An early Cambrian shallow-marine ichnofauna from the Puncoviscana Formation of northwest Argentina: the interplay between sophisticated feeding behaviors, matgrounds and sea-level changes

Abstract

An early Cambrian ichnofauna consisting ofHelminthoidichnites tenuis,Helminthopsis tenuis,Multinaisp.,Oldhamia alata, andPilichnuscf.dichotomusis documented from shallow-marine deposits ranging from the upper offshore to the offshore transition in the Puncoviscana Formation of northwest Argentina. Although the ichnogenusOldhamiais more common in Cambrian deep-marine environments, this occurrence provides further evidence that it is also present in shallow-marine environments. The burrow networkMultina(senior synonym ofOlenichnus) 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 minersOldhamia alataandPilichnuscf.P. dichotomusoccur 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 byOldamia alataandPilichnuscf.dichotomus, is a product of the Cambrian explosion.