Bull. nat. Hist. Mus. Lond. (Geol.) 54(2): 155-163 



Issued 26 November 1998 



The rhynchonellide brachiopod Isopoma 

 Torley and its distribution 



M. MOHANTI 



Department oj Ueology, Utkal University, Vani Vihar, Bhubaneswar-75 1 004 (Orissa), India 



C.H.C. BRUNTON 



Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD 



SYNOPSIS. The Devonian rhynchonellid brachiopod genus Isopoma Torley, 1934, is described, with comments on species 

 recorded from various geographic regions. Isopoma maymyoensis sp. nov. is erected for some specimens from the Late Eifelian 

 to Early Givetian Padaukpin beds of Burma. Isopoma is mainly Eifelian to Givetian, Middle Devonian, in age. There are two 

 species in the Pragian and Emsian, Lower Devonian, but possible occurrences in the Frasnian, Upper Devonian, have yet to be 

 proved. Biogeographically the genus belongs to the Rhenish-Bohemian Region of the Old World Realm and has Rhenish 

 affinities. 



INTRODUCTION 



Having discovered specimens of Isopoma Torley, 1934, incorrectly 

 identified, we determined to investigate this poorly known genus to 

 determine its stratigraphic and biogeographic distribution. One of us 

 had previously identified the genus in north Spain (Mohanti, 1972) 

 and, while investigating that fauna, studied specimens of Isopoma 

 from various localities in Germany housed at the Senckenberg 

 Natural History Museum at Frankfurt. 



MATERIAL 



We have studied specimens representing Isopoma brachyptyctum 

 (Schnur), /. gryps Schmidt, /. orthoglossa (Torley), /. ? ren Schmidt, 

 and /. xestum Torley from the Devonian of Germany, as indicated 

 under the species descriptions. We have also studied specimens in 

 the collections of the Natural History Museum, London, including 

 specimens of Rhynchonellal lummatoniensis Davidson, 1865, which 

 we assign to Isopoma and for which a lectotype is selected, and the 

 collections from Burma figured and described by Anderson, Boucot 

 & Johnson (1969). Some specimens in this collection identified as 

 Uncinulus subsignata (Reed), are described here as Isopoma 

 maymyoensis sp. nov. Mohanti also studied Isopoma hertae from 

 the Cantabrian Mountains. Spain, housed in the National Natural 

 History Museum, Leiden, Netherlands, which he had described in 

 1972. Our comments on other species of Isopoma are based largely 

 upon information from the literature. 



SYSTEMATIC DESCRIPTIONS 



Most of the material described here is housed in the BMNH collec- 

 tions of The Natural History Museum, London, and these have 

 registration numbers prefixed by B, BB, or BD. Register numbers of 

 specimens in the Forschungsinstitut Senckenberg, Frankfurt, Ger- 

 many, have the prefix SMF 



© The Natural History Museum. 1998 



Order RHYNCHONELLIDA Kuhn, 1949 



Superfamily PUGNACOIDEA Rzhonsnitskaya, 1956 



Family ASEPTIRHYNCHIIDAE Savage, 1996 



DIAGNOSIS. Pugnacoidea lacking dental plates, dorsal median 

 septum or septalium: fold and sulcus developed anteriorly. 



Discussion. Savage (1996) introduced the family Asepti- 

 rchynchiidae, which included Aseptirhynchia Soja, 1988. Brunni- 

 rhyncha Havlicek, 1979, Carolirhynchia Havlfcek, 1992; Chalimia 

 Baranov, 1 978 andlsopoma Torley, 1934. XuHan-kui and YaoZhao- 

 kui (1984: 561, table 3) described a new family, the Katuniidae, in 

 which they pi acedlsopoma Torley, 1934. Savage (1996), in his major 

 revision of Palaeozoic rhynchonellides, placed Katunia with weak 

 dental plates in the Leiorhynchinae. while Isopoma was assigned to 

 the new family, theAseptirhynchiidae, which lacks both dental plates 

 and a dorsal median septum. Genera in the family are united by their 

 similar external morphologies, ie. their posterior smooth shells, and a 

 tendency towards the anterior development of a fold and sulcus in 

 which a few ribs developed. In some, weak additional ribs occur 

 flanking the sulcus. The genera are differentiated principally on their 

 internal characters. In two, Chilimia and Aseptirhynchia, dental 

 plates are either weakly present or developed only in early ontogeny. 

 Shell wall thickening is variable: in Chilimia the walls appear to be 

 thin (Baranov, 1978: text-fig. 1); in Aseptirhynchia the ventral valve 

 walls became thickened and overgrow the juvenile dental plates, but 

 the dorsal valve remains relatively thin; in Isopoma both valves 

 became thickened, but the hinge plates remain free, and the teeth and 

 sockets are particularly strongly developed; in Carolirhynchia and 

 Brunnirhyncha the valve wall thickening fused the hinge plates to the 

 dorsal interior, leaving only a groove between the median edges. A 

 further distinction in Chilimia is that the hinge plates only became 

 medially disjunct at their anterior extremities. 



Distribution. Aseptirhynchia occurs in the Emsian of Alaska. 

 Carolirhynchia was reported originally from Pragian and Emsian 

 beds in the Barrandian area of Bohemia, although Savage (1996) 

 reported the genus as restricted to the Eifelian. Chilimia is reported 

 from the Middle Devonian of northeast Russia. Isopoma was origi- 

 nally described from the Middle Devonian of Germany, but is fully 

 described below. Brunnirhyncha is of Early Famennian age from the 

 Bruno area of the Czech Republic. 



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