Estimating gaits of an ancient crocodile-line archosaur through 1 trajectory optimization, with comparison to fossil trackways

Abstract

13 Fossil trackways provide a glimpse into the behaviour of extinct animals. However, while providing 14 information of trackmaker size, stride and even speed, the actual gait of the organism can be 15 ambiguous. This is especially true of quadrupedal animals, where disparate gaits can have similar 16 track patterns. Here predictive simulation using trajectory optimization can help distinguish gaits 17 used by trackmakers. First we demonstrate that a planar, five-link quadrupedal biomechanical model 18 can generate the qualitative trackway patterns made by domestic dogs, although a systematic error 19 emerges in track phase (relative distance between ipsilateral pes and manus prints). Next we use 20 trackway dimensions as inputs to a model of Batrachotomus kupferzellensis, a long-limbed, 21 crocodile-line archosaur (clade Pseudosuchia) from the Middle Triassic of Germany. We find 22 energetically optimal gaits and compare their predicted track phase to the track phase of the fossil 23 footprints. The optimal results agree with trackways at slow speeds, but differ at faster speeds. 24 However, all simulations point to a gait transition around a nondimensional speed of 0.4 and another 25 at 1.0. The trackways likewise exhibit stark differences in track phase at these speeds. In all cases, 26 including when simulations are constrained to fossil track phase, the optimal simulations at the first 27 gait transition do not correspond to a trot, as often used by living crocodiles. Instead, they are a 28 diagonal sequence gait similar to the slow tölt of Icelandic horses. This is the first evidence that 29 extinct pseudosuchians may have exhibited different gaits than their modern relatives, and of a gait 30 transition in an extinct pseudosuchian. The results of this analysis highlight areas where the models 31 can be improved to generate more reliable predictions for fossil data, while also showcasing how 32 simple models can generate insights about the behaviour of extinct animals. 33