The earliest evidence of true crabs? Insights on the evolution of Brachyura from an exceptional exposure of Carnian Psilonichnus (Upper Triassic, Braies, Dolomites, Italy)

A new exceptionally exposed ichnofauna is reported from the Upper Triassic Travenanzes Formation of Braies in the Dolomites (Italy). The ichnosite is extraordinary in that it provides a continuous exposure of 32,000 m2 of intensely bioturbated dolostone. The bioturbated surface is characterized by vertical burrows (Psilonichnus upsilon) occurring at a density of 5 openings / m2. With more than 160,000 burrow openings, the Braies ichnosite preserves the largest exposure ever described for Psilonichnus. The Psilonichnus-bearing horizons represent a carbonate tidal flat that bordered an arid coastal plain, as evidenced by facies analysis and confirmed by the presence of Taenidium barretti. Comparison with modern ocypodid burrows indicates that the Psilonichnus from Braies are compatible with a crab tracemaker (Decapoda: Brachyura), which would represent the earliest fossil evidence for true crabs, although other arthropods cannot be fully discounted as the producers.