Paleoecological study of the earliest Cambrian biological activity evidenced by the diversification of ichnofossils

In order to better understand the diversification of the animal activities in the earliest Cambrian than studying only body fossils, ichnofossils were comparatively studied in three different localities (Newfoundland, Canada, Yunnan, China and Gobi-Altai, Mongolia).Special attention was paid for the following topics; 1) To understand the developmental mode of the animal behaviors, the diversification patterns of ichnofossils were studied, 2) To know the adaptation processes to different environments, changes of the ichnofossil assemblages in different sedimentary facies were surveyed, 3)To reveal the body size changes of trace maker animals, size distributions of the tubeshaped ichnofossil Planolites were investigated, 4) To clarify the buildup processes of animal activities, densities of ichnofossils covering the bedding planes were measured.The diversification patterns of the ichnofossils in the earliest Cambrian Chapel Island Formation, Newfoundland show that major diversification of the ichnofossils occurred twice; firstly at the almost basal part of the Pc-C boundary, secondly around the boundary of the Treptichnus pedum and Rusophycus avalonensis zones.The ichnofossil assemblages are relatively common to different depositional facies, suggesting that there was no conspicuous specialization of these assemblages to particular facies.This observation indicates that animal behaviors developed in a wider environmental spectrum ii in the earliest Cambrian than in later ages.On the other hand, ichnofossil assemblages considerably differed between the late Early Cambrian Balang and Chintingshan formations in China.One possible explanation for this change would be that the animals have started to adapt and to be diversified their behavior corresponding to the environments since the late Early Cambrian.The size (width) of Planolites is assumed to represent the size of the trace maker.In Newfoundland, the average size of Planolites in the T. pedum Zone is smaller than in the R. avalonensis Zone.Thus, there should have been an increase in the size of Planolites producers near the boundary of the two zones.On the other hand, the size distribution of Planolites in the Zhujiaqing Formation, Yunnan and the Bayan Gol Formation, Gobi-Altai showed that the variously sized animals already appeared in the T. pedum Zone.Besides, the densities of ichnofossils on the bedding planes were low in the T. pedum Zone in Newfoundland, although they became high in the R. avalonensis Zone.The densities in Yunnan and Gobi-Altai, however, were already high in the T. pedum Zone.Therefore, the animal activity became intense in the R. avalonensis Zone in Newfoundland, whereas they had already been intense in the T. pedum Zone in Yunnan and Gobi-Altai.In addition, the occurrences of specific ichnofossils show considerable differences among threes localities.Gyrolithes isp. was commonly observed in iii Newfoundland, but was not observed in Yunnan and Gobi-Altai.Didymaulichnus miettensis frequently occurred in Yunnan and Gobi-Altai, but not in Newfoundland.Therefore, there were geographical differences in the benthic animal activities between Newfoundland and Yunnan -Gobi-Altai.One possible factor of this difference can be based on the differences of the paleolatitude positions of Newfoundland and Yunnan -Gobi-Altai.The earliest Cambrian Newfoundland was located in high-latitude region, whereas Yunnan and Gobi-Altai were in low-latitude regions.Previous paleoclimatic studies show that climate in low latitudinal regions was warm whereas climate in high latitude was cold in the earliest Cambrian.In general, species generation time decrease in high temperature region, and thus biological diversification might become high in low latitude regions.Therefore, variously sized animals with intense biological activities could have firstly originated in low latitude regions in the T. pedum Zone.These animal activities subsequently reached to high latitude regions in the R. avalonensis Zone.

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