Morphology and revision of Late Devonian (early Famennian) Cyrtospirifer (Brachiopoda) and related genera from South China and North America

摘要

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 Cyrtospir fer Nalivkin in Fredericks, 1924, or Tenticospirifer Tien, 1938, as previously assigned. Some of these forms are reassigned to the new genus Plicapustula, with Spirifer (Sinospiriftr) 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 pseudodelfidia 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 Smospirifer 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 verneudi type, consisting of microspines arising from the shell surface with spine bases extending into the primary shell layer, with radial capillae; 2) the whitneyi-subextenstis 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 cyrtospirifermids 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.