Definition, Correlation and Age of the Cynognathus Assemblage Zone, Lower–Middle Triassic of Pangea

The Cynognathus Assemblage Zone (AZ) has been recognized for more than 100 yr and is one of the most durable tetrapod biostratigraphic concepts of the Triassic. Long treated as a single biostratigraphic unit, the Cynognathu s AZ is now divided into three subzones (in ascending order), A, B and C. The South African Cynognathus AZ can be correlated across Pangea to tetrapod assemblages in Algeria, Antarctica, Argentina, Australia, China, Germany, India, Japan, Kazkahstan, Namibia, Pakistan, Poland, Russia, Svalbard, Tanzania, the USA and Zambia. Various index taxa found in the subzones of the Cynognathus AZ in the Karoo basin provide multiple and reinforcing correlations, and independent age constraints combined with marine records of Parotosuchus associated with ammonoids indicate that subzone A is of Olenekian age, primarily Spathian, but perhaps as old as part or all of the Smithian. The early Anisian age of subzone B lacks robust support and is largely based on its stratigraphic position in the Karoo basin between late Olenekian (subzone A) and late Anisian (subzone C) assemblages. In China a subzone B assemblage of the lower Ermaying Formation is overlain by a late Anisian assemblage of the upper Ermaying Formation correlative to subzone C. The Russian Eryosuchus fauna, which is very likely of Anisian age, is at least in part correlative with subzone B, and palynostratigraphy in Australia also indicates an Anisian age for strata (Ashfield Slate) that contain subzone B and C tetrapods. Correlation of subzone C to the later Anisian is primarily supported by a radioisotopic age from China that indicates a subzone C tetrapod assemblage is ~ 243 Ma. The beginning of Perovkan time is close to the beginning of the Anisian, so the Nonesian is the time equivalent to subzone A, and subzones B and C are of Perovkan age. Nevertheless, how close the base of subzone B of the Cynognathus AZ may be to the base of the Anisian is not clear, and a stronger basis is needed to identify the beginning of the Anisian and the beginning of the Perovkan land-vertebrate faunachron in the time interval represented by subzone B of the Cynognathus AZ.