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Jakobs, G.K., 1992

Toarcian (Lower Jurassic) ammonite biostratigraphy and ammonite fauna of North America

Bibliographic Reference

Jakobs, G.K., 1992, Toarcian (Lower Jurassic) ammonite biostratigraphy and ammonite fauna of North America: Vancouver, British Columbia, Canada, University of British Columbia, Ph.D. dissertation, 697 p., illust., maps, photos.


Toarcian ammonite collections from British Columbia, Nevada, Oregon, the Yukon, and Alaska form the basis of a detailed taxonomic study. Fifty-seven species are described, allocated to 27 genera, one of which is new (Yakounia). Nine new species are introduced: Yakounia yakounensis, Y. pacifica, Y. freboldi, Y. silvae, Pleydellia maudensis, P. crassiornata, Phymatoceras hillebrandti, Leukadiella n. sp. A, and Leukadiella n. sp. B. Measured sections from the Queen Charlotte Islands and other key areas in western North America were used to distinguish six successive assemblage zones: Kanense, Levisoni, Ionica, Crassicosta, Hillebrandti, and Yakounensis. All six zones, which can be recognized from southern Alaska to Nevada, are defined here for the first time. The Arctic basins (Sverdrup and Brooks-MacKenzie) have a low diversity ammonite faunal sequence similar to that of Siberia and the zonation developed there can be used in Arctic North America. The Toarcian of western North America is most commonly represented by argillaceous sediments. On the craton, in the Sverdrup basin, a transgression in the Middle Toarcian suggests a link with eustatic sea level changes. On the terranes, two intervals of coarser grained sedimentation can be recognized, one during the Crassicosta Zone and one during the Yakounensis Zone and containing into the Aalenian. These intervals are also probably related to eustatic sea level changes. A paleobiogeographic study of the similarity of ammonite faunas between different areas used two methods: a Monte Carlo simulation, in which randomly generated data sets provide confidence levels for similarity coefficients, and complete linkage cluster analysis. The Monte Carlo method corrects for sparseness in the data set; cluster analysis does not and should be used with caution in similar studies. The ammonite fauna of western North America includes taxa with pandemic, Tethyan, Boreal, Pacific, East Pacific, and Athabascan affinities. Several possible migration routes exist that could explain the similarity of western North American faunas to those of western Tethys. An analysis of the similarity of the western North American fauna to other areas, at the generic level, shows that migration of endemic forms occurred via the Hispanic Corridor, coinciding with periods of high sea level.

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