Linking burrow morphology to the behaviors of predatory soil arthropods: Applications to continental ichnofossils

Predatory arthropods are known from terrestrial environments since the Silurian. Many of these animals have developed morphological and behavioral adaptations for living within soil environments. Ichnofossils are common in Paleozoic paleosols, yet most are of uncertain origin and may record a hidden diversity of predatory arthropods. These ichnofossils are especially important given the relatively poor preservation potential of soil invertebrates in the environments they inhabit. To better understand the morphology and uses of predatory soil arthropod burrows, laboratory experiments were conducted with centipedes, scorpions, whip scorpions, and spiders. Specimens were placed in sediment-filled terrariums for 1 to 36 weeks. The animals were observed continuously using digital recordings to monitor their behaviors and use of their burrows. Open burrows were cast and described qualitatively and quantitatively. The animals burrowed using various techniques including intrusion, compression, excavation, and backfilling. Some burrows were occupied for short intervals (2-5 days) before being abandoned, whereas others were permanently occupied. Burrows ranged from simple vertical shafts to complex, branching networks that served as temporary to permanent dwellings, and most were used as sites for ambush predation or as prey traps. The different predatory arthropods produced unique burrows that could be linked to specific behaviors. Distinct burrow features were linked to predatory activities including vertical shafts, multiple surface openings, branching tunnel networks, and expanded chambers. These data can be applied to continental ichnofossils to improve our understanding of the evolution of terrestrial predatory arthropods, their distribution through time, and interpretations of the paleoecology of ancient soil ecosystems.