Ammonoids of the Lower Triassic (Scythian) offer a unique opportunity to study a broad radiation of a group of invertebrate animals. This is, of course, due to the near extinction of ammonoids in the Late Permian. The survivorship of ammonoids across this threshold is extremely limited. Two of the surviving families led to no future radiations, the third, however, was the stem of the main radiation of Triassic ammonoids which include more than 400 genera. This particular threshold stock gave rise to approximately 140 genera during the Early Triassic (Scythian) alone.

Biochronology of the Lower Triassic (Scythian) began with the first proposal of a definition of this time unit by Mojsisovics, Waagen, and Diener (1895). Their proposal was based primarily on studies of ammonoid sequences in the Salt Range of West Pakistan by Waagen (1895) and in the Himalayas by Diener (1897). In the ensuing years much new data accumulated, clearly demonstrating that the original scheme encompassed no more than the lower half of the Lower Triassic (Scythian). In this period there gradually developed an elaborate sequence of zones. Each author of a particular regional faunal sequence laid claim toward "completeness" and "finality" for his arrangement. It is now very apparent that a scheme of 10 to 14 separate zones for purposes of worldwide correlation is impractical and unreal. It now seems probable that only four such zones serve for an international biostratigraphic scheme.

An evaluation of all Lower Triassic ammonoid genera now recognized is essential in reconstituting a biostratigraphic framework. Of the 136 genera recognized for this state, 116 are present in the Tethys, only 52 in the circum-arctic region, and 73 in an intermediate region. In the evolutionary radiation of Scythian ammonoids there is a continuing expansion of numbers of taxa from the lowest to highest zone. At each of these recognized zonal steps the predominance of Tethys in numbers and diversity of taxa over that of the circum-arctic region is maintained at a ratio of approximately 2 to 1. Endemic elements are to a large extent confined to Tethys.

It has long been accepted that the lowest Scythian zone is that of Otoceras-Ophiceras as defined by Diener (1897; 1912). Discussion of this zone has always focused primarily on the genus Otoceras and this fact has led to a lack of perspective. In the Himalayas Otoceras and Ophiceras are associated, Ophiceras being by far the predominant element in the

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Stages, zones and associated flora and fauna of the Triassic of Spitsbergen.

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fauna. The phylogenetic positions of Otoceras and Ophiceras are very different. The genus Otoceras is a derivative of Pseudotoceras, a genus which is part of an impressive radiation, mainly within Tethys, encompassing the family Araxoceratidae Ruzhentsev. Otoceras is the last surviving element of a Late Permian radiation. On the other hand, Ophiceras is a derivative of a xenodiscid stock and, in the earliest Scythian, underwent a very great evolutionary radiation. It is from this stock that most other Scythian taxa are directly or indirectly derived. There are thus two contrasting taxa that have commanded attention in recognition of this lowest zone, one a last surviving element, and the other the initiation of a significant widespread radiation. The duration of Otoceras into the Lower Triassic is most probably not the same within its known areas of distribution.

What has been ignored in the record of the Himalayan Otoceras-Ophiceras beds is the presence of a number of other taxa; these include Anotoceras, Proptychites, Prionolobus, Xenodiscoides, Glyptophiceras, Episageceras, and Vishnuites. There has been controversy as to the validity of at least some of these records. Careful review by the author of the literature, and some new collections, appear to substantiate Diener's original claims that these specimens were collected from the lowest zone of the Triassic.

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