There are a limited number of fossil turtle footprint reports in the literature and fossil tracks have never been closely compared with modern turtle tracks. Foster et al. (1999) attributed a series of 29 small (20 to30 mm) tracks found in the Jurassic Morrison Formation to an unidentified vertebrate, most likely a turtle. Those tracks were preserved in a sandstone unit as a series of scratch marks, with the individual scratches attributed to the digits of each manus or pes. Individual claw marks, defined as nearly point-like depressions, are not preserved in any of the 29 tracks described and illustrated by Foster et al. (1999). The scrapes left by the Morrison turtles are very similar to the tracks made by the Galapagos tortoises at the Philadelphia Zoo walking across a relatively dry, sandy substrate. The zoo tracks confirm the suggestion by Foster et al. (1999) that the Morrison tracks were indeed made by a turtle.
Wright and Lockley (2001) described a series of short, wide tracks with well-defined claw marks from the Cretaceous Laramie Formation of Colorado. These tracks were attributed to a turtle, and emphasis was placed on the presence of claw marks providing a means to investigate the interaction between the animal and the substrate. They further suggested that the animal was partially buoyed by water as it walked across the substrate. Although the claws of the tracks in the Laramie Formation are prominent, the illustrated tracks by Wright and Lockley (2001) are a reasonable extrapolation of the clawed tracks left by the zoo turtles examined during this work.
Based on the work by Wright and Lockley (2001), well-defined claw marks would seem to be appropriate indicators of turtle tracks. The lack of claw marks on the Hidden Valley Quarry tracks is not problematic, however, because not all zoo tortoise tracks exhibited claw marks. Furthermore, not all of the digits left claw marks in the substrate in some examples of the modern tracks. Presumably this is the result of the graviportal nature of the tortoise foot as opposed to the more characteristically clawed foot of other turtles such as trionychids.The Hidden Valley Quarry sample of seemingly random tracks and overprinting of tracks upon tracks in the context of other trace fossils invites speculation regarding possible behavior. Layers immediately below these tracks, as well as others several centimeters above the tracks have dense concentrations of burrows likely produced by worm-like organisms (Hasiotis 2002). After detecting underground movements by worms, wood turtles (Clemmys insculpta) have been shown to alter their movements during feeding in a manner that has been referred to as a stomp (Kaufman 1989). During this behavior, a wood turtle stomps its foot such that the vibrations generated drive the worms to the surface. The juxtaposition of a bioturbated horizon made by worms below the turtle trackway and the seemingly random distribution of tracks at the Hidden Valley Quarry is consistent with a similar feeding strategy to that of wood turtles.
In conclusion, comparisons between fossil tracks and those made by Galapagos tortoises (Geochelone elephantopus) suggest that the substrate of the imprinted surface was saturated with water. The tracksite includes 71 prints made by a turtle, but given that turtles generally live in proximity to one another, the tracks may have been made by more than one individual. The distribution of tracks suggests a possible feeding strategy similar to that exhibited by modern wood turtles (Clemmys insculpta). These fossil footprints represent the first report of turtle ichnofossils from this rock unit.