Neoichnology of mygalomorph spiders: improving the recognition of spider burrows in the geological record

Abstract

The female Mygalomorphae spiders are sedentary and long-lived organisms that spend most of their lives inside their burrows. Neoichnological studies provide relevant information that can help the recognition of these structures in paleosols. Body fossils of spiders are known since the Carboniferous and burrowing is a primitive behavior in Mygalomorphae spiders. However, trace fossils attributable to ground-dwelling spiders are still poorly documented in the geological record. In this work, we examine the burrows and burrowing behavior of Rachias intermedius Soares, 1944 (Araneae: Mygalomorphae: Nemesiidae) in its natural environment and discuss the characteristics that can be used as ichnotaxobases for recognition of fossil spider burrows. Three major architectures, straight shaft with a terminal ovoid chamber, J-shaped winding shaft with the terminal chamber, and Y-shaped with a terminal teardrop-shaped chamber, are described and compared to morphologically similar ichnogenera, like Capayanichnus, Loloichnus, Macanopsis, and Psilonichnus. Differences in burrow shape and architecture are linked with the spider's sex and ontogenetic stage. Pedipalps, chelicerae, and fangs are used for soil excavation, forming a variety of burrow wall ornaments represented by delicate sub-horizontal parallel ridges, irregular knobby micro-relief surface with soil structures attached to the wall, rounded pits, and millimeter-scale vertical striations along the burrow length. A thick inorganic clay lining covers the inner burrow wall, a feature that has not been described for spider burrows yet. These characteristics allow distinguishing spider burrows from burrows produced by other soil-dwelling arthropods. They should be used for spider burrow recognition in paleosols, mainly the millimeter-scale vertical striations that had not been documented before. The data discussed herein improve the knowledge about spider burrowing behavior and the mechanism that play the main role in preserving these burrows' features in the fossil record.