Reliability of digit length impression as a character of tetrapod ichnotaxobase. Considerations from the Carboniferous–Permian ichnogenusIchniotherium

Although the relative length of digit impressions is often adopted as a binding character in ichnotaxa discrimination and trackmaker identification, it cannot be considered unequivocally operative. The reliability of this character has been evaluated by means of principal component analysis on several well-preserved footprints (both manus and pes) referred to as Ichniotherium sphaerodactylum from the Early Permian Tambach Formation (Thuringia, Germany). The analysis reveals a substantial variability of digit impression length (especially in the manus), relativizing the validity of digit IV length as a diagnostic character as practised in different ichnological studies. Regarding the use of such a character, results clearly suggest the need to preliminarily conduct explorative analysis on the whole available ichnological material in order to detect which digit is the most consistent from an ichnotaxonomical standpoint, thus avoiding an arbitrary selection. Such a preliminary analysis should provide a better control on ichnotaxonomical splitting/lumping, particularly in the case of allied footprints, and can also be considered a valuable tool, if associated to functionality analysis, in improving and refining trackmaker identification