Abstract

In 2010, a siltstone slab containing three arthropod trackways was discovered in the Middle Jurassic Dansirit Formation in the Ruyan region, central Alborz Mountains, northern Iran. Quality of preservation is varied, but the best-preserved track sequences consist of a chevron-shaped pair of outer tracks surrounding two pairs of inner tracks. These trace fossils are named herein as a new ichnogenus and ichnospecies, Porpaichnus dansiritensis. The depositional environment of the track-bearing stratum is enigmatic. If the sediment is of littoral origin, the footprints were likely made by a decapod crustacean. For a terrestrial deposit, the imprint morphology is possible evidence of an arachnid that used its pedipalps to supplement four pairs of walking legs. These morphological characteristics suggest the track-maker is may have been an early member of the Infraorder Mygalomorpha. If so, the 170 Ma trackways are the earliest known evidence of a tarantula-like spider. Although the oldest known tarantula fossils date to the Miocene, fossil evidence of mygalomorph spiders dates to the Cretaceous, and molecular clock evidence suggests that divergence of spider families occurred in the late Paleozoic.