Ethological interpretation of the ichnogenus Pragichnus Chlupáč, 1987 (Ordovician, Czech Republic)

The deep, root-like burrow Pragichnus fascis is probably an agrichnion made for obtaining methane and hydrogen sulphide to farm endosymbiotic bacteria. It occurred in settings of shifting sandy substrates overlying black clay or silt. Size, morphology and the presumed ethological function place it in the Chondrites-Zoophycos ichnoguild. Dissolved gas was probably obtained by active “pumping”, because disused parts of the structure were actively back-filled. Densest populations of Pragichnus probably correspond to periods of maximum degassing of the underlying fine-grained, organic-rich material. In some places, the aim of the animal was probably to reach the black clay directly by the terminal parts of its probes.