INFLUENCE OF EARLY JURASSIC POST-EXTINCTION MICROBIAL COMMUNITIES ON THE PRESERVATION OF DINOSAUR TRACKS: HARTFORD BASIN, CONNECTICUT

The occurrence of Microbially Induced Sedimentary Structures (MISS) in terrestrial sedimentary rocks deposited after mass extinction events is likely linked to post-extinction decreases in diversity and abundance of microbial mat-grazing metazoans. Because algae are particularly adapted to survive in saline and brackish waters that experience frequent changes in depth and water chemistry, MISS are well represented in modern and ancient tropical and sub-tropical playas and shallow lakes.

We identified MISS in cores and outcrops of Early Jurassic strata in the Hartford Basin (HB), notably East Berlin Formation at Dinosaur State Park (DSP), and cores of the lower Portland Formation. MISS observed at Dinosaur State Park (DSP) include sand cracks, gas escape structures, setulfs (sand shadow structures) and surface wrinkling of some of the c. 750 exposed footmarks. Beds displaying MISS are composed of thin laminae of fine to medium sand separated by irregular, crinkly, clay- and carbonate-rich interlayers. Small mound-like deformational features measuring a few centimeters high, unrelated to dinoturbation are interpreted as irregular, low-relief, stromatolites. Other MISS include small-scale deformation attributed to the escape of gasses produced by biochemical decay, and physical disturbance by wind, water currents, or dinoturbation.

Microbial mats, including gelatinous cyanobacteria coatings, are here invoked as a key factor in the preservation of tracks at DSP. MISS are most common on the surface that preserves most of the tracks at DSP. As transient features exposed to erosion, the preservation of dinosaur footmarks is not only greatly enhanced by microbial binding, such stabilization and consolidation may be required. Modern examples of long-term preservation of animal tracks are attributed to microbial binding of sediment.

Our observations are applicable to other outcrops and cores in the HB and related rifts. Sandstone and mudstone beds displaying millimeter-scale fluctuations in grain size with irregular, crinkly laminae are interpreted to represent mat intervals, and have been recognized in both the East Berlin and lower Portland Formations. Microbial laminae are common in strata interbedded with CAMP basalt flows that induced profound climatic instability.