Reinterpretation of palmate and semi-palmate (webbed) fossil tracks; insights from finite element modelling

A track from the Late Cretaceous previously described as being generated by a semi-palmate bird was studied with the aid of high resolution laser scanning. Substrate conditions at the time of track formation were diagnosed (fine-grained, soft, waterlogged sediment) and used to constrain a finite element track simulator. The indentation of a non-webbed virtual tridactyl foot in such conditions created a resultant track with features analogous to ‘webbing’ between digits. This ‘webbing’ was a function of sediment deformation and subsequent failure in 3D, specific to rheology. Variation of substrate conditions and interdigital angle was incrementally stepped. Apparent webbing impressions were clearly developed only within a limited range of sediment conditions and pedal geometry.The implications of this work are that descriptions of ‘webbed’ tracks should account for the possibility that webbing was indirectly formed through sediment failure and not necessarily the direct impression of a webbed foot. Additionally, dating the earliest occurrence of webbed feet in the fossil record, and potentially extending phylogenetic ranges, should be treated with caution when based upon evidence from tracks.