Study On Taphonomy Of The Palaeoscolecids From The Cambrian(Stage 4),Balang Formation Of Guizhou,China

Palaeoscolecids are found in many places of the Cambrian Balang Formation in Guizhou,Southwest China.Among them,the number of palaeoscolecidan fossils is the largest in Lazizhai section in Jianhe county and Geyi section in Taijiang county of the Balang Formation.Based on the identification of palaeoscolecidan fossils characteristics collected from different areas of the Balang Formation,it is confirmed that there is only one species of Wronascolex geyiensis Peng et al.,2016.In this paper,we mainly focus on the fossils from Lazizhai section in Jianhe,Guizhou,Southwest China,which is the most abundant area of Wronascolex geyiensis in the Balang Formation.A preliminary study on paleoecology and taphonomy were carried out.Wronascolex geyiensis in Lazizhai section,Jianhe,Guizhou,Southwest China of Balang Formation is mainly preserved in three layers.Based on the analysis of the rock structure,sedimentary characteristics,fossil preservation and taphonomic characteristics of the three layers,the following conclusions are obtained: In layer 1,Wronascolex geyiensis has high abundance of fossils which forms a soft body enrichment layer and the individuals are mostly preserved in curly shape.The trunk and tail of most Wronascolex geyiensis are well preserved even the intestines are preserved.Wronascolex geyiensis mainly preserved in a thicker event sedimentary layer along with the fossils of a nekton named Bradoriida.On the polished rock surface,cyclothem composed of event layers of sediment and organic layers(background layers)can be seen.The event layers of sediment with brighter color is mainly composed of coarse grained terrigenous debris,and the organic layers with darker color are algae-bearing mudstone.The repetition frequency of event layers of sediment within a fixed thickness is large,and the thickness are larger than that of the organic layers.The preservation characteristics of Wronascolex geyiensis in layer 2are extremely similar to that in layer 1.A lot of Wronascolex geyiensis with intestines and relatively complete trunk are preserved and the fossils are mainly preserved in thicker event layers of sediment as well.The above characteristics indicate that the fossils preservation methods of the layer 1 and layer 2 are autochthonous burial caused by rapid deposition events.Wronascolex geyiensis traverses the microlayer,was found in the layer 1 and layer 2,indicating that it has the ecological characteristics of free shuttle at low angles in the seabed soft sediments and free movement on the bottom surface.Layer 3 has lower abundance of Wronascolex geyiensis.Most of the individuals broke their trunk,without preserved introverts,intestines and tails.They without preserved with other animals common fossils,individuals are scattered in mudstone layer.On the polished rock surface,cyclothem composed of event layers of sediment and organic layers(background layers)can be seen.The thickness of the event layers are less than or equal to the gray organic layers,the above characteristics indicate that the rock layers are dominated by background sedimentation,and no large-scale rapid sedimentation has occurred.It may be caused by the transient exposure of the body of Wronascolex geyiensis on the surface of the basement during static water deposition,and then gradually buried by sediments.Wronascolex geyiensis are mainly preserved in thinner event layers of sediment or background layers and the broken fossils may be molting specimens.The fossil preservation methods of layer 3 are autochthonous-near-autochthonous burial.Wronascolex geyiensis in Balang biota contains 2 burial models as autochthonous rapid burial and near-autochthonous burial that went through short-term still water transportation.Wronascolex geyiensis in the three layers all have the phenomenons of clay mineral filling,strawberry pyrite filling and idiomorphic-hypidiomorphic pyrite filling.The formation of the strawberry pyrite would consume organic matter and there are certain extent of damage on the fine structure of the soft body with specimens that have filling phenomenon of strawberry pyrite,so the fossil preservation was affected by the burial location in the diagenetic process,mineral crystallization in deposition,and the filling mode.According to analyzing the clay mineral,strawberry pyrite,idiomorphic-hypidiomorphic pyrite in the Wronascolex geyiensis body on fossils,we consider that the most appropriate environment for preservation of soft-bodied fossils should be the oxygen-deficient environment with the sulfate in pore water no contact organisms under autochthonous and nearly autochthonous burial conditions,thus further supplying the new evidence for taphonomy study of Balang biota.