LINOLDHAMININAE, A NEW SUBFAMILY OF LYTTONIIDAE WAAGEN, 1883 (BRACHIOPODA) FROM THE GUADALUPIAN (MIDDLE PERMIAN) XIALA FORMATION IN THE XAINZA AREA, NORTHERN TIBET

Abstract

Members of the Lyttoniidae Waagen, 1883 (Lyttonioidea Waagen, 1883, Brachiopoda) are characterized by numerous peculiarities, including their disproportionately inequivalve, rudimentary articulatory apparatus and asymmetrically developed muscle scars. The ventral valve of lyttonioids usually possesses a posterior flap and complementary septal apparatus and the dorsal valve has corresponding lobes and slits to fit around the ventral septa (Noetling, 1905; Fredericks, 1926; Wanner, 1935; Williams, 1953; Grant, 1976; Williams et al., 2000). Externally, the Lyttoniidae have been previously described as being smooth or having some growth banding, lines, or disturbances only (Cooper and Grant, 1974; Williams et al., 2000). Spines were previously reported by Sarytcheva (1964), but subsequently not confirmed by Cooper and Grant (1974, p. 387) and Williams et al. (2000). Radial ornamentation has never been reported prior to this study. In 2002, two specimens with Linoproductus -like costellae and Oldhamina -like septal apparatus were found by one of the authors (CLR) from the Middle Permian Xiala Formation at the Xinji Section in the Xainza area (89–91°E, 30–31°N) in northern Tibet (Fig. 1.1, 1.2) during 1:250,000 mapping. We consider that the present specimens represent another distinct phylogenetic branch within the Lyttoniidae, although only two specimens were found. Further collecting is currently impossible because of the inaccessibility of the remote area in northern Tibet. In this paper we aim to propose a new subfamily of Lyttoniidae based on the specimens found from northern Tibet. Figure 1 —Map showing the fossil locality and the Permian columnar section in the Xainza area, northern Tibet (biostratigraphical and lithostratigraphical data after Cheng, personal commun) The Lhasa Block was attached to the northern peri-Gondwanan margin during the Late Paleozoic. Subsequently this block rifted northwards in late Early Permian or Middle Permian, and incorporated into the Cathaysian Province …