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The basic biostratigraphic unit of the Ordovician of the northeastern USSR is the horizon. Subdivisions of the horizons, termed “zones,” are proposed on the basis of graptolite occurrences. These zones are correlated with the unified standard graptolite scale of the USSR and with graptolite scales of other regions.

Graptolite zones are not established for the Tremadoc and most of the Arenig series. Graptolite development in upper Arenig rocks of the northeastern USSR is very similar to that of North America, China, and Australia, which are assigned to the Pacific Zoogeographie Province. In these areas, the genera Isograptus, Cardio-graptus, and Phyllograptus are widely developed, whereas in England Didymograptus prevails.

The Llanvirn of the northeastern USSR, North America, China, and Australia is characterized by the graptolite genera Americograptus, Paraglossograptus, and Cardio-graptus, as well as the abundant Didymograptus. In England these genera are absent, and the Didymo-graptus bifidus Zone is difficult to distinguish from the overlying D. murchisoni Zone (together composing the Llanvirn). In the Landeilo and lower Caradoc, graptolite development is more uniform throughout the world. Provincial isolation is distinctly expressed again from the middle Caradoc through the Ashgill sequence. Graptolite associations indicate that the northeastern USSR, North America, China, and Australia were a single Pacific paleozoogeographic province in the Ordovician. England and the other European countries are assigned to the Atlantic Province.

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