I am really starting to wonder if the Anasibirites Beds could preserve an opportunistic bloom. If the Prionitids (and not all the other ammonoids) began or found a way to prey on the micro gastropods of the time they may have had a great advantage and a large amount of food. Whether the beds in the Confusion Range preserve a mass mortality event or just the preservation of an unusually high amount of dead shells would need an answer.
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