The precise temporal calibration of dinosaur origins

Abstract

Dinosaurs have been major components of ecosystems for over 200 million years. Although different macroevolutionary scenarios exist to explain the Triassic origin and subsequent rise to dominance of dinosaurs and their closest relatives (dinosauromorphs), all lack critical support from a precise biostratigraphically independent temporal framework. The absence of robust geochronologic age control for comparing alternative scenarios makes it impossible to determine if observed faunal differences vary across time, space, or a combination of both. To better constrain the origin of dinosaurs, we produced radioisotopic ages for the Argentinian Chañares Formation, which preserves a quintessential assemblage of dinosaurian precursors (early dinosauromorphs) just before the first dinosaurs. Our new high-precision chemical abrasion thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages reveal that the assemblage is early Carnian (early Late Triassic), 5-to 10-Ma younger than previously thought. Combined with other geochronologic data from the same basin, we constrain the rate of dinosaur origins, demonstrating their relatively rapid origin in a less than 5-Ma interval, thus halving the temporal gap between assemblages containing only dinosaur precursors and those with early dinosaurs. After their origin, dinosaurs only gradually dominated mid-to high-latitude terrestrial ecosystems millions of years later, closer to the Triassic-Jurassic boundary.

The African cynodont Aleodon (Cynodontia, Probainognathia) in the Triassic of southern Brazil and its biostratigraphic significance

“…Based on radiometric dating of the overlying unit (Santa Cruz Sequence; [43]) and tetrapod correlation with the Chañares Formation of Argentina (Ischigualasto-Unión Basin; [44–45]), the Dinodontosaurus AZ is considered to be Ladinian—early Carnian in age. The only potential non-Brazilian specimen of A .…”

“…Recently, however, radioisotopic dates from the Chañares Formation have placed its typical vertebrate faunal association (from the upper half of the Lower Section; sensu [44]) in the early Carnian (236.1±0.6 Ma; [45]). Concomitantly, preliminary results from the Santacruzodon AZ (Santa Cruz Sequence) have also recovered an early Carnian age (236±1.5 Ma; [43]).…”

“…In addition, the traversodontid Menadon , apparently restricted to the Santacruzodon AZ in Brazil (and also recorded in Madagascar), also has a close phylogenetic relationship with Protuberum , from the Dinodontosaurus AZs, being positioned basal to the former taxon in some phylogenetic hypotheses (e.g., [120, 168]). To this scheme, the age obtained for the Santacruzodon AZ as early Carnian (236±1.5 Ma; [43]) is the same that the typical faunal association of the lower section of the Chañares Formation [44, 45]. However, traditional correlations have been done directly between the Dinodontosaurus AZ with the Chañares Formation (e.g., [51, 143]).…”

Dating the origin of dinosaurs

“…Now, Marsicano et al (10) report high-precision chemical abrasion thermal ionization mass spectrometry U-Pb zircon ages for the Chañares Formation, which indicate a much younger, early Carnian age (234-236 Ma) for this unit-some 5-10 million years younger than previously assumed. Their samples bracketed the main fossil-bearing interval of the Chañares Formation.…”

“…The Manda beds of Tanzania, long considered early Middle Triassic in age, have yielded skeletal remains of an undisputed dinosauriform (Asilisaurus) (16) and an enigmatic taxon, Nyasasaurus, which is known only from a fragment of a remarkably dinosaur-like humerus and some vertebrae (17). Precise temporal calibration of the Manda beds and correlative units elsewhere is critical for testing whether dinosaurs and their closest relatives have a long but as yet largely unrecorded history or, as suggested by Marsicano et al (10), dinosaurs appeared and rapidly diversified at middle to high paleolatitudes in Gondwana at the beginning of the Late Triassic.…”