Paper No. 23
Presentation Time: 1:30 PM-5:30 PM
COMIOTECHIA AND XINSHAOELLA? (RHYNCHONELLIDA-LADOGIIDAE) FROM THE MIDDLE? AND LATE FRASNIAN OF THE EURAMERICAN TROPICS (NORTH AMERICA AND GERMANY): FIRST KNOWN OCCURRENCES OUTSIDE OF THE RUSSIAN PLATFORM AND SOUTH CHINA
To date, Upper Devonian (Frasnian-Famennian) brachiopods assignable to genera of the family Ladogiidae (Rhynchonellida) have not been reported outside of the Russian Platform and South China. Specimens recovered from Frasnian carbonate platform successions in the Cordilleran continental margin of western Euramerica (Nevada and western Canada) and Germany are closely similar to the ladogiid genera Comiotoechia and Xinshaoella. Previously, Comiotoechia was not known outside of the Lower Frasnian of the Russian Platform, and Xinshaoella is restricted to Early-Middle Famennian shelf deposits of South China. Late Frasnian specimens resembling Xinshaoella are shown from the Mackenzie Shelf of the southern NWT of western Canada, as are Middle-Late Frasnian specimens closely resembling Comiotoechia from Nevada, Alberta, and Germany. Middle Frasnian (Montagne Noire [MN] Zone 7?) Comiotoechia? were recovered from a patch reef cavity-filling assemblage from the Guilmette Formation in southern Nevada. Late Frasnian specimens of Comiotoechia were recovered in platform-derived debris flow skeletal packstones of the upper Mount Hawk Formation (MN Zone 12) in the Alberta Rocky Mountains, and in an Upper Kellwasser Extinction bioevent crisis fauna (MN Zone 13c-Upper linguiformis Zone) recovered in the Beringhauser Tunnel section in west-central Germany. Late Frasnian (MN Zone 13a) specimens closely resembling Xinshaoella were recovered from reef-platform derived skeletal grainstones of Member 1 of the Kakisa Formation on Kakisa River and Kakisa Lake area of the southern NWT, western Canada. In all three North American localities abundant specimens of the uncinulloid genus Hypothyridina are directly associated with Comiotoechia and Xinshaolella. Adult specimens of ladogiids reprted here superficially resemble juvenile growth stages of Hypothridina, but are distinguished from Hypothyridina by their more numerous ribs and lack of the well defined and angular medial fold and sulcus in shells of comparable size. These North American and German occurrences fill a gap in the record of the Frasnian and Famennian ladogiids and suggest that Xinshaoella evolved from Comiotoechia in the latest Frasnian, prior to the extinction of the latter genus in the very latest Frasnian.