Abstract

The Lower Triassic (Scythian) Stage was a time of great emergence and extremely limited epicontinental seas. Geosynclines are restricted to Tethys and the circum-Pacific region. Shelf seas occupied much of the circum-Arctic region. The marine invertebrate fauna is unique because of its improverished nature. Ammonoids are the predominant group, with bivalves and brachiopods ranking second and third in abundance. Marine Lower Triassic formations include a very wide spectrum of lithofacies, lacking only true reefs. The impoverished nature of Scythian faunas cannot be related directly to lithofacies.

The type area of the lowest Triassic zone, that of Otoceras-Ophiceras, is in the Himalayas. At that locality Ophiceras is the predominant element in the ammonoid faunas of this zone. Otoceras is the second most common element but far inferior to that of Ophiceras. Though generally overlooked, this fauna also includes Glyptophiceras, Anotoceras, Vishnuites, Proptychites, Prionolobus, and Xenodiscoides. All of these genera are so far recognized on the basis of very few specimens. Survivorship of Otoceras into the Lower Triassic is believed to have been longer in Tethys than in the circum-Arctic region.

Scythian ammonoids show a progressive expanding radiation from an Ophi-ceratidae ancestry. Throughout the stage, at every zonal level, the largest number of genera are found in Tethys, while the circum-Arctic faunas are quite impoverished by comparison.