Defining the morphological quality of fossil footprints. Problems and principles of preservation in tetrapod ichnology with examples from the Palaeozoic to the present

1. What they did

Fossil footprints are named and classified by their shape, but the shape of a track depends on four things: the anatomy of the foot, how the animal moved, the state of the ground when the track was made, and everything that happened to the track afterwards until someone studies it. The authors, eleven specialists in fossil footprints (ichnologists), argue that the field has lacked clear words and methods for judging how well a track shows the real foot.

They:

• Redefined the terms for the stages a track goes through: registration (while the foot is in contact with the ground), biostratinomy (from the end of registration until final burial), and diagenesis (from burial, through exhumation and collection, until study).
• Separated two meanings of “preservation”: morphological preservation (M-preservation), how faithfully a track shows the anatomy of the foot, and physical preservation (P-preservation), whether the track survives at all.
• Updated a numerical preservation scale first published by Belvedere and Farlow (2016), which began in a study of how emus leave footprints.
• Set out rules for which track features may be used to classify tracks, and for choosing type specimens when naming new track types.
• Tested the scale on 21 examples from the Carboniferous to the present day: three track names based on poorly preserved tracks, and 18 well-established track types of amphibian-like animals, reptiles, dinosaurs, birds, mammals and humans. Every pictured track was given a grade, printed beside it in the figures.

2. What they found

The preservation scale

• 3.0 (optimal, elite): all digits, palm or sole, claws and pads present and sharp, with clear walls; no deformation or overprinting. Good enough to name a new track type and to recognise features of the trackmaker’s foot.
• 2.0 (well, good): nearly complete, fairly sharp; some claw or pad marks may be missing. Good for comparisons, but new species-level names are not recommended.
• 1.0 (poor, intermediate): digits and sole recognisable but incomplete, faint, blurred or distorted. May fit an existing track type; direction of travel and perhaps posture can be read.
• 0.0 (poor, very poor): digits and sole not recognisable. Shows only that an animal passed; a trackway may still give alignment and probable direction of travel.
• Half grades (0.5, 1.5, 2.5) can be used. Optional letters mark features such as partial track (p), cut by another feature (c), anomalous width (w), flattened (f), deformed in many directions (a) or one direction (m), skin impressions (s), drag marks (d) and split digit tips (b).
• Most fossil tetrapod tracks grade between 0.0 and 1.0; grade 2.0 is rather uncommon and 3.0 is rare. Grades often vary along a single trackway, between front and hind feet in the same set, and even between digits of one print.

What changes a track’s shape

• During registration: water-saturated ground can widen digits; tracks pressed through to lower layers (undertracks) lose detail; slopes, as on dune faces, distort tracks and trackways; dragging digits and tails, split digit tips and foot injuries all alter the shape without reflecting the trackmaker’s identity. Skin and scale impressions can survive even on poor tracks.
• Before burial: other animals’ tracks, invertebrate burrows, mud cracks, rain drops, wind and water erosion, salt crystals and microbial mats.
• After burial: compaction can flatten tracks, tectonic forces stretch or squash them in one direction, and weathering and breakage after exposure damage them.
• Undertracks are not always worse than the tracks on the surface the animal walked on; sometimes the reverse is true.

Examples

• Three track names were judged invalid (nomina dubia) because their original specimens are too poorly preserved (grades 0.5 to 1.5). Misuse of one led to a claim of the earliest reptile footprints: what had been read as claw marks were actually digit tips dragging, and the tracks fit a type made by amphibian-like animals.
• Dinosaur tracks: in one theropod trackway, the best-preserved track (grade 2.5) was the one of intermediate depth; shallower and deeper tracks showed less detail.
• Pleistocene canid tracks from Argentina: front feet were better preserved (grade 2.0 to 3.0) than hind feet (1.0 to 2.5), even within the same set; one hind print was cut by a crack in the rock.
• Present-day human footprints on tidal flats (ten made by co-author D. Marty, 23 cm foot length, 70 kg, and five by unknown people): ground with an intermediate amount of water gave the best tracks (grade 2.0 to 3.0), with all toes and a clear outline. Dry ground gave irregular, cracked outlines (grade 1.0); water-saturated ground let the foot sink, leaving only an elongated hollow (0.5 to 1.5). Microbial mats growing over tracks afterwards lowered the grade as they thickened (from 2.0 to 2.5 with little growth to 0.5 to 1.0 with heavy growth).

3. What they concluded

• Footprint classification should rest on the shape that reflects the foot’s anatomy and, secondarily, on the trackway pattern. Features caused by movement, ground conditions and later processes have little or no value for naming tracks.
• Only tracks graded 2.0 to 3.0 should be used to classify tracks. Names based on poorly preserved tracks (0.0 to 1.5) should be treated as doubtful.
• New track types should preferably be based on whole trackways (ideally six or more consecutive prints or sets) with at least one hind print, and front print for four-legged animals, graded 2.0 to 3.0.
• Grades should be given only to tracks shown in photographs or 3D models, and printed beside each track in the figures so readers can check the judgement. The authors do not recommend grading every track at large sites.
• The scale was quick and easy to use, worked for all their examples, and the authors recommend it for all future studies that classify vertebrate footprints.

4. How tracking was or was not used

This is a study of reading footprints, fossil and modern. The authors identified and graded tracks from photographs and specimens by the completeness and sharpness of digits, pads, claws, palms and soles, measured prints and trackways, and compared front and hind feet and left and right sides along trackways. No trailing was involved.

5. Tracker or observer certification

Not reported. The grades are the authors’ own judgements as fossil footprint specialists; no test of how consistently different people assign the grades is reported. The authors ask that grades be printed beside each pictured track so that readers can check them.

6. Other technologies, skills or innovations

• A numerical scale (0.0 to 3.0) with letter codes for recording how well a track shows the foot.
• Photographs and, where available, 3D models of tracks; the authors recommend 3D models for new descriptions.
• A table linking each kind of track feature (I to VII) to its likely causes during and after registration.