Discovery of Middle Jurassic Trace Fossil Tisoa Serres in N orthern Tibet and Its Palaeoenvironmental Significance

Key words: 
• Northern Tibet /
• middle jurassic /
• trace fossil /
• Tisoa /
• palaeoenvironment

Abstract: The trace fossil Tisoa Serres was unreported previously from China. Over 70 Tisoa specimens have been found in Middle Jurassic Sew a Formation in Qiang tang area, no rthern Tibet. The outcrop is located at 89°54′E in longitude and 32°07′N in latitude. Tisoa specimens consist typically of two closely parallel, cylindrical tubes that constitute the long axis of conical or cylindrical calcareous concretions. Individual tubes 2′～7mm in diameter and lying 1～4 mm apart, a rerarely branched. Calcareous concretions which covered the tubes are 10～25 mm. in di-ameter. One well-preserved specimen shows that Tisoa actually consists of a single U-shaped tube which linked at the basal part, and that the two parallel tubes constituting most specimens are actually the up-per part of a single U-shaped tube. An irregular layer of pyrite in a thin layer of microcrystalline calcite is commonly present along most tubes. The tubes are of ten filled with micrite and sparite, some with iron sulfide, mica and argillaceous as well as arenaceous quartz. Tisoa represents a dwelling burrow probably made by an arthropod. By analysis of sedimentology and palaeoecology, the upper part of Middle Jurassic Sewa Formation which bearing trace fossil Tisoa represents shallow water condition.