Dinosaur track preservation by microbial mats in Brazil

[Ben Creisler <bcreisler@gmail.com>]

From: Ben Creisler
bcreisler@gmail.com

A new online paper:


Ismar de Souza Carvalho, Leonardo Borghi & Giuseppe Leonardi (2013)
Preservation of dinosaur tracks induced by microbial mats in the Sousa
Basin (Lower Cretaceous), Brazil.
Cretaceous Research (advance online publication)
http://dx.doi.org/10.1016/j.cretres.2013.04.004
http://www.sciencedirect.com/science/article/pii/S0195667113000773


Dinosaur footprints and tracks in the Sousa Basin (Lower Cretaceous,
Brazil) occur in at least 37 localities, in distinct stratigraphic
positions. Footprints are rare in the Antenor Navarro (lower) and Rio
Piranhas (upper) formations, where lithofacies analyses point to
sedimentation in ancient alluvial fan to fluvial braided
palaeoenvironments. In the Sousa Formation, the generally finer grain
sized sediments rendered them more suitable for footprint
preservation, where lithofacies analyses point to sedimentation in
warm, small/shallow and temporary lakes, swamps and meandering fluvial
palaeoenvironments. Microbially induced sedimentary structures are
observed in many of the fine-grained lithofacies where dinosaur tracks
are also found, and the large number of these tracks in the Sousa
Basin (particularly in the Sousa Formation, Lower Cretaceous) may be
related to the role of the mats in their preservation. Observations on
recent microbial mats show that footprint morphology is related to the
mat thickness and to the water content of the mat and the underlying
sediment. In dry mats, generally poorly defined or no footprints are
produced, while in saturated ones the imprints are well-defined,
sometimes with well-defined displacement rims. The formation of
well-defined displacement rims around the prints of large dinosaurs
occurs in thick, plastic, moist to water-unsaturated microbial mats on
top of moist to water-unsaturated sediment. These aspects are commonly
observed in the tracks of the Passagem das Pedras site in the Sousa
Basin. The footprint consolidation and its early lithification
probably occurred due the existence of microbial mats that allowed a
more cohesive substrate, preventing the footprints from erosion. The
sediments were initially stabilized by early cementation and by the
mat fabric over the tracks. Successive flooding, and subsequent
sediment influx allowed the large number of layers with dinosaur
tracks and sedimentary structures.