MISSISSIPPIAN TETRAPOD FOOTPRINT ASSEMBLAGES FROM PENNSYLVANIA AND NOVA SCOTIA AND THE OLDEST RECORD OF AMNIOTES

Only two substantial tetrapod footprint assemblages of Mississippian age are known: Tournaisian Blue Beach and Hurd Creek Members of Horton Bluff Formation in Nova Scotia, Canada and late Visean middle member of Mauch Chunk Formation in eastern Pennsylvania, USA. The older Horton Bluff assemblage is from brackish to freshwater lake-margin sediments within a restricted marine bay, and is associated with an invertebrate ichnofossil assemblage dominated by arthropod walking and resting traces. Bones of tetrapods are also present in the Horton Bluff, but never in the same facies or strata as the footprints. The Mauch Chunk tetrapod footprint assemblage is from fluvial red beds that yield an invertebrate ichnoassemblage dominated by backfilled burrows of deposit feeders. Despite these striking differences, the Horton Bluff and Mauch Chunk tetrapod footprint assemblages share key features: (1) low ichnodiversity (4-5 ichnogenera); (2) temnospondyl footprints (Palaeosauropus and subordinate Batrachichnus) are most common; (3) other footprint ichnogenera (Hylopus, Matthewichnus, Attenosarus) are rare; and (4) both well represent the Batrachichnus ichnofacies. A striking difference is abundant Pseudobradypus in the Horton Bluff, whereas this ichnotaxon is absent in the Mauch Chunk. Pseudobradypus has been claimed to represent the oldest amniote footprint in the Lower Pennsylvanian of New Brunswick, Canada. However, the Horton Bluff records are much older and are more likely to have been made by basal tetrapods such as anthracosaurs that have foot structures homeomorphic with those of later amniotes. Furthermore, Mauch Chunk redbeds represent relatively “dry” paleoenvironments yet lack amniote tracks. Large Mississippian temnospondyl footprints from both the Horton Bluff and the Mauch Chunk indicate the presence of Mississippian temnospondyls much larger than any known from the notoriously incomplete bone record. The Horton Bluff and Mauch Chunk tetrapod footprint assemblages indicate fully terrestrial pentadactyl/tetradactyl walking by tetrapods by Tournaisian time. They also lack unambiguous amniote footprints or bones, which supports a post-Visean origin of amniotes.