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Palaeozoic |
Department of Geology & Geophysics, University of California, Berkeley, CA 94720, USA
Late Ordovician to Early Silurian graptolite-bearing sequences have been recognized in three primary western North American areas: southeastern Alaska, northern Canadian Cordillera and the Great Basin (Nevada-Idaho). A spectrum of marine environments is represented in strata in these sequences, ranging from those of volcanic islands and deep oceans to those of shelves and shelf margins. During the time of glacial maximum, the time of the extraordinarius zone, sea-level lowered and oxic conditions prevailed on the ocean floor. The prominent Late Ordovician near-extinction among graptolites denotes the pacificus—extraordinarius zone boundary. Normalograptids appear to be the only taxa in the extraordinarius zone. They are the prominent taxa in the superjacent persculptus and acuminatus zones. Normalograptids are joined by a few diplograptids in the persculptus zone, and by a few more diplograptids as well as Parakidograptus and Cystograptus in the acuminatus zone. The first monograptids appear in the atavus zone in western North America. The atavus zone fauna is primarily diplograptids. Significant sea-level rise and transgression across the shelf following after deglaciation did not occur in western North America until the acinaces zone. At that time, new monograptids, including Lagarograptus, Pristiograptus and Pribylograptus, appeared as did many new diplograptid taxa. Stratigraphic occurrence data indicate that the major post near-extinction reradiation took place in western North America as shelf seas expanded and shelf sea habitats became increasingly available. That pattern of new taxa appearing during transgression continued into the gregarius zone.
