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The systematics and paleobiology of Hettangian ammonites from the allochthonous terranes of British Columbia Longridge, Louise M.

Abstract

The Hettangian is a poorly understood 3 million year time interval following the Triassic-Jurassic mass extinction during which the biosphere struggled to rebuild its diversity. Hettangian ammonite faunas are examined from the two key areas of the terranes of British Columbia, Taseko Lakes and the Queen Charlotte Islands (QCI). In total, 90 different species or taxonomic groups are described representing 32 genera. Thirteen new species are recognized. Stratigraphic ranges are identified for each taxon and the North American Zonation is updated so that it is more representative of Canadian sequences. Except for the definitive occurrence of the Spelae Zone, all Hettangian zones of the North American Zonation are present in the BC terranes. A section on Kunga Island in the QCI demonstrates a spectacular radiolarian turnover across the Triassic-Jurassic transition interval (T-J boundary), provides a radiometric date to constrain the boundary, contains ammonites which permit correlations with other areas and can be correlated with a carbon curve which shows a distinct negative excursion just below the radiolarian turnover. This section is proposed as a potential Global Stratotype Section and Point for the basal Jurassic if radiolarians are selected as the primary standard for defining the T-J boundary or as a parastratotype section to assist with characterizing the interval if radiolarians are not selected. Detailed correlations are suggested between the Hettangian and lower Sinemurian of the terranes and other areas of North America, South America, New Zealand, western and eastern Tethys, and northwest Europe. The lateral distribution of ammonite faunas suggest there may have been some longitudinal separation between the craton and the Cadwallader, Wrangellia and Peninsular terranes during the Hettangian. They also support a significant northward displacement for Wrangellia relative to the craton since Early Jurassic time and suggest that the Hispanic Corridor connecting the Panthalassa and Tethys oceans may have been open during the Hettangian. Sexual dimorphism is recognized in several Hettangian genera including Kammerkarites, Eolytoceras, Sunrisites and Badouxia. Asymmetries in the vertical position of the internal elements of the phragmocone of B. columbiae were assessed for a potential counterbalance mechanism using computer modeling. No counterbalance mechanism was recognized.

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