Skolithos serratus in paleosols: Paleobiological, paleoecological, and paleobiogeographical insights

Abstract

The ichnogenus Skolithos is are recorded from continental to deep marine deposits and the simplicity of the burrows and the variety of potential burrowers make it challenging to identify a probable tracemaker and limit paleoenvironmental interpretations. The paleosols of the Triassic Caturrita Formation from southern Brazil present non-marine Skolithos piperocks dominated by Skolithos serratus with scratch marks arranged perpendicular to the axis of the tube on the inner wall of the burrow, giving the burrow the shape of a screw. The fossil finding provides crucial paleoenvironmental information about the texture and moisture conditions of the substrate and the producing organism. The micromorphological analysis and characterization of the dominant burrows and paleosols of Skolithos piperock allowed assignation to the ichnospecies Skolithos serratus and attribution of this ichnotaxon to the activity of tiger beetles (Coleoptera: Carabidae: Cicindelidae) which have morphological characters necessary to excavate a burrow with striated walls similar to a screw in response of the activity of the larva in the burrow throughout their life. The burrows represent a multifunctional structure that serves simultaneously as a domicile and a hiding place from predation and feeding for the larvae (pupa protection). The predominance of S. serratus in the Skolithos piperock suggests a preference of the producers for exposed, firm and predominantly sandy textured soils in little vegetated flood plains in a semiarid climate, similar to modern inceptisols developed in desiccated-overbank deposits of the Scoyenia ichnofacies. The presence of S. serratus in the studied paleosols suggests that the southwest of Gondwana has been a key center of diversification and dispersion of the subgenera Cylindera and Plectographa since the Late Triassic. Such center would include the geographic region bounded by the southern states of Brazil, northern Argentina, Uruguay and eastern Paraguay.