Possibly the oldest fish-made resting traces

Abstract

The Silurian Sharawra Member (middle to late Llandoverian) in Saudi Arabia exhibits unique surface traces with exceptional preservation: Seven small, wing-shaped traces occur on a sandstone slab showing well-defined hummocky cross-stratification having a few decimeters wavelength being covered by a thin mud layer. These traces are preserved as concave epirelief along with elongated internal striae and surrounded by slightly elevated levees. Preservation of these delicate, 10-20 mm long and similar to 10 mm wide traces suggests that they were formed after a storm event when mud had already settled to the sediment surface. The lack of wave-generated sedimentary structures implies deposition between fair-weather and storm-wave base probably preventing further rapid reworking by waves. Based on their geometry, internal pattern, and spatial distribution, these wing-shaped traces are interpreted to have been produced by relatively stationary bottom-feeding fish, morphologically similar to ancestors of Gobiidae, probably in the Actinopterygii class. The majority of fish-made trace fossils record feeding and locomotion behaviour, whereas the wing-shaped traces are interpreted to represent resting structures made by fish that pressed their fins downward on the muddy sediment surface and hence, represent undertracks. These traces are possibly the oldest fish-made structure documented thus far. These wing-shaped traces are described as Pisquiesichnus dashtgardi igen. et isp. nov. in this study.