Re-positioning the Triassic-Jurassic Boundary in the main Karoo Basin

MARSICANO, CLAUDIA; SMITH, ROGER; WILSON, JEFF

Teruel

Conferencia; 10th International Symposium on Mesozoic Terrestrial Ecosystems and Biota; 2009

The Triassic-Jurassic sequences in the southern African Karoo Basin comprise the Molteno, Elliot and Clarens formations capped by the Drakensberg volcanics. The basal Drakensberg basalts have a radiometric age of ca. 183 Ma. However, the lack of radiometrically datable minerals within underlying sediments has necessitated a reliance on biostratigraphic age control. In particular, the stratigraphic position of the Triassic-Jurassic (TJ) boundary has always been poorly constrained and is currently considered to lie within the middle part of the Elliot Formation. The Elliot and Clarens formations host one of the richest assemblages of early Mesozoic tetrapods, including dinosaurs, crurotarsal archosaurs, tritylodontids, early mammals and turtles. In addition to body fossils, tetrapod footprints are well represented and have always played an important role in the placement of TJ boundary. We have re-evaluated the stratigraphic position, sedimentological context and ichnology of the well-known Moyeni trackway locality in southern Lesotho, whose stratigraphic position we interpret to be close to the base of the upper Elliot Formation.   The Moyeni surface contains several distinct track-types, mainly represented by tridactyl footprints and trackways of bipedal dinosaurs. Analysis of the paleosurface has yielded unambiguous “chirotheroid”-type footprints generally accepted as having been produced by non-crocodylomorph crurotarsan archosaurs.  Recently, fragmentary remains this group of archosaurs were recovered from Upper Elliot strata in South Africa. The co-occurrence of footprints and body fossils of non-crocodylomorph crurotarsan archosaurs in strata attributed to the Early Jurassic is crucial in our understanding of the evolution and radiation of continental tetrapods. Three possible scenarios could explain their presence in this assemblage. First, the upper part of the Elliot Formation could be wrongly dated as Early Jurassic and should be reassigned a Late Triassic age. If this is proven correct, it would bring all the associated fauna including the chelonians, basal mammals and several basal crocodylomorphs, into the Late Triassic. Second, the bones/tracks could be misidentified, and they actually belong to an animal that is unknown from the Jurassic, a scenario that we consider unlikely. The third explanation is that the remains are correctly identified and the age is correctly assigned, but the accepted global extinction of non-crocodylomorph crurotarsal archosaurs at the end of the Triassic is incorrect. Deciding which scenario is correct has important implications for understanding the patterns of diversification of the Gondwanan tetrapod faunas during the early Mesozoic. If the Upper Elliot is Late Triassic in age, then the early diversification of several clades of tetrapods, e.g. chelonians, and mammals, occurred in the Triassic, altering estimates of the magnitude of the tetrapod faunal extinction at the Triassic−Jurassic boundary in this part of Gondwana. If the Upper Elliot is Early Jurassic in age, then this new faunal association again reduces the severity of the end-Triassic extinction.