The Middle Ordovician of the Oslo Region, Norway: 16. Graptolites of the Ogygiocaris series

Fifty-three graptolite forms representing sixteen genera are described from the lower part of the Oslo Region Middle Ordovician, the Ogygiocaris Series. Five new species are named and so, too, are five new subspecies of previously-described species. Two graptolite Zones, those of Didymograptus murchisoni and Glyptograptus teretiusculus, are recognized within the rocks of the Ogygiocaries Series. The congregation of forms characteristic of the Didymograptus murchisoni Zone occurs in 4aa1 , 4aa2, and the basal part of 4aa3. The typifying congregation of species of the Glyptograptus teretiusculus Zone occurs in the upper part of 4aa3 and in 4aa4. Five stratigraphic sections in the Oslo-Asker district and a total of thirty-eight collections form the basis for the zonal interpretation. Only three of the collections were obtained outside of the Oslo-Asker district. The graptolite Zones are of the same kind as those recognized by Oppel (1856-1858) in his work on ammonites from the Jurassic System. They are time-stratigraphic subdivisions of the most refined sort and may be recognized within one biogeographic province. A new species of amplexograptid is noted to possess lists that form a sort of clathria. The lists are like those of Gymnograptus linnarssoni and to some extent like those of Lasiograptus retusus. The species may represent a lineal antecedent of the lasiograptids, and its presence suggests that lasiograptids descended from amplexograptids, as is also indicated by similarity in thecal shape of the two genera.