Triassic Time and Ammonoids: Problems and Proposals

Abstract

Problems arising from the classification and dating of Triassic rocks and ammonoids are reviewed. It is emphasized that all chronostratigraphic units must be based on stratotypes in order to achieve an unambiguous time scale. Explanations are sought for anomalies encountered when comparing ammonoid age ranges determined in North America with alleged ranges based on European and Asian occurrences. Most can be resolved by taking into account recent research on penecontemporaneous solution processes or by invoking condensation. Both have resulted in the mixing of natural faunas at European and Asian localities. Review of problems of ammonoid classification and taxonomy indicates that the recognition of a variable species is only possible when the full range of variation has been established in a population, i.e. from a series of specimens of one age obtained from one place. For discriminating higher taxa the evidence indicates that no single character, of either the inner of outer whorl, is of overriding significance for determining phylogenetic relationships. The total absence of marine Triassic rocks on the Atlantic shores supports the conclusion that this Ocean did not exist in Triassic time, but distribution of ammonoids provides some evidence to suggest that Laurasia and Gondwanaland may have been separated in the Middle Triassic. It is suggested that Triassic limestone in red Hallstatt facies may be encountered by the Deep Sea Drilling Project in Wharton Basin, west of Australia. Appendixes include: I, a discussion of the Triassic Time Scale; II, a new classification of all Triassic Ammonoidea; III, diagnoses of new taxa (seven new families, three new subfamilies, and nine new genera).