The enigmatic trace fossil Tisoa de Serres, 1840

A new attempt is made to shed light on the enigmatic trace fossil Tisoa that was introduced from the Lower Jurassic of France about 180 years ago. Morphological variability and diagenetic overprint were the reasons for the erection of numerous ichnotaxa now regarded as synonymous with Tisoa. Tisoa is an ichnogenus of the Arenicolitidae and comprises simple, vertical to horizontal, narrow U-shaped burrows with paired tubes and a very high length/width ratio. Two ichnospecies are distinguished: T. siphonalis, the type ichnospecies, occurs in firmground and has a sharply defined burrow margin and passive fill (Glossifungites Ichnofacies), while T. habichi comb. nov. occurs in softground and has an irregular outline and active or passive fill (Skolithos Ichnofacies). Tisoa is known worldwide from the Lower Ordovician and occurs in a wide range of marine environments from paralic settings to the deep sea, based on 63 evaluated records from the literature. Tisoa and Bathichnus reach a length of up to 9 m and thus are the deepest invertebrate burrows recorded so far. Tisoa is often associated with major stratigraphical surfaces such as omission and erosion surfaces. T. siphonalis commonly occurs in organic-rich and oxygen-poor substrate and in association with cold seep deposits. There, the extremely long burrows acted as venting conduits, which led to the precipitation of minerals in form of concretions and obliteration of the enclosed burrow. T. habichi is known from sandy, well-oxygenated substrates. Polychaetes (bristle worms) are good candidates for producing Tisoa, partly being able to employ chemosynthetic symbiosis to gain nutrition and able to burrow to extraordinary depths. This review aims to raise awareness of Tisoa, which is more common than previously anticipated, and to utilize its presence in palaeoenvironmental reconstructions. It demonstrates once again the importance of a solid ichnotaxonomical evaluation as basis for further interpretations.