Trace fossils and ichnofacies of Middel Ordovician Gongwusu Formation, Zhuozishan, Inner Mongolia

Abundant trace fossils are discovered from the Middle Ordovician Gongwusu Formation in Zhuozishan, Inner Mongolia, with 14 ichnogenera (including 1 new ichnogenus) described in this paper, namely, Arthrophycus, Biscopulatichnus ichnogen. nov., Chondrites, Circulichnus, Granularia, Helminthopsis. Nereites, Paleodictyon. Phycodes. Scolicia. Sublorenzinia. Taenidium. Volkichnium and Zoophycos. Based on the characteristics of the trace fossils, four trace fossil assemblages have been established, which are briefly described in ascending order as follows: 1. Taenidium-Phycodes assemblage, ranging from Beds 2-4, dominated by Taenidium and Phycodes, also including the ichnogenera Granularia, Arthrophycus. Biscopulatichnus, Chondrites, and Volkichnium. 2. Volkichnium assemblage, occurring in Bed 5 of the formation, in which the radiate trace fossil Volkichnium is most abundant, co-existing with the ichnogenera Arthrophycus, Granularia, Phycodes and Sublorenzinia. 3. Helminthopsis assemblage, ranging from Beds 6 to 8, characterised by the occurrence of abundant winding trace fossil Helminthopsis, together with a small number of the netlike Paleodictyon. 4. Granularia-Circulichnus assemblage, restr ted to Bed 9, mainly composed of pellet-walled Granularia, together with Circulichnus, Helminthopsis, Nereites, Phycodes, Volkichnium, and Zoophycos. It is believed that the above-mentioned four assemblages correspond broadly to Seilacher's (1967) Zoophycos ichnofacies, which indicates the circalittoral to bathyal, anoxic quiet water environment. This is also supported by the occurrence of abundant graptolites and some small, thin-shelled brachiopods as well as their good preservation in the formation. Description of new ichnotaxa Biscopulatichnus ichnogen. nov. Type ichnospecies: Biscopulatichnus wuhaiensis ichnosp. nov. Etymology: Combination of Latin "Bi" with "scopilas". Diagnosis: Traces with unequal ribs radiating in two opposite directions and resembling two brooms with their axes longitudinally connected; without areola. Biscopulatichnus wuhaiensis ichnogen. et ichnosp. nov. (Pl.IV, figs. 4, 6; Text-fig. 6) Etymology: Wuhai, the type locality of the fossil. Holotype: Pl. IV, fig 6. Description: Traces with unequal ribs radiating in two opposite directions; ribs of the two 'brooms' 1.8-2.5 cm long, numbering 4 and 5 respectively. Circulichnus spiralis ichnosp. nov. (Pl. I, figs. 2, 6) Etymology: From "spiralis", Latin, helical. Holotype: Pl. I, fig. 2. Description: Helical unbranching burrows 1.1 mm in diameter; one circle 4.2 cm in diameter. Granularia simplex ichnosp. nov. (Pl. I, figs. 3, 4) Etymology: From "Simplex", Latin, simple. Holotype: Pl. I, fig. 4. Description: Sinuous cylindrical pellet-wa led burrows 3 mm in diameter. Discussion: The new ichnospecies differs from G. lumbricoides in the unbranching and sinuous burrows. Taenidium irregulare ichnosp. nov. (Pl. III, figs. 1, 2) Etymology: From "irregularis", Latin, irregular. Holotype: Pl. III, fig. 1. Description: Freely curved meniscate burrows with intermeniscate segments generally longer than wide, and even more deeply concave menisci; burrow ranging irregularly from 0.7 to 1.2 cm in diameter. Discussion: As compared with Tacniditm cameronense (Brady, 1947), this new ichnospecies has even more deeply concave menisci, with the urrow changing irregularly in width.