5th Swiss Geoscience Meeting, Geneva 2007
Comparative taphonomy of human footprints left in
microbial mats of tidal flats and dinosaur footprints of
Late Jurassic biolaminites; a key for the interpretation
of fossil vertebrate tracks
Marty Daniel*, **
*Office de la culture, Section d’archéologie et paléontologie, Paléontologie A16, Hôtel
des Halles, C.P. 64, CH-2900 Porrentruy 2 (daniel.marty@palaeojura.ch)
**Dept. of Geosciences, University of Fribourg, Chemin du Musée 6, CH-1700 Fribourg
Generally, tidal-flats are ill-suited for the conservation of skeletal remains of
terrestrial vertebrates, but they are important for the preservation of their
footprints. Dinosaur footprints for example, are abundant in tidal-flat deposits
throughout the Mesozoic. They fill gaps in the skeletal record and improve the
knowledge of dinosaur locomotion and palaeoecology (e.g., Lockley 1998).
The use of fossil footprints for ichnotaxonomy, for interpreting the
palaeoecology of the trackmaker, or for reconstructing the palaeoenvironment is
closely related to the understanding of footprint formation, taphonomy, and
preservation processes. For this purpose, human footprints have been studied
in a wide range of present-day tidal-flat environments, where microbial mats are
ubiquitous (review in Gerdes & Krumbein 1994), and may lead to the formation
of biolaminites (Gerdes et al. 1991). Microbial mats play an important role
during the formation and preservation of vertebrate footprints (Marty et al.
submitted). Due to different constellations in water content and nature of the
microbial mat and underlying sediment, a wide range of true track morphologies
was produced by the same human trackmaker. After formation, true tracks are
in most cases subjected to modification due to physicochemical and biological
taphonomic processes leading to modified true tracks. A (modified) true track
may be consolidated by desiccation, lithification, or ongoing growth of the mat.
The latter process may lead to the formation of overtracks. Amongst
consolidated or (partially-) lithified footprints found on present-day tidal flats,
poorly-defined true tracks, modified true tracks, and overtracks were most
frequently encountered whilst unmodified and well-defined true tracks were
rather rare (Marty et al., submitted).
These observations made on human footprints of recent tidal-flat environments
are compared with dinosaur footprints from Late Jurassic biolaminites,
excavated on the Transjurane highway (Canton Jura, NW Switzerland; review
of the tracksites in Marty et al. 2007), using surface documentations, crosssectioned footprints, and sedimentological analyses of the encasing sediment.
This comparison facilitates evaluating the relative abundance of true tracks,
modified true tracks, undertracks, and overtracks, even if an unambiguous
identification is not always possible. It is suggested that modified true tracks and
overtracks make up an important part of fossil footprints and that they may be
as common as undertracks. Even though only unmodified, well-defined true
tracks should be used for ichnotaxonomy, poorly-defined true tracks, modified
true tracks, and under- and overtracks are important for the reconstruction of
5th Swiss Geoscience Meeting, Geneva 2007
the palaeoenvironment and of the physicochemical and biological sedimentary
processes acting within.
Figure 1. A: Only slightly-modified, well-defined true track. Potential modern
analogon for D. B: Moderately-modified, poorly-defined true track with a small
displacement rim. Potential modern analogon for E. C: Cross-section of the
consolidated microbial mat of A and B. D: Right sauropod pes print with digit
impressions. Slightly-modified, well-preserved true track. E: Right sauropod pes
and manus prints with small displacement rims. F: Thin section of the filling of
the pes print in E showing microbial laminations. Scale bars: 5 cm in A, 10 cm in
B and D, 1 cm in C and F, 20 cm in E. The footprints in A and B have been left
by adult humans on the modern supratidal flats of Hassi Jerbi (southern
Tunisia) whilst the thick microbial mat was still moist (A) to unsaturated (B); they
were consolidated when the pictures were taken in May 2005; arrows indicate
the first digit. D, E, and F are from the Transjurane tracksites.
REFERENCES
Gerdes, G. & Krumbein, W. E. 1994: Peritidal potential stromatolites – a
synopsis. In: Phanerozoic stromatolites II (Ed. by Bertrand-Sarfati, J. & Monty,
C.). Kluwer Academic Publishers, 101-129.
Gerdes, G., Krumbein, W. E. & Reineck, H.-E. 1991: Biolaminations –
Ecological versus depositional dynamics. In: Cycles and events in stratigraphy
(Ed. by Einsele, G., Ricken, W. & Seilacher, A.). Springer-Verlag, 592-607.
Lockley, M. G. 1998: The vertebrate track record. Nature, 396, 429-431.
Marty, D., Strasser, A. & Meyer, C. A. submitted: Formation and taphonomy of
human footprints in microbial mats of present-day tidal-flat environments:
implications for the study of fossil footprints. Ichnos.
Marty, D., Ayer, J., Becker, D., Berger, J.-P., Billon-Bruyat, J.-P., Braillard, L.,
Hug, W. A., Hug, W. A. & Meyer, C. A. 2007: Late Jurassic dinosaur tracksites
of the Transjurane highway (Canton Jura, NW Switzerland): overview and
measures for their protection and valorisation. Bull. appl. Geol., 12, 75-89.