Ammonoids have long provided the basis for age assignments and correlations of Lower Triassic (Scythian) marine strata. Although the early Triassic marine faunas were of low diversity and abundance relative to those of the late Permian, ammonoids are widely distributed in Scythian strata. Areas with Lower Triassic fossiliferous marine rocks, including ammonoids, are shown in Figure 18.1. Some 136 ammonoid genera are recognized for the Scythian (Kummel, 1973), and various zonal schemes show detailed subdivisions of that important geologic interval (Tozer, 1967, 1974; Silberling and Tozer, 1968). However, ammonoids are not always present in sufficient numbers for biostratigraphic use.

Coincident with the widespread use of acetic acid for dissolution of conodont-bearing carbonate rocks, the recovery and study of these small phosphatic elements provided a distinctive and cosmopolitan microfossil alternative for Triassic marine biostratigraphy.

Lower Triassic conodonts and evolution of a zonation

Early studies of conodont elements focused primarily on the stratigraphic distribution of diagnostic forms, and the literature burgeoned. By 1950, widespread conodont faunas from Ordovician rocks through Permian rocks had been described in the United States. A full description of a Lower Triassic conodont fauna, however, was not published until the work of Müller (1956) in Nevada, who also suggested the potential for a timesignificant biostratigraphic succession. Later, Sweet's prototype Lower Triassic zonation, based on comprehensive collections from West Pakistan, stimulated worldwide study of equivalent faunas (Sweet, 1970).