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Flattened fossil footprints



Ben Creisler
bcreisler@gmail.com


A new paper:

Martin G. Lockley & Lida Xing (2015)
Flattened fossil footprints: implications for paleobiology.
Palaeogeography, Palaeoclimatology, Palaeoecology (advance online publication)
doi:10.1016/j.palaeo.2015.03.008
http://www.sciencedirect.com/science/article/pii/S0031018215001303



Highlights

Large theropod track casts are flattened in some sand-dominated sequences
Flattening occurs if casts are deformed by vice-like pressures between
massive beds
Flattened casts have deceptively wider digit traces and reduced hypex dimensions
Flattened theropod tracks casts distort original track morphology
Flattened theropod tracks casts may not be suitable for ichnotaxonomic studies


Abstract

Studies of natural casts of dinosaur footprints associated with very
thin mudstone and siltstone intervals in thick sand-dominated
sequences often reveal casts that are significantly flattened due to
the differential effects of overburden pressures on different
lithologies. They are in effect squeezed, vice-like, between two
thick, non-compactible sand layers. Thus, the sand filled tracks
(casts) are flattened or widened as the ductile layers are compressed.
Such flattening, here described from five localities, is a previously
unreported phenomenon with implications for vertebrate ichnology.
Present evidence suggest that significant flattening is not evident in
most sequences in which mudstone and siltstone intervals are thicker,
even though overburden pressures may have been comparable. Examples
from the Jurassic of North America and the Cretaceous of China show
that the flattening (widening) of tridactyl theropod tracks leads to
predictable changes in track cast morphology, which may influence
interpretations of track maker identity, and ichnotaxonomy. In the
theropod dominated samples described here, such extramorphological
changes differentially affect the shape of the whole cast and
individual digit trace casts making them appear more “fleshy” and
sometimes deceptively convergent with ornithopod tracks.
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