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An extinction at the base of the anasibirites beds, and a small climate gradient left the anasibirites beds to be very cosmopolitan. Perhaps they were just the most common ammonoid in the latest smithian so any unfavorable water or climatic condition would have killed only the anasibirites fauna.
Still so much to learn and figure out in all this.
Galfetti, T., H. Bucher, A. Brayard, P. A. Hochuli, H. Weissert, K. Guodun, V. Atudorei, J. Guex, 2007, Late Early Triassic climate change: Insights from carbonate carbon isotopes, sedimentary evolution and ammonoid paleobiogeography,
Palaeogeography, Palaeoclimatology, Palaeoecology 243, 394–411
Kidwell, S.M. 1991. The stratigraphy of shell concentrations. In: P.A. Allison and D.E.G. Briggs (eds.), Taphonomy: Releasing the Data Locked in the Fossil Record, 211–290. Plenum Press, New York.
DIANA L. BOYER, DAVID J. BOTTJER and MARY L. DROSER, 2004, Ecological Signature of Lower Triassic Shell Beds of the Western United States, PALAIOS, v. 19; no. 4; p. 372-380
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