Insights from functional morphology and neoichnology for determining tracemakers: a case study of the reconstruction of an ancient glacial arthropod-dominated fauna

Apportioning trace fossils to producers is significant for palaeoecological and palaeoenvironmental studies. A tracemaker is usually attributed through the comparison of the morphology of the trace to the body plans of modern and extinct animals. However,hypotheses of tracemaker identifications may be tested with the use of further methods. This contribution aims to evaluate the potential tracemakers of a trackway-dominated ichnofauna preserved in the siltstone-claystone rhythmites of the Pennsylvanian Rio do Sul Formation in the southern Brazil. These deposits are typical of the deglaciation phase of the Late Paleozoic Ice Age of Gondwana in the Paraná Basin. Thus, the parameters applied in this work are: (1) functional analyses of the traces to infer the biomechanics of the locomotion of the producer and (2) neoichnology, whereby modern traces whose producers are known are compared to those of the ichnofaunain question. The conclusions obtained with these methods strongly support millipedes as the producers of Diplichnitesgouldi and Diplopodichnus biformis. The malacostracan crustacean-related ichnogenera are Umfolozia sinuosa andGluckstadtella elongata, produced by syncarids or peracarids, while isopods are suggested as thetracemakers of Glaciichnium isp.and Protovirgularia dichotoma. In terms of branchiopodcrustaceans, Cruzianaproblematica is attributed to notostracans. Helminthoidichnitestenuis, Mermia carickensis and Treptichnuspollardi are considered traces produced by insect larvae. The palaeobiology of the tracemakers that produced the ichnofauna preserved in the rhythmites of the Rio do Sul Formation permits inferences that terrestrial (millipedes) and aquatic (crustaceans and insect larvae)animals colonized the sediments. This conclusion points to a dynamic depositional environment, with changing physical and chemical parameters conditioning the biota.