HORSESHOE CRAB (XIPHOSAURA) TRACES CRESCENTICHNUS AND SELENICHNITES FROM THE LATE CRETACEOUS DAKOTA GROUP AT DINOSAUR RIDGE, COLORADO, USA

We present new evidence of Horseshoe Crab (Chelicerata: Xiphosaura) activity in the Late Cretaceous Dakota Group at Dinosaur Ridge, Colorado. The trace fossils (Crescentichnus and Selenichnites) were discovered in upper intertidal to lower supratidal deposits of the upper Muddy Formation. Utilizing close-range 3D photogrammetry, we accurately recorded the characteristic crescent-shaped and semicircular morphologies of Crescentichnus and Selenichnites, respectively. Additional minor surficial features were also observed and include pes and “pusher” imprints, and genal spine and telson drag marks. Notably, these trace fossils are preserved within fossil epibenthic microbial mats that once were truncated by erosional pockets. This preservation pattern strongly suggests the involvement of microbial activity— specifically biostabilization— which facilitates the early precipitation of cements and enhances substrate stability. Biostabilization is affected by sticky extracellular polymeric substances secreted by microbial organisms. The coherent nature of the extracellular polymeric substances leads to ductile behavior of the otherwise clastic sediment. Furthermore, the Horseshoe Crab tracks occur alongside both subaerial and subaquatic marine trace fossil assemblages which record a mixed Skolithos Ichnofacies syndepositionally overprinted by a Charirichnium-Magnoavipes ichnocoenosis. This study expands our existing knowledge of the paleoichnologic data recorded at Dinosaur Ridge and enhances our understanding of xiphosuran traces in North America during the Cretaceous period.