A THEROPOD RESTING TRACE THAT IS ALSO A LOCOMOTION TRACE: CASE STUDY OF HITCHCOCK'S SPECIMEN AC 1/7

MARTIN, A. J. and RAINFORTH, E. C. (2004)

The most common fossil evidence for dinosaurs in the eastern U.S. is their tracks, occurring abundantly in Upper Triassic and Lower Jurassic strata of the Newark Supergroup, and first and most extensively studied by Edward Hitchcock between 1836 and 1864 from the Early Jurassic of the Connecticut River Valley. Although the vast majority of these tracks are simply related to locomotion, a few examples indicate resting, in which a dinosaur sat on a sedimentary surface. Perhaps the most famous of such resting traces is AC 1/7, acquired by Hitchcock between 1858 and 1864 and residing in the Amherst College's Pratt Museum (Massachusetts). The trace, herein referred to Fulicopus lyellii, is preserved in hematitic shale and shows pes-metatarsal and ischiadic impressions of a theropod dinosaur; Hitchcock referred the specimen to Anomopeus major (currently attributed to an ornithischian) on the basis of its metatarsal impressions.  Gierlinski (1996) suggested that this resting trace indicated the presence of feathers on the ventral surface of the tracemaker; the "feather impressions" are small grooves and ridges extending away from the imprint, proposed to have formed as the feathers dragged through the mud. This hypothesis prompted our reexamination of the specimen, which generated another hypothesis: the "feather impressions" are pressure-release structures caused by movement of the tracemaker after it rested. Evidence for our hypothesis includes: (1) a broad, heightened ridge with superimposed millimeter-thin ripples (making a "feathered" appearance) along the left side of the right metatarsal print; (2) similar but smaller ridges along the left sides of the right pes and ischiadic prints; and (3) the right side of the resting trace is relatively deeper than the left. These structures and the overall pattern of the trace are thus consistent with those made by a theropod that stopped, sat back on its metatarsals, then shifted its weight to the right when it stood up and resumed forward movement. This interpretation does not preclude the possibility that the tracemaker had feathers; however, feather drag marks would have had a very different appearance compared to the traces exhibited by this specimen.

Geological Society of America, Abstracts with Programs, 36 (2):96.