REEVALUATION OF THE AGE AND ENVIRONMENT OF DEPOSITION OF A GIANT EURYPTERID TRACKWAY FROM NORTHWESTERN PENNSYLVANIA

Palmichnium kosinskiorum Briggs and Rolfe is an interpreted arthropod trackway recovered from purported marine sediments of the Shenango Formation along the banks of Spring Creek, in Elk County, Pennsylvania. This trace is interpreted to be a trail made by a lower Mississippian eurypterid. Discovered in 1948, the type specimen has been periodically displayed within the Benedum Hall of Geology at Carnegie Museum of Natural History in Pittsburgh. The trackway was recovered from the upper surface of a block of coarse-grained to conglomeratic, trough cross-bedded sandstone. Recent field efforts have focused on re-identification of the original site where the type specimen was discovered. The bedding surface where the type was removed, four sets of track remain verifying the type’s original location. The length of the trail on the type specimen indicates that the original trail was more than 3 m long.

The recent field work verifies that the trackway of Palmichnium kosinskiorum was recovered from an allocthonous block of the Pottsville Formation, and not an in-place outcrop of the Shenango Formation, as proposed in the naming publication. The P. kosinskiorum block is one of more than a dozen out-of-place giant boulders forming a train of giant blocks up the valley wall. These boulders can be traced back to an outcrop of the Pottsville Formation several hundred feet up the valley. Thus, P. kosinskiorum can be shown to be lower Pennsylvanian, and not a lower Mississippian trackway.

With the age of P. kosinskiorum confirmed as Pennsylvanian the depositional origin of this specimen can be revised as well. Original interpretations that this specimen was recovered from the lower Mississippian Shenango Formation presumed that the trail was made in a marine environment. However, the trough cross-bedding in the conglomeratic boulders now confirmed to have yielded the type specimen appears to have been deposited in a high-energy fluvial environment. This environment of deposition is consistent with other Carboniferous eurypterid finds.