Reconstructing the locomotion of extinct vertebrates offers insights into their palaeobiology and helps to conceptualize major transitions in vertebrate evolution1,2,3,4. However, estimating the locomotor behaviour of a fossil species remains a challenge because of the limited information preserved and the lack of a direct correspondence between form and function5,6. The evolution of advanced locomotion on land—that is, locomotion that is more erect, balanced and mechanically power-saving than is assumed of anamniote early tetrapods—has previously been linked to the terrestrialization and diversification of amniote lineages7. To our knowledge, no reconstructions of the locomotor characteristics of stem amniotes based on multiple quantitative methods have previously been attempted: previous methods have relied on anatomical features alone, ambiguous locomotor information preserved in ichnofossils or unspecific modelling of locomotor dynamics. Here we quantitatively examine plausible gaits of the stem amniote Orobates pabsti, a species that is known from a complete body fossil preserved in association with trackways8. We reconstruct likely gaits that match the footprints, and investigate whether Orobates exhibited locomotor characteristics that have previously been linked to the diversification of crown amniotes. Our integrative methodology uses constraints derived from biomechanically relevant metrics, which also apply to extant tetrapods. The framework uses in vivo assessment of locomotor mechanics in four extant species to guide an anatomically informed kinematic simulation of Orobates, as well as dynamic simulations and robotics to filter the parameter space for plausible gaits. The approach was validated using two extant species that have different morphologies, gaits and footprints. Our metrics indicate that Orobates exhibited more advanced locomotion than has previously been assumed for earlier tetrapods7,9, which suggests that advanced terrestrial locomotion preceded the diversification of crown amniotes. We provide an accompanying website for the exploration of the filters that constrain our simulations, which will allow revision of our approach using new data, assumptions or methods.
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Orobates gait solutions can be explored on the interactive website: (https://go.epfl.ch/Orobates; Supplementary Video 2 provides guidance for using the website). Data for Fig. 2 and Extended Data Figs. 3, 4, are provided in Supplementary Data 12 and 13, respectively. X-ray videos are available from the Jena Collection of X-Ray Movies database (https://szeb.thulb.uni-jena.de) upon request. The digital marionette of Orobates and any other data are available from the corresponding author upon reasonable request.
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We thank R. Petersohn, I. Weiß, S. Curth, B. Weißflog, J. Stampe, R. Häfner, U. Feiler-Kress, O. Demuth, S. Clemens, T. Blochberger and M. Krüger for support during data acquisition and analysis of extant animals; and N. Schilling, T. Martens, A. Henrici, S. Sumida and D. Berman for input during the conceptualization and feedback at various stages of the project. The project received funding from the Volkswagen Foundation (AZ 90222 to J.A.N. and M.S.F.) and the Daimler and Benz Foundation (32-08/12 to J.A.N.). J.A.N. was also supported by the German Research Council (DFG EXC 1027). T.H. and K.M. were supported by the Swiss National Science Foundation through the NCCR Robotics. J.R.H. was supported by the UK Natural Environment Research Council (NE/K004751/1).
Nature thanks S. Gatesy, S. E. Pierce and the other anonymous reviewer(s) for their contribution to the peer review of this work.