| Summary: | 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:
Yakouma 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, lonica, 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 t.he 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 continuing 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 westeril 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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