AN ORDOVICIAN (EARLY LLANVIRN) TRILOBITE FAUNULE FROM NANZHENG, SOUTHERN SHAANXI

Abstract

Only seven trilobite species were previously described by Lu (1975) from the basal part of the “ Yangtzeella poloi beds” at Zhajiagou, about 300 m southwest of the Xialiangshan village, Nanzheng County,southern Shaanxi (Lu,1975, text fig.10). All are from an 8 m marlite (Bed 7, Lu, 1975,p. 49, text fig.11), which is rich in biocalcilutite nodules. They constitute a nileid dominate assemblage, including Nileus armadilloformis Lu, N. walcotti Endo(= N.liangshanensis Lu, see Zhou et al., 1998), Symphysurus carinatus Lu, S. subquadratus Lu, Dikelokephalina gibbosa Lu, Neseuretus concavus tenellus Lu and Ogmasaphus hannanicus (Lu) (= Pseudasaphus hannanicus Lu, see Zhou et al., 1998). New material was collected along the Zhajiagou section from a silty marlite of 2 m thick. The horizon, according to Lu (1975,p.49), lies at the base of Bed 8, which is altogether 12 m in thickness, representing the lower part of the “Yangtzeella poloi beds”. After critical identification, the collection contains following 13 trilobite species: Carolinites sp., Bathycheilus sinensis sp. nov., Hemisphaerocoryphe elliptica (Lu) (= Ellipsocoryphe elliptica Lu, see Zhou et al ., 1998), Prionocheilus sp., Ovalocephalus cf. intermedius (Lu and Zhou), Sinocybele cf. baoshanensis Sheng, Pseudocalymene sculpta sp. nov., Lonchodomas sp., Liomegalaspides usuii (Yabe), Yanhaoia huayinshaensis (Lu)(= Pterygometopus huayinshanensis Lu, see Zhou et al., 1998), Ogmasaphus hannanicus (Lu) and Mioptychpyge zhenbaensis (Zhou)(= Basiliella zhenbaensis Zhou, see Zhou et al., 1998).The faunule is dominated by the bathycheilid Bathycheilus, which makes up 25% of the total collected specimens. The encrinurid Sinocybele (20%) and leiostegiid Pseudocalymene (14%) rank respectively second and third. Trilobites are all randomly distrbuted and poorly sorted with no distinct reworking. Some of them are disarticulated, but about 40% of collected specimens are articulated exoskeletons. The evidence from taphonomy and lithology suggests that benthic forms of this trilobite faunule may have lived on a muddy substrate in a generally quiet environment below storm wave base (70 m). In southern Shaanxi, the “Yangtzeella poloi beds” have recently been referred to a Llanvirn litho stratigraphic unit, the Guniutan Formation (Lai et al., 1990);a view is temporarily followed in the present paper. In the composition and diversity, the trilobite faunule is quite different from that of the underlying Bed 7 and may represent a new assemblage. Some trilobites, such as O. hannanicus, H. elliptica and Y. huayinshanensis, have recently been recorded from the Lenodus variabilis conodont Zone in the Kalpin area, northwestern Tarim (Zhou et al., 1998); O. cf . intermedius and S. cf . baoshanensis are morphologically similar to nominal Llanvirn species respectively from Zhuozishan, Inner Mongolia and Baoshan, western Yunnan. We, therefore, consider that the trilobite assemblage is of early Llanvirn age. Except for the epipelagic genus Carolinites, of the 12 benthic trilobite genera in the faunule, five (Ovalocephalus, Pseudocalymene, Prionocheilus, Bathycheilus and probably Sinocybele are typical of peri Gondwana faunas, three (Liomegalaspides, Mioptychpyge, Yanhaoia) endemic to the Yangtze and Tarim blocks, and two (Ampyx, Lonchodomas) widespread. However, a faunal link with Baltoscandia is indicated by the occurrence of Hemisphaerocoryphe and Ogmasaphus. Seven from the 13 species, namely Carolinites sp., Bathycheilus sinensis sp.nov., Hemisphaerocoryphe elliptica (Lu), Prionocheilus sp., Ovalocephalus cf. intermedius (Lu and Zhou), Sinocybele cf. baoshanensis Sheng, and Pseudocalymene sculpta sp.nov., are described in the paper. Diagnostic characters of the two new species are given as follows. Family Leiostegiidae Bradley, 1925 Genus Pseudocalymene