The trace fossil Tisoa siphonalis in its type area; characteristics and environmental significance

The type material of Tisoa siphonalisde Serres, 1840 is lost but nearly complete specimens occur at the herein designated type locality Hameau de Valz (Department of Gard, France), where Tisoa is present in dark Pliensbachian organic-rich mudrock. The upper part of the trace is encased in a concretion. The concretion's carbonate content implies 65% initial porosity, a value typical of soft mud. Such a consistency is also indicated by shallowly produced biodeformational structures that dominate the fabric; in association with small traces they are diagnostic of low-oxygenated bottom water and anoxic conditions just below the seafloor. At its type locality, Tisoa represents a nearly vertical protrusive spreite burrow, exceeding 2 m in length and having nearly parallel limbs 0.1-1.5 cm apart. The spreite is only weakly expressed; the inter-limb material was apparently not processed during deepening of the U-tube but placed directly in the spreite or pressed aside. Tisoa deviates slightly but consistently from a vertical orientation and commonly shows a low-amplitude helicoidal course. The U-tube exhibits a thick pyrite lining implying the former presence of mucus. The steep chemical gradient between oxygenated water in the U-tube and anoxic host sediment evidently fostered microbial activity. The extraordinary penetration depth of Tisoa suggests exploitation of extreme redox conditions such as could be found in the methanogenesis zone. The Tisoa producer probably deepened the U-tube incrementally, continuing when the chemical gradient between tube and host sediment declined due to circulation of the oxygenated water used for respiration. The Tisoa producers might have fed on suspended material, microbes flourishing along the tube wall, or acquired nutrition via chemosymbionts.