BOTTOMS UP: A SEDIMENTOLOGICAL ANALYSIS OF THE MANUS-ONLY COFFEE HOLLOW SAUROPOD TRACKS

Dinosaur footprints provide clues for reconstructing the behavior of these ancient animals, as well as the circumstances under which their tracks were preserved. Among the most controversial dinosaur traces are sauropod trackways in which prints of only the forefoot (manus) are preserved. Were such trackways made by "punting" sauropods with floating hindquarters, or is the lack of hindfoot (pes) prints due to deeper penetration of manus than pes prints in undertrack sequences? The now-destroyed Coffee Hollow site (Glen Rose Formation, Kendall County, Texas) preserved three manus-only sauropod trackways. The exposed footprint surface showed footprints that were very shallowly impressed into a foraminiferal packstone. There were also remnants of a thin layer, a few centimeters thick, overlying this track surface. The remnants are a laminated fine-grained limestone with birdseye structures suggesting a microbial mat. Some remnants appeared to be concordant with the track surface, and petrographic analysis of a vertical section through one of the footprints revealed flame structures in this bed. These observations suggest that the laminated limestone was the true track layer, and that the visible footprints were indeed undertracks that formed beneath a microbial mat. However, in sauropod trackways at other sites in the Glen Rose Formation in which both forefoot and hindfoot prints are preserved, pes prints are consistently as deeply or more deeply impressed than manus prints. Consequently differential undertrack depth preservation alone does not account for manus-only sauropod trackways in this unit. Unlike the makers of manus-only or manus-dominated trackways in other ichnofaunas, the Glen Rose sauropods may in fact have been partly floating, buoyant behind, and heavy in front. This interpretation is consistent with the reconstructed buoyancy profiles of sauropods thought to be close relatives of the Glen Rose sauropods.