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Early Jurassic gregarious therapsid tracks from Nevada

Ben Creisler

A recent paper:

Stephen M. Rowland and Jennifer M. Mercadante (2014)
Trackways of a gregarious, dunefield-dwelling, Early Jurassic
Therapsid in the Aztec Sandstone of Southern Nevada.
PALAIOS 29(10): 539-552
doi: http://dx.doi.org/10.2110/palo.2013.067

We describe and interpret a tracksite in the Lower Jurassic Aztec
Sandstone in Valley of Fire State Park, southern Nevada. The site
contains approximately one hundred tracks of the ichnogenus
Brasilichnium, arranged in twelve, subparallel trackways, all on the
same foreset bedding plane. The Brasilichnium trackmaker was most
probably a fossorial, tritylodontid therapsid. Sedimentologicial
analyses indicate that the trackway surface is a wind-ripple horizon
with a primary dip of about 25°, and that the animals climbed straight
up the slip face of the dune. A combination of features leads us to
conclude that the footprints were impressed into a crust of moist,
cohesive sand, leaving two modes of preserved tracks: (1) shallow,
well-defined tracks without associated sand crescents, and (2) deeper,
less well defined tracks with associated sand crescents. We interpret
this assemblage of tracks to record gregarious behavior in a mixed-age
group of tritylodontid therapsids. In the correlative Navajo
Sandstone, other researchers have documented the presence of complex
networks of burrows concentrated in elevated mounds, reminiscent of
colonies of North American prairie dogs. The Brasilichnium trackmaker
is a good candidate to have excavated the burrows. Although we cannot
directly associate the Brasilichnium trackmaker with the burrow
complexes, we hypothesize that these gregarious, fossorial animals
lived in prairie-dog–town–like colonies. This study supports the
aridity food-distribution hypothesis, which posits that the patchy
distribution of food resources in arid environments creates selective
pressure for colonial behavior.