Reconstructing early Phanerozoic intertidal ecosystem: ichnology of the Cambrian Campanario Formation in northwest Argentina

The Campanario Formation is the middle unit of the upper Lower to Middle Cambrian Mesón Group of northwest Argentina. This formation is interpreted as having accumulated in macrotidal shallow-marine environments with extensive tidal-flat areas flanked seawards by subtidal sandbar complexes. Shallow subtidal and intertidal sand-flat deposits are dominated by vertical domiciles of suspension feeders and passive predators of the Skolithos ichnofacies (Skolithos, Arenicolites, Diplocraterion). The ichnogenus Syringomorpha also occurs in sand-flat facies, commonly forming high-density, monospecific assemblages. Mixed-flat deposits contain horizontal feeding, locomotion and resting traces as diagnostic components. A relatively low-diversity Cruziana ichnofacies is present in these lower-energy deposits. This ichnofauna includes presumed trilobite trace fossils (Cruziana, Rusophycus, Diplichnites), structures produced by sessile cnidarians (Bergaueria cf. B. perata) and shallow burrows and trails of vermiform organisms (Planolites, Palaeophycus, Helminthoidichnites). Tiering in these tidal-flat deposits is relatively simple. Six ichnoguilds (Cruziana problematica, Palaeophycus, Bergaueria, Rusophycus leifeirikssoni, Syringomorpha and Skolithos) have been defined. These ichnoguilds show a preferential palaeoenvironmental distribution following proximal-distal trends. Modern tidal flats are characterised by an abundant food supply derived from multiple sources, including nutrients brought in by the sea, terrestrially derived organic detritus and autochthonous food production (e.g. primary production, faecal pellets). The inhabitants of modern intertidal areas are exposed to a double set of predators. During submergence they are preyed on by marine organisms and during emergence they are visited by enemies from the land and air. Contrastingly, the Cambrian intertidal environments functioned as refugia in the absence of continental (i.e. air and land) predators. Despite the physical stress, Cambrian tidal flats must have been protected areas where abundant food was available at almost no risk. Due to the paucity of land vegetation (and land-derived detritus), Cambrian intertidal trophic webs were almost entirely based on the organically rich marine source and most likely a significant autochthonous production. Although the picture that emerges from the Cambrian is qualitatively different, Cambrian tidal flats may have resembled modern ones in their ecological role as sites of reproduction and protection. The depth and extent of bioturbation reveal colonisation of a relatively deep infaunal ecospace by coelomate metazoans. Arthropod incursions in Cambrian tidal flats support an early colonisation of intertidal environments and indicate that representatives of the Cambrian evolutionary fauna were able to colonise very shallow-water environments, thus suggesting a significant landward expansion of the Cambrian explosion and the "Agronomic Revolution".