Evidence from fossil tetrapod tracks of gregarious behavior, ichnopathologies, and locomotion

Vertebrate tracks provide evidence of behavior, locomotion, and paleoecology. We discuss the significance of tracks for providing evidence for gregariousness and other behavior, determining speed, assessing gait, and recognizing pathologies. The majority of the work to date on these topics pertains to dinosaurs. There are two types of tracksites related to gregarious behavior: (1) sites where several individuals moved across an area in subparallel alignment, which we term transgressive sites; and (2) where tracks of multiple individuals are concentrated at a location, which we term congregation sites. Factors to be considered in evaluation of evidence for gregarious behavior include: (1) ichnotaxonomy; (2) bedding plane; (3) time averaging; (4) subparallel orientation; (5) directional barriers; (6) ecological attractors; (7) intertrack spacing; (8) age structure; (9) speed and directional change; and (10) topography. We recognize two scales of congregation sites; (1) microcongregation sites (up to tens or hundreds of square meters), which are concentrations of the same ichnotaxon focused on a discrete and often recognizable locus; and (2) megacongregation sites are large tracksites (up to hundreds or thousands of square meters) with multiple trackways in which few, if any, demonstrate an obvious preferred orientation with no clear cause for the concentration. The first good track evidence for complex gregarious behavior in tetrapods is from the Early-Middle Triassic, but we expect that it may remain to be recognized in Permian tracksites. Paleopathologies have been recognized from fossil trackways on the basis of: (1) irregular gait or (2) pathologic foot impressions. Most reports to date are from dinosaurs or associated tracks. Consistent irregularities in pace and stride values are defined as irregular gaits, whereas limping gaits are the more extreme examples that are more likely to be cases of ichnopathology. The majority of reported ichnopathologies and putative ichnopathologies pertain to bipedal, tridactyl trackmakers, principally theropods. The narrow digits and long stride of theropods facilitate both the recognition of abnormal foot structure and of irregular pacing. Characteristics of trackways reflect the gait of the track maker and thus have great potential in the study of the evolution of locomotion. However, gait analysis has received relatively little attention. Formulae for calculating speed from tetrapod trackways have been very useful, but they have challenges, notably those related to the nature of footprint preservation and of calculating dimensions. Other paleoecological or locomotory evidence from trackways includes: (1) traces produced by swimming or underwater walking, principally in the Mesozoic and mainly relating to dinosaurs, crocodylomorphs, and turtles; (2) turning trackways; (3) associated body impressions; (4) crouching dinosaurs; and (5) tail drag traces.