Gyrolithes as a multipurpose burrow: an ethologic approach

Abstract

Spiral burrows perpendicular to the bedding plane are well known in the fossil record and are usually referred to the ichnogenus Gyrolithes. Common in Mesozoic and Cenozoic marginal-marine, brackish-water deposits, the only records of Gyrolithes in Paleozoic rocks are from the Cambrian. In Permian rocks of the Parana Basin (Rio do Sul and Rio Bonito formations), south Brazil, some short, irregularly spiraled burrows similar to Gyrolithes occur in marginal- marine, brackish-water siltstone and mudstone, closely associated with Thalassinoides, forming a Glossifungites suite. These burrows seem to play the same ethologic role of the corkscrew-shaped burrows that grade into modern Thalassinoides burrow systems. The corkscrew shape of Gyrolithes is assumed to represent a widespread architectural adaptation to deal with extreme salinity fluctuations in Mesozoic and Cenozoic brackish-water environments. However, spirals produced by thalassinidean shrimps in brackish water connected to modern Thalassinoides burrow systems are related to feeding activities. Instead of behavior to avoid extreme salinity fluctuations, the decapod spiral burrows may represent a feeding strategy to exploit organic-rich zones of the substrate in calm and protected environments. The discovery of spiral burrows in Permian marginal-marine stiffgrounds to firmgrounds indicates that this strategy was already adopted by some group of infaunal crustaceans in brackish-water ecosystems during the late Paleozoic.