A numerical scale for quantifying the quality of preservation of vertebrate tracks

From its beginning, vertebrate ichnology has described fossilized footprints in a qualitative, descriptive way. At the same time, considerable effort has gone into illustrating footprint morphology. In recent years, new technologies (e.g., laser-scanning and close-range photogrammetry) and methods (e.g., geometric morphometrics) have allowed more objective, quantitative approaches to vertebrate ichnol ogy. However, quantitative shape analyses need to be based on data of high quality, and comparisons are best made between tracks comparable in quality of preservation. Thus, determining which footprints constitute the most reliable sample for quantitative analyses is fundamental for the progress of ichnology. We introduce here a numerical scale to quantify the quality of preservation of vertebrate footprints, based on morphological details present in the prints. The numerical grades conceptually arise from theoretical assumptions about the fossil record, and also from experimental observations of footprints of the extant emu (Dromaius novaehollandiae). The numerical scale is ordinal in character, reflecting progressively better preservation of morphological details. Values range from zero (the worst prints, occurring only as aligned depressions) to three (the best preserved prints), plus some coded letters to specify the occurrence of certain morphological or preservational features (e.g., skin impressions). Introducing this numerical scale will facilitate easier and faster comparisons among footprints, reducing the need to refer to drawings or photographs of the traces to assess their quality. This will make possible more accurate evaluation of the track record, with comparisons made only from the best preserved footprints. This scale could potentially be used when creating new ichnotaxa, which should be based on well preserved tracks (such as those with numerical values of at least two, for simple-shaped tracks). Morphometric analyses might likewise be restricted to prints of the same quality, because the results of such analyses are highly dependent on the preservation of the specimens analyzed.