Radiocarbon dating of seeds and wood let to mostly consistently younger ages toward the top of the stratigraphic profile ( Table 1 ), reflecting a low energy setting with only mild perturbations. For the underlying strata PB-6 the mean of the two median probability ages is 17,300 cal yr BP ( Table 1 ). The age of the footprint is constrained by seven ages ranging between 13,195 ± 35 and 12,735 ± 40 14C yr BP. Thus, according to the median probability ages, it is safe to say that the Pilauco footprint is circa 15,600 cal yr BP. Additional radiocarbon dating obtained from a portion of a Notiomastodon platensis skull found near the trace, as well as a sample taken from a rib and its infilling sediments, indicate ages of 13,220 ± 60 and 13,240 ± 60–12,905 ± 40 14 C yr BP, respectively [ 10 ]. It is completely normal to find these age ranges, since wood, seeds, and/or bone material have separate depositional histories within the site, and these with the footprint itself, which cannot be directly dated.

Textural data indicate that the trackbed’s gravel and mud content appears to be slightly higher than the infilling and adjacent sediments. The infilling and surface sediments show a slight increase in the organic component (11.1 and 10.5 wt% of organic matter, compared to 9.3 wt% for remaining samples, but generally these variations are minor (9.7 ± 1.4 wt%).

Of the five digits, only the impression of digit I is properly delimited. It has a concave sub-circular section with the longitudinal axis obliquely oriented with a 25° torsion angle with respect to the plantar surface. A crest between the base of digit I and the edge of the anterior third can not be observed. On the contrary, an isthmus somewhat narrower than the diameter of digit I connects both regions. Adjacent to the change in angle, the base of digit I is slightly elevated above the surface plane. The remaining digits (II to V) are not present or did not leave evident impressions. An oval projection, located in the middle portion of the anterior region of the trace, could eventually correspond to the mark produced by the IV and/or V digits. One of the notable features of the trace is an ellipsoidal, conical elevation or promontory located in the center of the anterior third ( Fig 3 ).

The plantar surface impression is flat, undifferentiated, and lacks ridges and/or separations between the anterior third, the plantar vault, the plantar isthmus and the anterior portion of the ball. The outer edge of the footprint impression is practically straight along its entire length, and slightly curved at the height of the outer anterior portion of the heel. The medial arch is well defined, circumscribing a pronounced concavity in the inner margin. The ball has a sub-rounded back edge that is internally directed. In general appearance, the plantar surface of the footprint is equivalent to a neutral to slightly pronator type. The region of the anterior third is the widest, which is a typical feature of Hominipes modernus. The anterior edge presents a triangular invagination at the height of the IV digit. The distal end generates a small angle with respect to the external border, which is common in the impressions of this ichnospecies [ 22 ].

Due to its dimensions, general shape and taphonomic attributes, the ichnite of Pilauco corresponds to a right foot impression of an adult human, and discard other produces such as ground sloths ( S1 File ). The general length and width, together with the presence of a planar surface with a well-defined medial arch and a rounded posterior heel, allow to identify this trace as Hominipes modernus [ 10 , 22 ]. The print has an elongated and semi-oval shape, with internally curved longitudinal axis, forming a left-projected longitudinal arcade that extends from the medial arch to the ball ( Fig 3 ). The total linear length measured in the silicone replica from the anterior edge of digit I impression to the back of the heel is 279 mm, while the largest width, located in the mesial portion of the anterior third, (i.e. the “ball” of the foot), is 105 mm. The smaller width, at the height of the medial constriction, which is maintained toward the back of what could be interpreted as the heel, is 82 mm ( Fig 3 ). The estimated FI footprint index is 0.38, placing the Pilauco footprint within the range of the larger and more robust individuals described for the ichnospecies [ 22 , 38 ]( S1 File ).

Diagnosis: solitary or aggregated footprint impressions, plantigrade, with elongated axial plane, entaxonic, generally pentadactyle; with short, round, oval to slightly elongated digit prints, which may or may not be present because of taphonomic conditions. Print of digit I oval and extended in antero-posterior direction, and twice as long as the remaining digits. Anterior third region wider than the rest of the trace, and separated from the impressions of the digits by an axial ridge. Medial arch well demarcated and concave with respect to the inner edge of the impression ( S6 Fig ). Sub-circular heel mark (ball) projected internally, and narrower than the anterior third. Traces of cosmopolitan distribution, recorded from the early Pleistocene to the late Holocene, and assigned to bipedal hominids of the genus Homo, mainly Homo sapiens.

