Thalassinoides Bacae From The Zhangxia Formation(Cambrian Miaolingian Series) In Henan Province And Its Ecological Significance

Amounts of Thalassinoides bacae are preserved in lamellar microcrystalline limestone sandwiched between thick layers of oolitic limestone of Zhangxia Formation in western Henan Province.The field profile was measured,the occurrence and distribution of ichnofossils were observed.In lab,the samples were made into slice and grinding chip,using polar-microscope,reflect microscope,cathodoluminescence microscope,X-ray fluorescence spectrometer,scanning electron microscope and energy spectrum analysis to observe microscopic mineral composition of trace fossils in detail.The results show that: from the perspective of bedding plane,the shapes of burrows are dots or strips with branching,the cross-section of the burrow tube is round or oval,nearly round,and has a thick lining.In vertical section,the burrows are inclined or vertical to the bedding plane.Burrows are mostly concentrated in the bioturbation halo,and a few burrows are also isolated in the surrounding rock.According to the morphological,structural and other characteristics,the author believe that the thick-lining ichnofossil from Zhangxia Formation should be attributed to Thalassinoides bacae.Under the microscope,there are three types of fillings: sparry calcit,micritic calcite,rhomboid dolomite crystal.The burrow linings are mainly composed of micritic calcite;the bioturbation halo is mainly composed of dolomite;the surrounding rock is also composed of micritic calcite,but its grain size is larger than that of linings.XRF indicates that Fe and S are enriched at the junction between burrow fillings and linings.In combination with optical microscope and scanning electron microscope,bio-induced framboidal pyrites,single idiomorphic crystal and pyrite oxide are only formed in the boundary of sparry calcite fillings and thick linings.Thalassinoides bacae in Zhangxia Formation occurred in the restrict platform between shoals enclosed by high-energy oolitic beach,which has the characteristics of high local abundance and single species,belonging to the opportunism ichnofossil.The thick-lining structure is an ecological strategy adopted by the trace makers to deal with this special environment.Burrow linings can maintain the internal environment,grow and nurture microbes to improve diets.The discovery of high organic matter,iron oxides and biogenic framboidal pyrites in the inner linings provided important and reliable basis for the appearance of agrichnia.The trace markers of Thalassinoides bacae can build microenvironment and cultivate the behavior habit of microorganism,which also reflects that the ability of trace makers to cope with environmental changes has been greatly improved.