Arthropod tracemakers of Nereites? Neoichnological observations of juvenile limulids and their palaeoichnological application

The paleoichnologic record has provided numerous opportunities for a sort of reverse uniformitarianism in that some trace fossils have prompted interest in finding their modern analogs and their tracemakers. This is particularly the case for trace fossils associated with the Zoophycos and Nereites ichnofacies. Nereites is a commonly reported ichnogenus, yet its makers and manner of production remain obscure despite a consensus view that the tracemaker was a worm. To get the most out of the paleoichnological record, the behavior and resultant traces of extant animals must be studied carefully, even if this upsets the long-established ideas about the makers of trace fossils. Observations of modern juvenile limulids (Limulus polyphemus) show that they make modern traces similar to the ichnogenus Nereites on sandy tidal flats of Sapelo Island (Georgia, USA)-with the understanding that ichnogenera are morphologically based groups of trace fossils with no implication as to the maker. The results presented in this chapter confirms earlier work that suggests various arthropod makers as the makers of some Paleozoic Nereites-while throwing into question the established interpretation of Nereites as a burrow of vermiform tracemakers. Accordingly, Nereites-like traces should be reinvestigated on a case-by-case basis with regard to their characteristic bioprint, that is, the set of morphologic features that allows the identification of the maker of a trace.