have argued that Serpentichnus traces were not actually formed by animals. These sources argue that the grooves were "tool marks", meaning that they were formed by rocks or vegetation brushing against the riverbed while being carried by a current. References General references Carroll, RL (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co. p. 180 von Zittel, K.A (1932), Textbook of Paleontology, C.R. Eastman (transl. and ed), 2nd edition, vol.2, p. 225-6, Macmillan & Co. External links Lysorophia - Palaeos Lysorophia - phylogeny Carboniferous amphibians of North America Permian amphibians of North America Pennsylvanian animals of North America Cisuralian animals of North America Pennsylvanian first appearances Cisuralian extinctions