Formation and Taphonomy of Human Footprints in Microbial Mats of Present-Day Tidal-flat Environments: Implications for the Study of Fossil Footprints

Abstract

This study concerns the formation, taphonomy, and preservation of human footprints in microbial mats of present-day tidal-flat environments. Due to differences in water content and nature of the microbial mats and the underlying sediment, a wide range of footprint morphologies was produced by the same trackmaker. Most true tracks are subjected to modification due to taphonomic processes, leading to modified true tracks. In addition to formation of biolaminites, microbial mats play a major role in the preservation of footprints on tidal flats. A footprint may be consolidated by desiccation or lithification of the mat, or by ongoing growth of the mat. The latter process may lead to the formation of overtracks. Among consolidated or (partially) lithified footprints found on present-day tidal flats, poorly defined true tracks, modified true tracks, and overtracks were most frequently encountered while unmodified and well-defined true tracks are rather rare. We suggest that modified true tracks and overtracks make up an important percentage of fossil footprints and that they may be as common as undertracks. However, making unambiguous distinctions between poorly defined true tracks, modified true tracks, undertracks, and overtracks in the fossil record will remain a difficult task, which necessitates systematic excavation of footprints combined with careful analysis of the encasing sediment.

A new trackway possibly made by a trotting theropod at the Las Hoyas fossil site (Early Cretaceous, Cuenca Province, Spain): Identification, bio-dynamics and palaeoenvironmental implications

“…The shallow footprints at Las Hoyas show analogies with those described by Marty et al (2009) in dry to damp mats. In accordance with their results, the theropod footprint with a chipped surface inside (Figure 3.2) would suggest that the dinosaur cracked the mat after pressing its foot into the sediment.…”

“…The impact of the foot formed a displacement rim all around the track, whereas the superficial microbial mat led to cracking around the foot (Figure 4) forming striation marks (Dai et al, 2015). These features are analogous to those of human prints produced in water unsaturated mats (see figure 5E in Marty et al, 2009); these taphonomic details have also been recognized in ornithopod tracks of the Early Cretaceous Jiaguan Formation (Dai et al, 2015). Furthermore, the distortion of some dinosaur footprints at Las Hoyas (Figure 3.1 and 3.3) prompts us to think that they could have been produced in a more watery setting of a slippery surface able to shear the tracks.…”

“…Microbial mats favor the preservation of tetrapod tracks because a microbially-stabilized substrate behaves more plastically, the microbiallybound sediment preserves the imprint, and microbial mats often induce early cementation (Marty et al, 2009;Carmona et al, 2011;Dai et al, 2015). Further, a high spectrum of variation in footprint TABLE 1.…”

“…The rest of the measurements as in Figure 5.2 and Table 1. Track FL FW LII LIII LIV TE II-III III-IV II- shape has been experimentally demonstrated in present-day tidal flat environments with growing microbial mats and human trackmakers (Marty et al, 2009). These authors found that due to differences in water content and thickness of microbial mats related to the underlying sediment, the same trackmaker may produce a wide range of preservation types.…”

Fossil Vertebrate Tracks as Paleopenetrometers: Confounding Effects of Foot Morphology

“…Whether and how the track is preserved depends on the characteristics of the substrate (Milan and Bromley 2006;Lockley 2007) and on taphonomic processes (Marty et al 2009). As tracks are created, the mechanical and elastic response of the substrate (such as the Goldilocks effect; Falkingham et al 2011a, b) determine the form and size of the track.…”