RHYNCHONELLIDE BRACHIOPOD ISOPOMA TORLEY 



161 



Isopoma xestum Torley, 1934 Figs 17, 18 



1934 Isopoma xestum Torley: 82, pi. 3, figs 16, 17. 



COMMENT. This species is from the Upper Givetian Massenkalke 

 of Bilveringsen near Iserlohn on the east of the Rhine, Germany. The 

 figured specimens are small and less deep than typical /. 

 brachyptyctum and do not show ribbing. We have studied 1 1 speci- 

 mens (SMF XVII 347c) and an unregistered specimen, donated by 

 the late Dr Struve, from the Kohlenstein member of the Massenkalk 

 at the Kohlenstein quarry in the Iserlohn Bilveringsen area (BD 

 12785). The sectioned specimen was not well preserved so the 

 internal features remain poorly known and reference of the species 

 to Isopoma remains insecure. 



STRATIGRAPHIC AND GEOGRAPHIC 

 DISTRIBUTION OF ISOPOMA 



Twelve species of Isopoma are described or commented upon above; 

 /. aptyctum (Schnur), of supposed Frasnian age, is very poorly 

 known, and we have not been able to study specimens, so we do not 

 describe it here. Of these twelve reasonably established species, 

 eight are from the Eifelian to Givetian of the Middle Devonian, and 

 two (/. alecto (Barrande) and/, nekhoroshevi Bublichenko) from the 

 Pragian and Emsian of the Lower Devonian are doubtfully referred 

 to Isopoma. Two more species (/. ? ren Schmidt and /. xestum 

 Torley), from the upper Givetian might belong to Isopoma, but they 

 are also poorly known. We, therefore, suggest a well established 

 stratigraphical range for Isopoma in the Eifelian and Givetian, with 

 less securely established species in later Lower Devonian rocks. 



Isopoma appears to be absent from the Devonian of Morocco 

 (Drot, 1964) and Afghanistan (Durkoop, Mensink & Plodowski, 

 1967; Durkoop, 1970). Brice (1971), however, described specimens 

 from Afghanistan as Kransial cf. subsignata (Reed, 1908), and 



suggested they are conspecific with specimens from Burma figured 

 by Anderson, Boucot & Johnson (1969) as Uncinulus subsignata 

 (Reed). We do not think Brice's (1971) illustrations belong to what 

 would now be calledNalivkinaria subsignata, or to either Beckmannia 

 or Isopoma. 



The palaeogeographical map (Fig. 22) shows the distributions of 

 Isopoma species. 



PALAEOBIOGEOGRAPHY 



Devonian brachiopod biogeography has been discussed in detail by 

 Boucot (1988). Isopoma is typically a Middle Devonian brachiopod 

 belonging to the Rhenish-Bohemian Region of the Old World Realm 

 (Boucot, 1984) and has Rhenish provincial affinities (Struve, 1982a). 

 The largely warm, shallow marine environment of this biogeographic 

 region probably had complex current circulation patterns. Apart 

 from palaeogeography, sea-level changes and the development of 

 reefs and banks, the shallow seas and patterns of marine circulation 

 might have influenced the variable faunal associations of the Middle 

 Devonian of the Rhenish-Bohemian Region. Information on the 

 Rhenish-Bohemian Region brachiopods shows them as occurring in 

 parts of Europe, North Africa and Asia. The Lower and Middle 

 Devonian faunas of most of north Africa are of Rhenish-Bohemian 

 Region type (Boucot et al, 1983). The Jauf Formation of Lower 

 Devonian (Pragian-Emsian) age of northwestern Saudi Arabia con- 

 tains brachiopods and trilobites which also belong to the 

 Rhenish-Bohemian Region of the Old World Realm (Boucot, 1984; 

 Boucot etai. 1989). 



A Rhenish type of Eifelian brachiopod fauna occurs in Armenia 

 and the Tien Shan (Boucot et al., 1988: 365). The biostratigraphy 

 and biogeography of Devonian brachiopods in China have been 

 described by Hou Hong-Fei ( 1981 ) and Wang Yu et al. (1984). The 

 South China Province includes the south Tien Shan Mountains, 



Figs 19-21 Scanning electron micrographs of abraded valve exteriors illustrating macro-shell structures in three species of Isopoma. 19, /. brachyptyctum 

 (Schnur), Eifel, Germany; same specimen as Fig. 3: ventral valve anterolateral flank, BD 12784. x 100; 20, /. maymyoensis sp. nov., Burma; ventral 

 valve (see Fig. 13), posterior to median sulcus, BB 55547. x 50; 21, /. lummatoniensis {Davidson): Devon. England; ventral valve, posterolateral flank, 

 BB12802, x40. 



