VERTEBRATE BIOSTRATIGRAPHY AND THE TRIASSIC-JURASSIC BOUNDARY

For several decades, the working definition of the Triassic-Jurassic boundary (TJB) has been taken as the LO (lowest occurrence) of the ammonite Psiloceras (usually the species P. planorbis), and an eventual GSSP (Global Stratotype Section and Point) will likely follow this definition. However, there is no direct way to relate the LO of P. planorbis to the nonmarine fossil vertebrate record across the TJB. In the nonmarine strata of the Newark Supergroup of eastern North America, the position of the TJB has been placed just below the oldest basalts of the Central Atlantic Magmatic Province (CAMP) based on palynostratigraphy. This boundary, however, is demonstrably older than the TJB as it is defined in marine strata in Europe. The palynological turnover cited as the TJB in the Newark most resembles a late Norian change in the European sections and does not correspond to any palynological event at the marine-defined TJB. Vertebrate footprint evidence (lowest occurrence of Eubrontes) used to support the palynostratigraphic placement of the TJB in the Newark Supergroup is undermined by the occurrences of Eubrontes tracks in the Triassic of Europe, Africa and Australia and of bones of potential Eubrontes trackmakers in Triassic strata of North America and Europe. Radioisotopic ages indicate that the marine TJB is no older than 200 MA, but the palynostratigraphically-defined TJB in the Newark Supergroup is older than 201 Ma, based on isotopic dates of the oldest CAMP basalts. Magnetostratigraphic correlation to European marine sections also suggests that the palynostratigraphically-defined TJB in the Newark Supergroup is older than the marine TJB. Thus, all data indicate that the TJB in the Newark Supergroup is above the lowest CAMP basalt, probably in the Newark extrusive zone or just above it. Therefore, the position of the TJB in nonmarine strata is actually higher than generally thought, and does not obviously correspond exactly to any bioevent in the fossil record of vertebrates. A useful approximation of the TJB is the LO of the crocodylomorph Protosuchus, index taxon of the Wassonian land vertebrate faunachron (lvf) known from lowest Jurassic strata in North America (Arizona and Nova Scotia) and South Africa.