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The Atoka was named and defined as the formation between the Hartshorne Sandstone above and the Wapanucka Limestone below (1900, 1901).
The basal part of the Atoka is considered by some to correspond with the Kessler Member of the Morrow.
Minute fusiform fusulinids are known from the basal Atoka at Clarita, Oklahoma; and it is commonly believed that Fusulina is entirely a post-Atokan genus.
However, "Fusulina" was already described from beds assigned an Atokan age (in New Mexico and Wyoming); and now prolific Fusulina was found about 400 feet below the Hartshorne northwest of Clarita.
It would be logical not to expect lithologic boundaries, no matter how locally persistent, to coincide with major steps in evolution of embedded fossils, no matter how seemingly rapid. Hence, biostratigraphic boundaries postulated on the evidence of evolution of fusulinids need not correspond with lithologic boundaries, although they may do so at places. The first appearance of the fusiform fusulinid Fusulinella appears to coincide roughly with the basal sediments of the Atoka, at least in the type area of the Atoka; but the first appearance of Fusulina is undoubtedly well within the upper and perhaps even middle part of the original Atoka.
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