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In the Jemez Springs area of Sandoval County, northern New Mexico, siliciclastic red beds of the upper Paleozoic Abo Formation are well exposed and yield fossil plants and vertebrates. The local Abo Formation section is more than 190 m thick and rests disconformably on the Upper Pennsylvanian Guadalupe Box Formation and is conformably overlain by the Lower Permian DeChelly Sandstone (Yeso Group). Abo sandstone sheets are low sinuosity river deposits, and intercalated sandstone beds and lenses represent sheet splays and minor channel fills that formed during overbank flooding. The dominant Abo lithofacies is mudstone, which represents floodplain deposits, many with calcareous paleosols. Fossils are present in three stratigraphie intervals of the lower to middle Abo Formation. All three intervals yield eupelycosaur-dominated vertebrate fossil assemblages of Coyotean age (Coyotean = late Virgilian-Wolfcampian on the North American provincial marine timescale: Lucas 2006). The lowest interval also yi...

The Triassic chronostratigraphic scale was built on two centuries of research on ammonoid biostratigraphy and biochronology. Two Triassic stage bases and all of the Triassic substages are currently defined by ammonoid bioevents. The study of Triassic ammonoids began during the late 1700s, and in 1895, Edmund von Mojsisovics, Wilhelm Waagen and Carl Diener published an essentially complete Triassic chronostratigraphic scale based on ammonoid biostratigraphy. This scale introduced many of the Triassic stage and substage names still used today, and all terminology of stages and substages subsequently introduced has been based on ammonoid biostratigraphy. Early Triassic ammonoids show a trend from cosmopolitanism (Induan) to latitudinal differentiation (Olenekian), and the four Lower Triassic substage (Griesbachian, Dinerian, Smithian and Spathian) boundaries are globally correlated by widespread ammonoid biotic events. Middle Triassic ammonoids have provinciality similar to that of the Olenekian and provide a basis for recognizing six Middle Triassic substages. Late Triassic ammonoids provide a basis for recognizing three stages divided into five substages. The main uncertainty for the future of Triassic ammonoid biostratigraphy is not the decline of the ammonoids as a tool for dating and correlation of Triassic strata but, rather, the dramatic decrease in the number of specialists, due to the lack of replacement of experienced palaeontologists who started their activity in the 1950s and 1960s.