Ichnological evidence of a horseshoe crab hot-spot in the Early Triassic Buntsandstein continental deposits from the Catalan Pyrenees (NE Iberian Peninsula)

Abstract

Xiphosurids (horseshoe crabs) are aquatic chelicerate arthropods commonly related to marine environments. Although today only four marine species exist, in the geological record they were much more diverse (especially during Carboniferous and Triassic periods), and even inhabited non-marine settings. Here we analyze an exceptional xiphosurid trace-fossil site preserved in the lowermost Buntsandstein red-beds from the Early Triassic of the Catalan Pyrenees. Ichnological and sedimentological analyses were mainly conducted during fieldwork. Fossiliferous surfaces were cleaned and mapped in order to recognize distribution and orientation patterns. Two main ichnotaxa were identified: (1) Selenichnites isp. (>400 trace fossils), consisting of a lunate, anteriorly convex lobe (prosoma imprint) followed by a rounded to triangular impression (opisthosoma imprint) and a posterior straight thin groove (telson imprint); and (2) Kouphichnium isp. (about 70 trace fossils), consisting of long (up to 2 m), often curved, complex series of parallel rows of morphologically variable tracks and imprints. Selenichnites isp., interpreted as resting and/or feeding traces (cubichnia/fodinichnia), were mostly oriented against the current, preventing tracemakers being overturned. Kouphichnium isp. correspond to locomotion traces (repichnia). Trace fossils are found at the uppermost succession of fining- and energy decreasing- upwards fluvial red beds with no marine influence. These ichnofossils are the earliest Triassic evidence of life from the Catalan Pyrenees, and thus shed light on ecosystems composition at the dawn of the Mesozoic era, when biota recovered from the greatest mass extinction in Earth history.