3-D radar imaging unlocks the untapped behavioral and biomechanical archive of Pleistocene ghost tracks

Abstract

Footprint evidence of human-megafauna interactions remains extremely rare in the archaeological and palaeontological records. Recent work suggests ancient playa environments may hold such evidence, though the prints may not be visible. These so-called "ghost tracks" comprise a rich archive of biomechanical and behavioral data that remains mostly unexplored. Here we present evidence for the successful detection and 3-D imaging of such footprints via ground-penetrating radar (GPR), including co-associated mammoth and human prints. Using GPR we have found that track density and faunal diversity may be much greater than realized by the unaided human eye. Our data further suggests that detectable subsurface consolidation below mammoth tracks correlates with typical plantar pressure patterns from extant elephants. This opens future potential for more sophisticated biomechanical studies on the footprints of other extinct land vertebrates. Our approach allows rapid detection and documentation of footprints while enhancing the data available from these fossil archives.

Figure 1

Map showing the White Sands National Monument, Alkali Flat, and the study site. Digital elevation model is from Shuttle Radar Topography Mission 1 arc-second data with the surficial geology taken from U.S. Geographical Survey maps (relief in ft.). Note the precise location of the study site is withheld in accordance with the requirements of the National Parks Service (NPS) compliance with U.S. law. Interested parties may apply to the NPS for the specific site location. Data from https://www.usgs.gov/centers/eros and map made with ArcMap 10.1 (http://desktop.arcgis.com/en/arcmap/).

Figure 2

Photographs of the study site at WHSA. (a) “ghost tracks”: the surface expression of the tracks is poor as can be seen from the image and they can only be seen under specific moisture and salt conditions. Scale bar 500 mm from target to target. (b) Tracks at the study site excavated to reveal both human and mammoth tracks. (c) GPR equipment used in this study. (d) Gridded foam mats used to protect the surface during the GPR survey following Jacob et al.. (e) Excavated human tracks at the study site.

Figure 3

(a) The principal tracks and trackways observed at the study site which are split into Location-1 and Location-2 (shown in true spatial relationship). (b) GPR amplitude slice (2.0 to 4.0 ns). Human prints that were excavated and used for analysis are indicated with (+) while an unexcavated sloth trackway (identified in subsequent fieldwork) is indicated with (x).

Figure 4

3-D perspective-view GPR results at various depths revealing hidden tracks and volumetric variations including sub-track consolidation. (a) Just beneath the surface (0–5 cm). (b) 5–10 cm of surface clipped. (c) 10–15 cm of surface clipped. (d) A photograph of an excavated human print (left) shown beside a close-up view of the corresponding GPR anomaly (right) at a depth of 5–10 cm, collected prior to excavation.

Figure 5

Close up 3-D GPR perspective of mammoth track 1 (T 1) along with human and sloth tracks. The presence of sloth tracks was not known prior to the GPR survey and was later confirmed when the prints became partially visible after a period of precipitation. The sloth prints remain unexcavated at this location.

Figure 6

Location- 2 GPR slices at given travel-times with comparative data. Each 2 ns slice represents an estimated thickness of 5–6 cm. (a) Bottom or near bottom of true tracks, with a superimposed human footprint seen in the perimeter of T1. (b) Slice immediately beneath the tracks yield amplitude patterns similar to observed plantar pressure data. (c) Depths derived from excavation of the largest mammoth track (T1) with human print. (d) Mean plantar pressure data from African elephants (Loxodonta africana), curtesy of Panagiotopoulou et al.. This based on the mean records of five elephants and the methods are described in Panagiotopoulou et al.. Means for each of the five animals are based on between 1 and 24 pressure records.

Figure 7

(a) GPR fence diagram showing amplitude variations below the mammoth tracks. Note the ‘hook-shaped’ structure indicated by the white arrows consistently points in the mammoth’s travel direction. (b) GPR amplitude isosurface from T3 rotated to show several perspectives of the 3-D structure of the sub-track anomaly.