Walking in mud: Remarkable Pleistocene human trackways from White Sands National Park (New Mexico)
Introduction
The interaction of two or more animals, as evidenced by their fossilised footprints, preserves a moment in time. Gregarious behaviour has frequently been documented (Hatala et al., 2016b; Roach et al., 2016; Hatala et al., 2017), but multispecies interactions (e.g., Leakey and Hay, 1979), such as predator vs prey, are rare by comparison. When the taxa involved are extinct, such instances offer unique avenues for understanding inter-specific behavioural palaeoecology. Human footprint sites often contain animal tracks (e.g., Aldhouse-Green et al., 1992; Roach et al., 2016; Altamura et al., 2018) and perhaps most famously at Laetoli (Leakey and Harris, 1987), but demonstrating interaction or even strict contemporaneity is challenging. Most footprint sites are limited by the extent of the exposed ichno-surface which precludes palaeo-tracking over extensive distances (e.g., Ashton et al., 2014). The lack of evidence of interaction is therefore in part a function of preservation and exposure. As a result, our understanding of the behavioural interaction between different animals at various times in the geological record is limited. Such information is relevant to a wide range of questions such as early human hunting practices and their consequence therefore for the extinction of Pleistocene mega-fauna (e.g., Martin, 1973; Guthrie, 2006; Meltzer, 2015; Surovell et al., 2016).
In addition, the sample of fossil human tracks made by an individual track-maker is also typically limited by the size of natural exposures. Inferences are consequently often made, in some cases for entire hominin species, on the basis of small track samples, leading to debate. This point is illustrated by the on-going controversy over the biomechanics of the Laetoli track-maker (e.g., Meldrum et al., 2011; Crompton et al., 2012; Bennett et al., 2016a, b; Hatala et al., 2016a). Our understanding of natural track variability, and both the biomechanical and behavioural modifications associated with unstable or slippery surfaces, often the ones that record tracks, hampers these debates (Morse et al., 2013; Bennett and Morse, 2014). The occurrence of a double human trackway extending over at least 1.5 km at White Sands National Park (WHSA; Locality-3) provides a remarkable opportunity to explore intra-trackway variability. This site is distinct from that reported by Bustos et al. (2018) on the eastern, rather than western, side of Alkali Flat. We present new methods for investigating trackway variability and the implications of that variability for biometric and biomechanical inferences. Finally, we explore the behavioural implications that can be deduced from both the human trackway and the mega-fauna that crosscut and are subsequently crosscut by it.
Section snippets
Geomorphological context and geochronology
Ichnofossils of extinct Rancholabrean fauna occurring at White Sands National Park (WHSA) in New Mexico comprise one of the largest concentrations of Cenozoic vertebrate tracks in North America (Lucas et al., 2007, Fig. 1, Fig. 2, Fig. 3). Tracks and trackways of Proboscidea (mammoth/mastodon), Folivora (ground sloth), Carnivore (canid and felid), and Cetartiodactyla (bovid and camelid) are all known from this site along with humans (Bustos et al., 2018). A fossil mastodon is also known from
Materials and methods
Tracks were made by unshod humans, and are impressed into thinly bedded gypsiferous and siliciclastic muds and sands. They are visible only under specific moisture conditions which produce colour contrasts between the track infill and the surrounding sediment (Fig. 3A). Repeated field reconnaissance, and both aerial and geophysical survey allowed targets to be identified and excavated (Urban et al., 2018, 2019; Bennett et al., 2019). Once targets were located, they were gridded into 5 m squares
Double human trackway
This consists of two parallel human trackways that run in a north-north-west to south-south-east direction. The accessible length is approximately 800 m however it extends further to the north for at least another 600 m into the White Sands Missile Range (Fig. 2). The southern end becomes progressively harder to follow as it approaches the former dune line. Both the north- and south-bound trackways show little deviation in direction, although the gap between the two trackways varies from as
Discussion and implications
The double trackway reported here is remarkable within the human ichnological record, in terms of its length and the number of visible tracks. Morse et al. (2013) describe a trackway of 70 continuous tracks from Namibia over a distance of approximately 300m and Panarello et al. (2017) describe a human pathway extending over several hundred metres at Roccamonfina in Central Italy, but without continuous visible tracks (see also: Marcos, 2001). Recently, Hatala et al. (2020) described 400 tracks
Conclusions
The double trackway reported here is remarkable within the human ichnological record for its length and also for the morphological diversity of the individual tracks. We draw the following conclusions:
- 1.
Both the outward and return journeys were likely made by the same individual, an adolescent or small adult female (?) that appears to have been carrying a small child (<3 years old) on at least the north-bound (outward) leg. The journey was made with considerable haste over difficult and slippery
Author statement
Matthew R. Bennett: Conceptualization, Data curation, Writing - original draft; Visualisation, Formal analysis; Methodology, Software, Validation; David Bustos: Conceptualization, Data curation, Funding acquisition; Daniel Odess: Conceptualization, Visualization, Funding acquisition; Tommy M. Urban: Visualization, Data curation, Writing - review & editing; Jens Lallensack: Conceptualization, Software; Marcin Budka: Conceptualization, Software; Vincent L. Santucci: Fieldwork; Patrick Martinez:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
MRB acknowledges Natural Environment Research Council grants (NE/H004246/1 and NE/M021459/1). Views and conclusions are those of the authors and do not necessarily reflect policies of the National Park Service (NPS). Mention of trade names or commercial products does not constitute endorsement. Several federal laws in the United States prohibit the disclosure of specific archaeological and palaeontological locality information within U.S. National Park Service areas, namely: Archaeological
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