Invertebrate trace fossils and bioturbation at the lower Permian lagerstätte Bromacker in the Thuringian Forest

We investigate trace fossils and bioturbation degree of two sediment cores extracted from the Tambach Formation and from excavations at the Bromacker locality.

The degree of bioturbation of the investigated silt- and sandstones is rather low in most parts of the sections, averaging at about 8%. Thus, the low degree of bioturbation varies between grade 0 and 2 following the classification of Reineck (1963). Most frequent structures are burrows of 2-38 mm diameter. In general, the dominating small burrows of <5 mm penetrate the bedding planes perpendicular or with a steep angle (Skolithos), whereas larger burrows of a few cm in diameter are mostly horizontal. Burrows are simple, unbranching and show no internal or surface structures. They are characterized by passive infill, similar to (Palaeophycus) or slightly different from the host rock (Planolites).

Some additional trace fossils are preserved as convex hyporeliefs on sandstone slabs. Most abundant is Tambia spiralis (Müller, 1956), which was selected by Paläontologische Gesellschaft as Fossil of the Year in 2024. This trace fossil is characteristic of the Tambach Formation and geographically and stratigraphically almost restricted to it. Less common is the feeding trace fossil Striatichnium bromackerense Martens, 1982. Very rare are trackways and trails of Arthropoda, such as Diplichnites, Diplopodichnus and Lithographus.

The investigated sediments were deposited as fluvial overbank and channel sediments with short transport routes as low rounding degree of grains indicates. Degree and type of bioturbation point to a Scoyenia ichnofacies with strong seasonal effects under a wet-and-dry tropical climate.