Biofilm harvesters in coastal settings of the early Palaeozoic

The ichnogenera Syringomorpha and Daedalus are here interpreted as products of infaunal biofilm harvesters. This study investigated: (1) Syringomorpha nilssoni and Syringomorpha isp. from the Cambrian Series 2‐Miaolingian Campanario Formation, northwest Argentina; and (2) Daedalus halli from the Floian Grès et Schistes de la Cluse de l’Orb Formation, Montagne Noire, France. Syringomorpha nilssoni occurs in sandy to mixed intertidal to lower shoreface deposits, whereas Syringomorpha isp. in the lower intertidal zone. Daedalus halli occurs in a lagoon and intertidal to lower shoreface sands of a barrier island. Syringomorpha and Daedalus comprise a vertical J‐shaped causative burrow and deep spreite. These ichnotaxa form monospecific assemblages (bioturbation index BI = 3–5) in quartzose medium‐ to fine‐grained sandstone, recording colonization in high‐energy tide‐and wave‐dominated settings. Lower abundances (BI = 1–2) are observed in silty sandstone. The abundance of both ichnogenera in mature sandstone is inconsistent with a classic deposit‐feeding strategy because ‘clean’ sediments are commonly impoverished of organic detritus, this being particularly true in Cambro‐ Ordovician littoral settings lacking terrestrial plant detritus. Based on morphology, host sediment properties and comparison with modern structures, such those produced on intertidal and shallow subtidal setting by Arenicola marina and Paraonis fulgens , it is suggested that the diet of Syringomorpha and Daedalus producers may have consisted of biofilms colonising sand grains, associated eukaryotic microbes, and possibly meio- fauna. Whereas Syringomorpha is a product of the Cambrian explosion, Daedalus is associated with the Ordovician Radiation. In contrast to most ichnotaxa, which dis- play long temporal ranges, these two ichnogenera are restricted to the Cambrian and Ordovician‐Silurian, respectively. The underlying reasons for the relatively restricted stratigraphic ranges of these ichnotaxa are unclear, but space competition, and increased predation pressure may have played a role. The feeding strategy of the Daedalus and Syringomorpha producers was less efficient than suspension feeding and passive predation, trophic types epitomized by the dominant macroinfauna that persisted in water‐agitated nearshore sands during the rest of the Phanerozoic. □ Biofilm harvesters, Cambrian explosion, meiofaunal hunters, Ordovician Radiation, trace fossils.