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