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ABSTRACT: Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with (40)Ar/(39)Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement.
Science 01/2011; 331(6014):206-10. · 31.20 Impact Factor
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ABSTRACT: 40Ar/39Ar dating of the potassium–magnesium sulfate mineral langbeinite from Permian evaporites of the Salado formation near Carlsbad, New Mexico, provides quantitative evidence that some salts in these deposits have not recrystallized for 251 Myr since deposition. Survival of Permian salts supports the possibility that Bacillus bacteria recovered from nearby halite was isolated in a closed system and represents a sample of uncontaminated Permian life. Local recrystallization of langbeinite and other nearby minerals is also indicated by the dating, suggesting both the need and the opportunity to document closed system behavior more rigorously. The shoaling and desiccation event recorded by the Salado formation began at least 1 Myr before the Permian–Triassic boundary. Temporal correlation of the Salado with the Zechstein evaporites of north-central Europe supports previously inferred regression models for the origin of these deposits. Significant paleoenvironmental change at the Permian–Triassic boundary thus occurred on a time scale more protracted than that implied by geologically instantaneous events such as bolide impacts.
Earth and Planetary Science Letters.
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ABSTRACT: Cosmogenically enriched 38Ar/36Ar ratios are detectable in Ca-bearing terrestrial samples, and are resolvable from a uniform background value of 0.18826±0.00018 for materials lacking nucleogenic or kinetic mass fractionation effects. Relationships between 38Ar/36Ar and cosmogenic 3He in apatite and fluorite are systematic but difficult to reconcile with current estimates of terrestrial production rates for 38Ar and 36Ar from Ca, suggesting that production from Cl and/or other targets is significant, or that the estimates are inaccurate. In either case, the effects of time-varying production of 36Ar from β− decay of cosmogenic 36Cl produced from Ca, K, or Cl on the 38Ar/36Ar ratio must be accounted for.
Earth and Planetary Science Letters.
