Large horseshoe crab traces from the Early Triassic Nanlinghu Formation, China

Abstract Large trace fossils are commonly attributed to vertebrates; they tend to be larger animals and, during the Early Triassic of the Nanlinghu Formation, were an important component of the Chaohusaurus Fauna. The analyzed traces, preserved on two marlstone bedding planes, comprise complex, curved to straight trails and trackways characterized by parallel, discontinuous to continuous scratches often associated with granular sediment deposits. Morphometric analysis reveals significant sizes, with trail widths ranging from 3.10 to 29.78 cm and lengths from 50 cm to 5.2 m. Comparative ichnological analysis indicates not a vertebrate tracemaker but an invertebrate, probably an aquatic arthropod. The presented traces do not conform precisely to any established ichnotaxon, although they share closest morphological similarities with swimming traces attributed to limulids ( Kouphichnium variabilis (Linck, 1949)) and the arthropod scratch trace Monomorphichnus biserialis Mikuláš, 1995. Tracemaker identity, inferred from trace morphology and size estimates based on trackway dimensions, points to large limulids (Xiphosura) with calculated total body lengths potentially exceeding 70 cm, representing one of the largest Triassic horseshoe crabs recorded. The morphology of the traces is interpreted as evidence of near-bottom, possibly dorsal-up, swimming or subaqueous locomotion in a marine depositional setting. This discovery adds a new component to the complex trophic structure of the well-known Chaohu Fauna and provides significant insight into the behavior and gigantism of marine arthropods during the Early Triassic.