| Trace fossils play an important role in paleontology.On the one hand,they intuitively record information of various physiological activities of organisms.On the other hand,they indirectly support the paleontological classification of these organisms and the interactions between organisms and the environment.The Cambrian Explosion is one of theremarkble biological events throughout the geological history.The uneven distribution of exceptionally preserved Cambrian fossil Lagerst?tten,however,places an challenge to our understanding of the evolutionary history of intervals that lack body fossils.Therefore,Cambrian trace fossils have become an important auxiliary evidence for the reconstruction of the Cambrian Explosion.The Mantou Formation(Series 2 and Series 3 of the Cambrian System),widely distributed in the North China Platform,is a set of shallow marine clastic rocks with relatively fast deposition rate.In the sedimentary environment where the Mantou Formation is located,the overall water body is relatively turbulent,the salinity is normal,the oxygen content is sufficient,and the climate is warm.In addition,the sediments were not transported over long distances and contained a lot of nutrients.These ecological environments are suitable for various organisms to inhabit.The shallow coastal area where the Mantou Formation is developed is occupied by an opportunity population with low differentiation and strong adaptability in a relatively short period of geological time.As the biological abundance continues to increase,various biological behaviors such as swimming and benthic continue to transform the sedimentary substrate,and a large number of biological disturbances form a dense layer of trace fossils on the surface of the substrate.Cambrian marine clastic rocks in the Lushan area of Henan Province,China have complete stratigraphic sequences and clear outcrops.More importantly,a large number of coastal and shallow marine trace fossil assemblages are preserved in the Mantou Formation in the Lushan area.The Lushan trace fossils are herein investigated in this thesis.In this paper,combined with excavations in the filed and comprehensive analysis in the lab,a systematic study on the taphonomy,paleobiology,and paleoecology of trace fossils from the Mantou Formation is carried out.Current progresses are summarized as following: 1.The Mantou Formation at the Xiatang Section of Lushan is rich in trace fossils,especially in the thin-to-medium fine sandstone and siltstone(partially interbedded with thin siltstone)in the 138–179 m interval above the bottom of this formation.Trace fossils are the most abundant in the second member of the Mantou Formation.A total of 27 genera and 29 species are identified.According to behavioral classification,the Lushan trace fossil assemblage includes: 2 genera of Domichnion(Bifungites,Skolites),7 genera of Repichnion(Beaconichnus,Didymaulichnus,Dimorphichnus,Diplichnites,Gordia,Palaeophycus,Planolites),2 genera of Cubichnia(Bergaueria,Rusophycus),6 genera of Fodinichnion(Belorhaple,Cruziana,Palaeophycus,Phycodes,Planolites,Skolites),1 genus of Agrichnia(Chondrites),and 2 genera of swimming traces(Monomorphichnus,Parallehelcura).2.Shallow-middle-class benthic trace fossil assembalge records a variety of ecological characteristics of paleontological communities.The burrowing fossils in this community include Planolites,Palaeophycus,and Skolithos.Arthropod-producing traces include Beaconichnus,Cruziana,Dimorphichnus,Diplichnites,Monomorphichnus,and Rusophycus.3.Popular Cambrian trace fossils in the shallow environment include Gordia,Lophoctenium,Phycodes,Torrowangea.Trace fossils in the Mantou Formation in the Lushan area are highly diverse(assigned into the Cruziana ichnofacies),including abundant of atypical fossils.4.Some trace fossils in the Mantou Formation represent arthropod animals.These creatures have flat bodies,long outer limbs with bifurcated ends,and long spines on their bodies.They are not only good at digging burrows,but also can walk on the surface of sediments and move quickly by swimming.They are mixed swimming and benthic animals.In addition,non-arthropod trackers in the Mantou Formation include worms or worm-like animals,annelids,gastropods,and polychates.5.Traces created by animals in the low-energy environment are often shallower,and the latent caves are often open or semi-open.In high-energy environments,trace-makers often tend to dig deeper into the stratum and build a solid burrow(represented by thickened lining fossil morphological features).6.Trace fossils in the Mantou Formation can be divided into three parts from the bottom upward.The first and second members of the Mantou Formation are mainly composed of the opportunity to save the semi-open opportunity of the species Phphycodes montanus.The size of the trace fossils is small.The middle part of the second member of the Mantou Formation produces a rich "Cruziana ichnofacies".The size of the latent cave is relatively large and the wall is thin.The third member of the Mantou Formation belongs to a confined platform environment with low bioturbation.The remaining fossils are mainly near-level subterranean caves,including Phycus and Plphycodes.The size of the fossils is medium.7.The occurrence of trace fossils in the Mantou Formation in the Lushan area is affected by the nature of substrate.According to the consolidation state of the substrate,it can be divided into soft bottom and hard bottom,respectively.Among them,the soft bottom is preserved as a large number of burrows formed by the tracers on the unconsolidated substrate,and the submerged caves usually have reinforced linings.The hard substrate is consolidated,compacted,and dehydrated sediment.The remnant fossils controlled by solid bottom generally do not have linings,and the depth of the burrows is shallow,the fidelity is high,the decoration is clear,the filling is completely different from the surrounding rocks,and most of them are open-system burrows with low community differentiation.The degree of deformation is also often small. |
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