SOUTH AMERICAN OCCURRENCES OF THE ADAMANIAN (LATE TRIASSIC:LATEST CARNIAN) INDEX TAXON STAGONOLEPIS (ARCHOSAURIA: AETOSAURIA) AND THEIR BIOCHRONOLOGICAL SIGNIFICANCE

Abstract

Aetosaur fossils from the Upper Triassic Ischigualasto Formation of Argentina formerly assigned to Aetosauroides scagliai Casamiquela and Argentinosuchus bonapartei Casamiquela are reassigned to Stagonolepis robertsoni Agassiz (small specimens) and S. wellesi (Long and Ballew) (larger specimens). Numerous features of the skull, vertebral column, appendicular skeleton, and particularly the armor, of these Argentinian aetosaurs are identical to those of Stagonolepis and differ from other aetosaurs. Identification of Stagonolepis in South America has important implications for the correlation of nonmarine strata across Late Triassic Pangea. Stagonolepis is an index taxon of the Adamanian land-vertebrate faunachron of latest Carnian age. Its occurrences in the Ischigualasto and Santa Maria formations, as well as at the type locality in the Lossiemouth Sandstone of Scotland and in the Blasensandstein of the German Keuper, cross-correlate with a well-established tetrapod biostratigraphy of the Chinle Group in western North America. Thus, all Stagonolepis records are of Adamanian (latest Carnian) age, not Otischalkian (early or “middle” Carnian) age, as proposed by some earlier workers. This correlation also demonstrates that the oldest known dinosaurs are not from the Ischigualasto and Santa Maria formations. Previous workers obtained radioisotopic dates of 227.8 Ma from the Ischigualasto Formation, providing a maximum numerical date for Stagonolepis-bearing units. This date must be late Carnian, not of Ladinian age, and thus supports the apparent age of the Ladinian-Carnian boundary (232 Ma) of time scales based on the Newark Supergroup in eastern North America.