Functional Morphology,Behavioural Ecology And Macroevolution Of The Complex Trace Fossil Rhizocorallium

The ichnogenus Rhizocorallium,a complex trace fossil with various ichnospecies,ichnosubspecies and varieties of different morphplogies.As one of the earliest described and recorded complex trace fossils.It has long stratigraphical range(Cambrain-Neogene),occurring in a wide range of evironments(from the terrestrial,marginal-marine,shallow-marine,to the deep-sea environment)and substrates(slilceous rock,carbonates,siliciclastic rock,and volcanic clastic rocks).Therefore,compared with other trace fossils,Rhizocorallium better reveals the morphological charateristics,behaviour,and ethology of ancient organisms,as well as the co-evolution between organisms and the environment in the geological history.In this paper,110 specimens collected from the Silurian,Devonian,Permian,and Triassic of the South China Plate,the Triassic of Spain,and the Neogene of Taiwan,and nearly1000 field photos of Rhizocorallium were analyzed quantitatively and semi-quantitatively.The morphological differences between different ichnospecies,ichnosubspecies,and varieties belonging to Rhizocorallium were systematically described.Based on about 300 papers on Rhizocorallium published between 1840 and2021,the Phanerozoic database of Rhizocorallium was constructed,and the spatial and temporal distribution of Rhizocorallium was summarized systematically.(1)Through a systematic analysis of the host strata and sedimentary background of trace fossils from five sections(Silurian Guodingshan section in Hanyang,Wuhan,Devonian Ganxi section in Sichuan,Devonian Xichuan section in Nanyang,Triassic Longmendong section in Sichuan,Triassic Boyun section in Yunnan)and four auxiliary sections(Devonian Dushan section in Guizhou,Permian Laibin section in Guangxi,Triassic Huesa section in Spain,and Neogene Fanziao Section in Taiwan),it is found out that Rhizocorallium is mainly distributed in siliciclastic and carbonate rocks of the shallow-marine environment.Moreover,the morphological and ichnoassemblage characteristics of Rhizocorallium under different environmental conditions are obviously different.In the stable shallow-marine environment,there is usually a highly diversified K-selected ichno-community,while in the turbulent shallow-marine environment affected by storm activities,there is usually a less diversified r-selected ichno-community.It is suggested that the intensive intersecting distribution of Rhizocorallium in different beds of the Triassic Boyun section is affected by the multi-stage storm erosion activities,and the opportunistically foraging Rhizocorallium can finally form the "lam-scram" fabric.(2)Based on semi-quantitative and quantitative analytical methods,the burrow morphological elements of Rhizocorallium ichnospecies,subspecies,and varieties were coded and measured.A total of 10 semi-quantitative morphological codes were established to characterize the opening and distal end morphology of the U-shape marginal tube and the shape of the overall burrow path in Rhizocorallium.Using such morphological code combinations,the morphological differences among ichnospecies,subspecies,and varieties of Rhizocorallium are accurately-represent,and it is proposed that the differences in the morphological code combinations reflect the different behavioural types of the trace makers.Meantime,the diameter and,length of the marginal tube and burrow width were selected as quantitative analytical parameters,and the quantitative morphological differences among different ichnospecies,ichnosubspecies,and varieties of Rhizocorallium were revealed by comparing the changes in the quantitative parameters among different specimens.(3)Four categories of excavation patterns were proposed according to the four main spreite structures of Rhizocorallium,the protrusive spreite structure is formed by the continuous excavation along the bed,the retrusive spreite structure is formed by the escape of the tracer makers,the J-shaped spreite structure is formed by the avoidance of obstacles during excavation,and the branched spreite structure indicates the active detection and pursuit of food sources by the trace maker.(4)Taking Rhizocorallium from the Devonian Xichuan and Ganxi sections as an example,it is found that Rhizocorallium in the two sections are active-filling burrows by microscopic-morphological characterization.However,there are obvious differences in lithological characteristics between the marginal tube and the spreite area of the Rhizocorallium burrow in the Xichuan section.A large number of quartz grains and body fossils are developed in the marginal tube,which are consistent with the overlying sediments.Compared with the closed spreite environment,the open marginal tube area tends to be transformed by the later environment.Meanwhile,four types of pyrite framboids(pyritohedral,octahedron,cube,and spherical)have been found within Rhizocorallium.Moreover,the ratio of pyrite framboids diameter(D)to submicron crystal diameter(d)is less than 10,suggesting that they were formed by microbial-activities.Relatively large pyrite framboids with smooth surface mainly occur in the marginal tube,while small pyrite framboids with sheath are abundant in the spreite.EDS(energy spectrometer)analysis showed that the S/Fe ratio in the pyrite increased with decreasing crystal diameter,indicating different redox conditions in the marginal tube and the spreite.These features indicate that Rhizocorallium is not simply a foraging trace,but is probably a multi-functional,complex burrow system utilized for living,caching,and farming microorganisms.(5)Based on the Phanerozoic database of Rhizocorallium,the temporal and spatial distribution of Rhizocorallium and the morphological evolution of burrows are systematically summarized.The burrow morphology of Rhizocorallium changed from simple to complex,and the host environment changed from the shallow-marine to the marginal-marine,terrestrial,and deep-sea environments.At the same time,considering the characteristics of atmospheric temperature and chemical oceanographic parameters during different Phanerozoic periods,it is found out that the environmental fluctuations are related to the changes in the distribution patterns of Rhizocorallium.In particular,the abundance and palaeolatitudinal distribution of Rhizocorallium burrows changed before and after the five mass extinction events.After the Late Ordovician mass extinction,Rhizocorallium disappeared completely from the shallow-marine environment,and did not reappear until the Early Silurian.After the Late Devonian mass extinction,the palaeolatitudinal distribution of Rhizocorallium showed a transition from the low palaeolatitudes to the middle-high palaeolatitudes,the burrow density was relatively low,and the burrow mainly occurred in the normal shallow-ocean environment.After the P-T mass extinction,Rhizocorallium first appeared in the high palaeolatitudes.With the process of biological recovery,Rhizocorallium began to expand to the middle and low palaeolatitudes,the density of burrows gradually increased,and the host environment also transitioned from the marginal-and shallow-marine environment to the deep sea.The Late Triassic and Cretaceous-Paleogene(K-Pg)extinction events had relatively little influence on the palaeolatitudinal distribution and burrow abundance of Rhizocorallium.At that time,Rhizocorallium had been widely developed in different environments at different palaeolatitudes.Therefore,the morphological changes and macroevolution of Rhizocorallium reflect the responses of its trace makers to major Phanerozoic bio-environmental events,which reveal the co-evolution of organisms and the environment in the geological history.