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Triassic swimming reptile tracks from Utah

Ben Creisler

A new recent paper not yet mentioned:

Tracy J. Thomson and Mary L. Droser (2015)
Swimming reptiles make their mark in the Early Triassic: Delayed
ecologic recovery increased the preservation potential of vertebrate
swim tracks.
Geology (advance online publication)
doi: http://dx.doi.org/10.1130/G36332.1

Fossil tetrapod swim tracks have been reported from deposits
throughout the world, ranging in age from the Carboniferous
(Mississippian) to the Neogene (Pleistocene). A normalized analysis of
these occurrences demonstrates that lower Triassic strata contain an
anomalously high number of occurrences. Lower Triassic swim tracks
also tend to be better preserved, showing exceptionally detailed
features such as scale striae and crescent-shaped claw margins.
Preservation of these features required a firm and semicohesive
substrate in order to maintain track detail before and after burial.
Swim-track localities from the lower Triassic Moenkopi Formation in
Utah (USA) are characterized by sedimentary and trace fossil features
that demonstrate the widespread development and persistence of
firmground substrates in a large delta plain complex. Within this
delta, complex low-diversity invertebrate trace fossil assemblages
consist of locally high densities of diminutive, millimeter-scale
traces characteristic of stressed brackish-water faunas. We suggest
that the depauperate infauna characteristic of such environments was
repressed due to delayed biotic recovery following the end-Permian
mass extinction, resulting in extremely low intensities of
bioturbation. Lack of biogenic mixing promoted semiconsolidation of
dewatered mud substrates and the widespread production and persistence
of firmgrounds capable of recording and maintaining swim tracks. Thus
a combination of factors, unique to the Early Triassic, increased the
preservation potential of detailed swim tracks: (1) depositional
environments that promoted the production of firmground substrates,
(2) delayed ecologic recovery resulting in the lack of
well-bioturbated sediment, and (3) the swimming behavior of various
Early Triassic tetrapods.