Transition between terrestrial-submerged walking and swimming revealed by Early Permian amphibian trackways and a new proposal for the nomenclature of compound trace fossils

Exceptionally preserved Early Permian tetrapod trackways from the Orobic Basin (Central-Western Southern Alps) offer a unique opportunity to investigate in detail locomotion in fossil vertebrates that lived on continental European landmasses. Herein are reported the results of a study on several tetrapod trackways that display a large variety of behavioral, gait and substrate related extramorphologies. They clearly document the transition from terrestrial-underwater walking to swimming and are assigned to the compound ichnotaxon Batrachichnus C Lunichnium. The use of the "C" symbol is here introduced for the first time as nomenclatural indication of a Compound trace. Producers were probably small-sized temnospondyl or lepospondyl (microsaurs) amphibians. Comparisons with living urodelan anatomy and mechanics provide evidence for conservatism of locomotor mechanics in evolutionary history among amphibians. The derived model for locomotor kinematics in Early Permian amphibians provides a reference for interpreting transitional land-to-water trackways. The shift from walking to swimming behavior in early tetrapods, as in extant urodelan amphibians, is described as a complex balance between different dynamics.