Studies on continental Late Triassic tetrapod biochronology. II. The Ischigualastian and a Carnian global correlation

Abstract

The Ischigualastian represents a key land vertebrate faunachron for the correlation of Late Triassic terrestrial deposits worldwide, based on the abundant and diverse tetrapod fauna of the Ischigualasto Formation, NW Argentina. The first appearance data of the rhynchosaur Hyperodapedon and the dicynodont Jachaleria define its lower and upper limits, respectively. Fossil taxa that characterize the Ischigualastian include Hyperodapedon, the cynodont Exaeretodon, the aetosaur Aetosauroides, and herrerasaurid dinosaurs. On the basis of faunal similarities, the Ischigualastian can be traced throughout south Pangea to encompass the fossil assemblages of the Hyperodapedon assemblage zone, Santa Maria Formation, south Brazil; the Pebbly Arkose Formation, Zimbabwe; and the Lower Maleri Formation, India. This correlation contradicts tetrapod-based Late Triassic biochronologies that divide the Carnian epoch into two land vertebrate faunachrons, Otischalkian and Adamanian, on the basis of the succession of faunas in some North American continental sequences. However, the extension of these biochronologic units into south Pangea is based mainly on dubious records of index fossils, the taxonomic status of which are not clearly understood. By reassessing the taxonomic status and distribution of tetrapod taxa, this article defines an updated correlation basis for the Late Triassic of Pangea. q

The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades

“…Triassic (Langer, 2005a), Alemoa local fauna (sensu Barberena et al, 1985, andAzevedo et al, 1990 Huene (1942), the specimen was found in ''Sanga 6'' in the Alemoa area. The exact stratigraphic position of the specimen may never be known; however, it was found with other taxa in the ''Alemoa local fauna'' of Barberena et al (1985) and Langer (2005a). Langer (2005aLanger ( , 2005b considered the ''Alemoa local fauna'' to correlate with the lower portion of the Ischigualasto Formation.…”

“…The exact stratigraphic position of the specimen may never be known; however, it was found with other taxa in the ''Alemoa local fauna'' of Barberena et al (1985) and Langer (2005a). Langer (2005aLanger ( , 2005b considered the ''Alemoa local fauna'' to correlate with the lower portion of the Ischigualasto Formation. If this correlation holds, Rauisuchus would be considered late Carnian because the base of the Ischigualasto Formation is dated at 228 Ma .…”

The origin and early radiation of dinosaurs

“…Early dinosaur-bearing strata from southern Brazil are probably of similar age based on correlations to the Ischigualasto Formation using vertebrate biostratigraphy (e.g., Schultz et al, 2000;Langer, 2005;Langer et al, 2007a). The Chinle Formation of the Colorado Plateau in western North America is traditionally considered late CarnianNorian in age (e.g., Litwin et al, 1991;Lucas, 1998), but new U-Pb radioisotopic age constraints indicate that even the oldest fossiliferous strata are Norian in age (Riggs et al, 2003;Irmis and Mundil, 2008;Mundil et al, 2008).…”

“…Staurikosaurus is known from the Santa Maria sequence in Brazil, which is approximately the same age as the Ischigualasto Formation (Langer, 2005;Langer et al, 2007a). Staurikosaurus is represented by a single partial skeleton, which is substantially less complete than specimens of Herrerasaurus, including the mandible, most of the vertebral column, pelvic girdle, and partial hindlimbs (Colbert, 1970).…”

“…Until the discovery of Panphagia, Saturnalia tupiniquim from the upper Santa Maria Formation of southern Brazil (Langer et al, 1999(Langer et al, , 2007cLanger, 2003) was the most basal sauropodomorph known. This taxon is from strata that are biostratigraphically correlative with the Ischigualasto Formation (Langer, 2005;Langer et al, 2007b). Saturnalia is known from several specimens that together preserve most of the skeleton.…”