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Ichniotherium  tracks with a relatively short pedal digit V (digit length ratio V/IV < 0.6) form the majority of yet described Late Carboniferous to Early Permian diadectomorph tracks and can be related to a certain diadectid clade with corresponding phalangeal reduction that includes  Diadectes  and its close relatives. Here we document the variation of digit proportions and trackway parameters in 25 trackways (69 step cycles) from nine localities and seven further specimens with incomplete step cycles from the type locality of  Ichniotherium cottae  (Gottlob quarry) in order to find out whether this type of  Ichniotherium  tracks represents a homogeneous group or an assemblage of distinct morphotypes and includes variability indicative for evolutionary change in trackmaker locomotion. According to our results, the largest sample of tracks from three Lower Permian sites of the Thuringian Forest, commonly referred to  I. cottae , is not homogeneous but shows a clear distinction in pace length, pace angulation, apparent trunk length and toe proportions between tracks from Bromacker quarry and those from the stratigraphically older sites Birkheide and Gottlob quarry. Three Late Carboniferous trackways of  Ichniotherium  with relatively short pedal digit V from Haine’s Farm, Ohio, and Alveley near Birmingham, United Kingdom, that have been referred to the ichnotaxa “ Baropus hainesi ,” “ Megabaropus hainesi”  and “ Ichniotherium willsi ,” respectively, share a marked outward rotation of foot imprints with respect to walking direction. Apart from this feature they are in many aspects similar to the Birkheide and Gottlob records of  I. cottae . With the possible exception of the Maroon Formation (Early Permian, Colorado) sample, Early Permian  Ichniotherium  trackways with a relatively short pedal digit V fall into the morphological spectrum of the three well defined “Hainesi–Willsi,” “Birkheide–Gottlob” and “Bromacker” morphotypes. With their more obtuse pace angulations and higher body-size-normalized pace and stride lengths the Bromacker type tracks imply higher walking speeds of their trackmakers compared to all other  Ichniotherium  tracks. More generally, a trend towards higher locomotion capability from the last common ancestor of all  Ichniotherium  producers to the last common ancestor of all “ Ichniotherium  with relatively short pedal digit V” and from the latter to the trackmakers of the mid-Early Permian Bromacker type can be deduced—with the reservation that overall sample size is relatively small, making this scenario a preliminary assessment. Whether the presumed advancements represent a more general pattern within diadectomorphs remains open until the non-European  Ichniotherium  trackway record improves. Ichnotaxonomic implications are discussed.

On the morphological variability ofIchniotheriumtracks and evolution of locomotion in the sistergroup of amniotes