Tetrapod swim techniques interpreted from swim trace fossils from the Lower Triassic Baranów Formation, Holy Cross Mountains, central Poland

Abstract

Swimming tetrapods may leave their traces under water if their digits or limbs stir the bottom sediment. Resulting trace fossils are evidence of a swim behavior. Tetrapods swim techniques depend on the functional morphology of the swimmers. Examination of swim trace fossils may reveal the swim techniques employed and swimmers’ functional morphologies behind the behaviors. The present paper analyzes swim trace fossils of tetrapods from the fluvial Lower Triassic Baranów Formation in the Holy Cross Mountains (central Poland). The examination focuses on swim techniques. An attempt is made to correlate the inferred technique with functional morphology of the swimmer. It is concluded that the two types of swim traces occur in the Baranów Formation. These record two different swim techniques—paddling with limbs and body/tail undulation. The distinct types of swim trace fossils point to two types of swimmers: fully terrestrial archosaurs paddling with their limbs and amphibious tetrapods swimming with undulatory movements—likely utilizing their laterally flattened tails.