Abstract

An early Cambrian ichnofauna consisting of Helminthoidichnites tenuis, Helminthopsis tenuis, Multina isp., Oldhamia alata, and Pilichnus cf. dichotomus is documented from shallow-marine deposits ranging from the upper offshore to the offshore transition in the Puncoviscana Formation of northwest Argentina. Although the ichnogenus Oldhamia is more common in Cambrian deep-marine environments, this occurrence provides further evidence that it is also present in shallow-marine environments. The burrow network Multina (senior synonym of Olenichnus) is preserved at the base of tempestites, representing the activity of post-storm colonizers. A drowning surface separating offshore-transition deposits below from upper-offshore deposits above contains widespread evidence of trace fossils in direct association with matgrounds. The undermat miners Oldhamia alata and Pilichnus cf. P. dichotomus occur on this surface, revealing exploitation of organic matter in the biomat. Low sediment rate during drowning and paucity of bioturbation by sediment bulldozers may have promoted the establishment of the matground. In comparison with the simpler animal-matground interactions characteristic of the Ediacaran, the combination of Cambrian evolutionary innovations and the presence of microbial mats promoted more sophisticated interactions. Complex feeding trace fossils revealing that systematic undermat mining, as displayed by Oldamia alata and Pilichnus cf. dichotomus, is a product of the Cambrian explosion.

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