RELATIVE ABUNDANCE OF THEROPOD DINOSAUR TRACKS IN THE EARLY JURASSIC (HETTANGIAN) MOENAVE FORMATION AT A ST. GEORGE DINOSAUR TRACKSITE IN SOUTHWESTERN UTAH: BIAS PRODUCED BY SUBSTRATE CONSISTENCY

An abundance of Early Jurassic traces and body fossils are being collected from the lacustrine Whitmore Point Member of the Moenave Formation in southwestern Utah. A wide variety of dinosaur behaviors and sedimentary structures have been preserved at these unique sites allowing for a detailed look at this previously poorly understood formation.

To date, approximately 25 track-bearing horizons have been recognized from and in the area of the St. George Dinosaur Discovery Site at Johnson Farm. All of these track horizons are dominated by (in decreasing order of commonality) Grallator (small theropod), usually followed by Batrachopus (small crocodylomorph), and Eubrontes (large theropod) tracks. However the “Main Track Layer” is dominated by Eubrontes with Grallator being quite uncommon. Traces at the base of this main track-bearing horizon are preserved as natural casts in a fine-grained, beach-deposited sandstone that overlies a basal mudstone. This surface is covered by large and small mudcracks with some scoured surfaces. The majority of the Grallator tracks, aside from those found on scoured surfaces, occur within larger Eubrontes tracks. A thick crust of dried mud covering wet, soft mud below produced the obvious bias on this main track-bearing horizon. Eubrontes track makers were heavier, breaking through this crust to produce footprints in the soft mud below. Grallator trackmakers were lighter and rarely broke through the mud crust on their own, and usually only produced track when stepping into a freshly made Eubrontes track. Based on dominance of Grallator tracks in other track-producing horizons at the site, Grallator track makers were no doubt more abundant on the main track-bearing horizon as well.