Abstract

We introduce new terminology and a new classification scheme for describing tetrapod tail traces, focusing on the interpretation of dinosaur tail traces. Our classification divides tail traces into (1) tail impressions—there is no evidence of forward motion; (2) protracted tail traces—they persist for at least one stride length; and (3) abbreviated tail traces—they persist for less than one stride length. Protracted tail traces are simple or compound, based on the amount of interruption of the tail trace, which we quantify by the percent interruption metric (PIM). Abbreviated tail traces are also simple or compound. Classifications are modified further by sinuosity, which we describe as low or high. The PIM approximates vertical tail motion, and sinuosity approximates lateral tail motion. Sediment variations, preservation, and lateral motion resulting from locomotion must be taken into consideration when interpreting tail traces. This new classification scheme is applied to a partial theropod trackway with associated tail trace from the Upper Jurassic Morrison Formation, Bighorn Basin, Wyoming, United States. The tail trace is protracted and simple with low sinuosity and a low PIM; we interpret this as the result of relatively low tail motion. We hypothesize that significant differences exist between ornithopod and theropod tail trace patterns. We also suggest that protracted tail traces associated with bipedal dinosaur trackways are not the result of the use of the tail as a stabilizing third leg; some may represent incidental contact of the sediment by the tail owing to backward rotation about the pelvis during deceleration.

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