TAPHONOMY OF THE JOANNA TRACK SITE, CRETACEOUS GLEN ROSE FORMATION: IS THE SHRIMP MIGHTIER THAN THE DINOSAUR?

Dinosaur trackways are common in the Cretaceous Glen Rose Formation of central Texas. While the trackways in Dinosaur Valley State Park are well known, many other sites can be found in this region. Recently, a new track site was discovered during construction in Glen Rose, Texas. The Joanna Track Site features tridactyl tracks which have been partly obliterated by invertebrates burrowing through the thick mud which buried them.

We measured and described the interval from 0.3 m below the track layer through 2.7 m above it in a vertical outcrop directly adjacent to the track site. Samples were collected in 10 cm intervals through this section. A larger sample, roughly 30 cm tall and 20 cm wide and deep, comprising 15 cm below and above the upper surface of the track layer was also collected. To prevent disintegration, the fragile rock, it was coated in epoxy as it was removed. This sample was dry cut with a diamond saw into a few vertical slabs, 3-4 cm thick. These are to be dry sanded and polished to reveal microstratigraphic details. A continuous series of thin sections will also be made through the thickness of the sample.

The track layer marl is riddled with Thalassinoides which nearly erased evidence of dinosaur tracks. The Thalassinoides originates from the top of a highly bioturbated 30 cm bed of ostracod and foraminifera-rich mud immediately overlying the track surface. This layer contains fragments of an unidentified crustacean—possibly the burrow makers along with turtle shell, shark teeth, and small vertebrae. Capping this is a thin bed of pyritic mud, 20 cm of burrowed mud with bivalves in life position, then 20 cm of grainstone with mud-filled burrows. Beneath the tracks, there are 10 cm of fissile mud with bits of marl, then a 20 cm ostracod packstone with mud-filled burrows.

An upward increase in deeper water fauna supports the conclusion that the succession was deposited during a period of increasing water depth. The mud bed overlaying the track layer suggests rapid burial, but burrowing has damaged the quality of preservation. Since Thalassinoides extends from the mud into the track layer, it can be inferred that the burrows occurred after burial by 30 cm (compacted thickness) of mud. While deep burial preserved the tracks, deeper burrowing nearly destroyed them.