Furin et al (2006) re-calibrated the Rogers et al. (1993) 40Ar/39Ar age. As they explained, this age also needs to be adjusted to account for the bias in the 40Ar/39Ar timescale, which systematically yields ages that are ~1% too young (e.g., Min et al 2000; Mundil et al. 2006; Kuiper et al. 2008). Therefore, the actual age of the Herr Toba bentonite from the Ischigualasto Fm dated by Rogers et al. (1993) is closer to 231 Ma (see pg. 1011 of Furin et al. 2006). Combined with the additional revisions of the Late Triassic timescale (e.g., Muttoni et al. 2004; Furin et al. 2006), this indicates a middle to late Carnian age for the base of the Ischigualasto Formation. Thus, the presence of sauropodomorphs at the base of the Ischigualasto Fm does not necessarily support a Middle Triassic diversification of Saurischia.

Randall Irmis


References:
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Kuiper, K. F., A. Deino, F. J. Hilgen, W. Krijgsman, P. R. Renne, and J. R. Wijbrans. 2008. Synchronizing rock clocks of earth history. Science 320:500-504. DOI: 10.1126/science.1154339

Min, K., R. Mundil, P. R. Renne, and K. R. Ludwig. 2000. A test for systematic errors in 40Ar/39Ar geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite. Geochimica et Cosmochimica Acta 64:73-98. DOI: 10.1016/S0016-7037(99)00204-5

Mundil, R., P. R. Renne, K. K. Min, and K. R. Ludwig. 2006. Resolvable miscalibration of the 40Ar/39Ar geochronometer. Eos Transactions AGU 87:F2218.

Muttoni, G., D. V. Kent, P. E. Olsen, P. Di Stefano, W. Lowrie, S. M. Bernasconi, and F. M. Hernandez. 2004. Tethyan magnetostratigraphy from Pizzo Mondello (Sicily) and correlation to the Late Triassic Newark astrochronological polarity time scale. Geological Society of America Bulletin 116:1043-1058. DOI: 10.1130/B25326.1

Rogers, R. R., C. C. Swisher, III, P. C. Sereno, A. M. Monetta, C. A. Forster, and R. N. Martínez. 1993. The Ischigualasto tetrapod assemblage (Late Triassic, Argentina) and 40Ar/39Ar dating of dinosaur origins. Science 260:794-797. DOI: 10.1126/science.260.5109.794