MORPHOLOGY AND REVISION OF LATE DEVONIAN (EARLY FAMENNIAN) CYRTOSPIRIFER (BRACHIOPODA) AND RELATED GENERA FROM SOUTH CHINA AND NORTH AMERICA (original) (raw)
Study of abundant well-preserved specimens of cyrtospiriferid brachiopods from early Famennian triangularis Zone shelf deposits from South China shows that none of them can be attributed to the genera Cyrtospirifer Nalivkin in Fredericks, 1924, or Tenticospirifer Tien, 1938, as previously assigned. Some of these forms are reassigned to the new genus Plicapustula, with Spirifer (Sinospirifer) gortanioides Grabau, 1931 serving as the type species. Restudy of the Upper Devonian cyrtospiriferids of North America and South China indicates that genera of the subfamily Cyrtospiriferinae all developed a delthyrial covering usually composed of two or more thin plates referred to as a composite pseudodeltidium. The delthyrial covering of Cyrtiopsis Grabau, 1923 and other genera of the subfamily Cyrtiopsinae is a convex pseudodeltidium. The lack of a pedicle foramen penetrating the pseudodeltidia of Frasnian Cyrtospirifer indicates that most species had an atrophied pedicle. The pseudodeltidia of most early Famennian cyrtospiriferids are perforated by a pedicle tube or a central hypothyrid foramen as in Sinospirifer Grabau, 1931, or a hypothyrid-to-submesothyrid foramen as in Lamarckispirifer Gatinaud, 1949 and Plicapustula n. gen. The orientation and concavity of the interarea, used with other shell characteristics, are useful characters in species-level taxonomy of Famennian cyrtospiriferids. Four major types of radial micro-ornament characterize Late Devonian species of Cyrtospirifer and other genera of the Cyrtospiriferinae. These are 1) the verneuili type, consisting of microspines arising from the shell surface with spine bases extending into the primary shell layer, with radial capillae; 2) the whitneyi-subextensus type with small pustules both in the grooves and on the plications; 3) the subarchiaci type characterized by pustules only on plications; and 4) the hayasakai type with coarse elongated pustules on plications and capillae in grooves. There is a notable increase in complexity of the medial plication patterns of shells of Late Devonian cyrtospiriferinids. Simple sinal plication patterns characterize shells of Frasnian species of Cyrtospirifer and related genera, whereas most early Famennian cyrtospiriferinids developed complex patterns. We have documented three variations in the structure of the cardinal processes of the Late Devonian cyrtospiriferids. These consist of 1) unsupported; 2) supported by callus (secondary shell material) from below; or 3) supported by a median septum. In Hunan-South China, cyrtospiriferid diversity was low in the late Frasnian, but a postextinction radiation of cyrtospiriferids in the very early Famennian resulted in the evolution of numerous new genera within the interval of the undifferentiated triangularis Zone shortly after the Frasnian–Famennian (F–F) mass extinction. Cyrtospiriferid brachiopods of the subfamilies Cyrtospiriferinae and Cyrtiopsinae are important elements of the early Famennian post-extinction survivor and recovery faunas that migrated and radiated rapidly to occupy niches vacated by extinct clades of late Frasnian brachiopods.
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The cyrtospiriferid brachiopod genus Tenticospirifer Tien, 1938, is revised based on restudy of the type species from the Frasnian (Late Devonian) of the Russian Platform. As revised the genus includes cyrtospiriferid species with pyramidal ventral valves, catacline ventral interareas, a narrow delthyrium, few sinal plications, and lack a median dorsal septum and pseudodeltidium. All species retained in the genus are of Givetian and Frasnian age. All Famennian age species described from South China and North America are rejected from the genus. It appears that Tenticospirifer evolved during the early Givetian in western Europe and remained endemic to that region during the remainder of the Givetian. Successive migrations of Tenticospirifer from eastern Laurussia to North America, then to South China and possibly Australia, coincided with middle and late Frasnian eustatic sea level rises, respectively. The North American species Spirifera cyrtinaformis Hall and Whitfield, 1872, and related species identified as Tenticospirifer by North American workers, are reassigned to Conispirifer Lyashenko, 1985. Its immigration to and widespread dispersal in carbonate platforms of western Laurussia, northern Gondwana and tropical island arcs (?) coincided with a major late Frasnian eustatic sea level rise. The new family Conispiriferidae is proposed with Conispirifer Lyashenko, 1985, selected as the type genus. The new family also includes the new genus Pyramidaspirifer with Platyrachella alta Fenton and Fenton, 1924, proposed as the type species. The affinity of the new family remains uncertain pending restudy of key genera currently included in the Superfamily Cyrtospiriferoidea. Available data from the Devonian brachiopod literature indicate that species of Pyramidaspirifer are restricted to late Frasnian deposits of central and western North America.
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Preliminary review of taxonomy of the brachiopod order AĘpida and its stratigraphic distribution in the late Frasnian Kellwasser Crisis of several regions of Laurussia, westem Siberia and South China point to their moderate diversity and stepdown but irregular extinction pattern. The distinctive character of the late Frasnian atrypid fauna is empha-sised by several relict genera, marked by recurrent and possibly aberrant characters (mainly in ornamentation types), tendency to size reduction and homeomorphy in some taxa. The transgressive/hypoxic Lower Kellwasser Event and preceding eustatic changes during the Palmatolepis rhenanaZone had only a regional destructive effect, and were linked rathęr to an enhanced dispersal of the last generic set of aĘpids. The VariaĘpi-nae, Spinatrypinae and Iowatryp,a-group seem to belong to the latest surviving atrypids. The final demise of the remaining atrypids (and some other articulate brachiopods, e.g., gypidulids) coincided with the fransgress...
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