Neither Bones Nor Feet: Track Morphological Variation and ‘Preservation Quality’

As purely sedimentary structures, fossil footprints are all about shape. Correctly interpreting the significance of their surface topography requires understanding the sources of morphological variation. Differences among specimens are most frequently attributed to either taxonomy (trackmaker) or to preservation quality. ‘Well-preserved' tracks are judged more similar to pedal anatomy than ‘poorly preserved' ones, but such broad-brush characterizations confound two separate episodes in a track's history. Current evaluations of track quality fail to distinguish among behavioral, formational, intravolumetric, and postformational sources of variation. On the basis of analogy with body fossils, we recommend restricting assessments of track preservation quality to modifications that take place only after a track is created. Ichnologists need to try to parse the relative influence of factors affecting disparity, but we currently lack an adequate vocabulary to describe the overall shapes and specific features of formational variants.