Raederichnus dondasi a new trace fossil from the early Paleozoic of Argentina reveals shoaling behavior in early fish

Although vertebrate carcasses––particularly those assigned to fishes––are abundant in the fossil record, the literature rarely mentions fishes as trace producers. Herein we present evidence that was possibly overlooked in previous studies. Study of more than 100 large, shallow, teardrop-shaped, imprints that are aligned, with few overlapping each other, from early Paleozoic (probably Silurian based on trace fossils) outcrops in western Gondwana (southeastern Argentina), are identified as the oldest example of shoaling behavior. To understand the nature of the behavior, we considered analog (vertebrate and invertebrate) extant and extinct taxa. We compare and discuss the superficial similarities with certain traces, in particular Selenichnites isp. and erect a new ichnotaxon, Raederichnus dondasi isp. nov. for the Argentinian material. We report for the first time from the Balcarce Formation Psammichnites isp., in association with Herradurichnus scagliai, both forming minor components of the ichnoassemblage. Raederichnus dondasi and the accompanying traces are preserved in three-dimensional dunes, developed in a tide-dominated shallow marine environment. Given the morphological resemblance, and paleoecological context, we consider that Raederichnus dondasi might have been produced by fish while “resting”. Finally, this aggregation of trace fossils suggests shoaling related to feeding or breeding on the shallow marine bottom surface.