the Perunica microplate (Bohemia) is presented with a description of three new species, Palaeozygopleura lukesi sp. nov., Cimrmaniela sveraki gen. et sp. nov. and Cimrmaniela smoljaki gen. et sp. nov. The stratigraphic distributions of the most of Bohemian palaeozygopleurid species are either corrected or refined, based on new records or modern stratigraphic studies. A complete list of the geographic occurrences of all known palaeozygopleurid gastropods from the Perunica microplate is also given together with notes on their ontogeny.
Klug, C., Kroger, B., Kiessling, W., Mullins, G.L., Servais, T., Frýda, J., Korn, D. & Turner, S. 2009: The Devonian nekton revolution. Lethaia, 10.1111/j.1502-3931.2009.00206.x
Traditional analyses of Early Phanerozoic marine diversity at the genus level show an explosive radiation of marine life until the Late Ordovician, followed by a phase of erratic decline continuing until the end of the Palaeozoic, whereas a more recent analysis extends the duration of this early radiation into the Devonian. This catch-all approach hides an evolutionary and ecological key event long after the Ordovician radiation: the rapid occupation of the free water column by animals during the Devonian. Here, we explore the timing of the occupation of the water column in the Palaeozoic and test the hypothesis that ecological escalation led to fundamental evolutionary changes in the mid-Palaeozoic marine water column. According to our analyses, demersal and nektonic modes of life were probably initially driven by competition in the diversity-saturated benthic habitats together with the availability of abundant planktonic food. Escalatory feedback then promoted the rapid rise of nekton in the Devonian as suggested by the sequence and tempo of water-column occupation. □Devonian, diversity, ecology, food webs, nekton, plankton, radiation.
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