Experimental trackmaking

The estimated speed calculated from the footprint data is similar for all trackmakers in dry and wet substrate, despite differences in their physique. However, the trackmakers walking on a saturated substrate show a slight decrease in their speed associated with an increase in the footprint depth (Table 2).

The morphology of the experimental footprints varies along with the water content of the substrate (Fig 4). The measured lengths and footprint morphology throughout each trackway were consistent. Hence, only a single print of each trackway experiment is described.

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larger image TIFF original image Download: Fig 4. Experimental footprints. Each row corresponds to a trackmaker and each column to a dry, wet and saturated water content of the sediment, respectively. Notice that in all the experiments trackmaker A has a deeper forward imprint of the digits and a large medial longitudinal arch (white arrow), trackmaker B has a slightly more even distribution of the footprint’s depth, but shows no major signs of the medial longitudinal arch (white arrow), and trackmaker C leaves footprints only on wet and saturated water content substrates (concepts used here are schematized as a replacement of the absent dry water content imprint). This can be mostly explained by light body weight (at least 10 kg lighter than the other trackmakers), as well as a well-balanced center of gravity (trackmakers A and B have centers of gravity forwardly deviated). In general, the hallux is a prominent structure that can be identified on each experiment, while the imprint of the lateral digits is variable. On the saturated water content experiments the hallux can drag sediment forward (pointing triangle) or backward (star). Trackmaker C on saturated water content substrate left footprints remarkably similar to the sedimentary structure excavated in Pilauco. https://doi.org/10.1371/journal.pone.0213572.g004

On the dry substrate (Fig 4), trackmaker A presents deeper distal impressions consistent with higher body weight (Table 2) and a remarkable development of the medial longitudinal arch compared to the other trackmakers. Lateral digits drag the sediment backward and no displacement rims are observed. The hallux impression is well rounded. Trackmaker B shows a more uniform distribution of the footprint depth compared to trackmaker A. The medial longitudinal arch is rather absent and replaced by a wider medial border, related to a flat-footed condition. Lateral digit prints are easily distinguishable, but they slide slightly laterally producing a square-like shape. The hallux impression has a clear, well-rounded shape. No displacement rims are observed. Trackmaker C did not leave any footprint marks on the dry substrate setting. While substrate desiccation might have contributed to this result, it is important to note that trackmaker C has a noticeably lighter body weight (10–14 kg less than trackmaker A and B respectively, Table 2). His center of gravity appears to be better balanced than trackmakers A and B, which are forward deviated [34]. These factors might have a more significant effect than the eventual substrate desiccation. No displacement rims are observed.

On wet substrate (Fig 4), trackmaker A left deeper footprints compared to the other trackmakers. The medial longitudinal arch is well-developed. No displacement rims are observed. All digits sunk forward into the substrate, but are easily distinguishable showing a well-rounded shape. Trackmaker B left more uniformly distributed footprint-depth compared to trackmaker A. The medial longitudinal arch is rather absent and replaced by a wider medial border. The hallux impression is well-rounded, but lateral digits lose definition, and only digits II to IV are easily identifiable. No displacement rims can be observed. Trackmaker C left a generally shallow footprint, with similar proximal and distal depths. The medial longitudinal arch is slightly marked. The impression of the hallux and lateral digits are easily distinguishable and have well-rounded shapes. The hallux and the rest of the lateral digits compress digit II. No displacement rims are observed.

Finally, in the saturated substrate (Fig 4), trackmaker A sunk a foot forward, producing a sediment wall through which the hallux passed dragging. This movement displaced some of the sediment forward, the hallux acting as a shovel carrying material and depositing it toward the distal surface of the impression (pointing triangle on Fig 4). The lateral digits are dragged into the distal surface, but the effect was less notorious and digits cannot be identified as separated structures. A marked displacement rim was observed all around the impression, becoming larger on the distal surface. Trackmaker B buried the pedal digits into the sediment. When lifting the foot up and forward it dragged the sediment forward, leaving the mark of each digit on the distal sediment wall. However, the hallux was dragging even further than the lateral digits. Some of the sediment trapped on the hallux’s dorsal surface dropped in front of the footprint (pointing triangle on Fig 4). Once the foot was lifted up, the sediment collapsed inside the imprint, filling up the hallux and digit II impressions. A marked displacement rim was observed all around the mark and is particularly notorious on its medial border. Trackmaker C left an elongated hallux mark, but the lateral digits are not identifiable. There is a sediment lump right behind the place where the digits should have left their mark. This lump dropped from the dorsal surface of the digits when the foot was lifted up and set forward (star on Figs 4 and 5). A marked displacement rim can be observed on the medial, proximal, lateral and latero-distal borders of the trace, but is absent in front of the hallux.