Graptolite species have long been considered to have been world-wide in occurrence. Consideration of interregional correlation has revealed that 3 faunal regions may be recognized for the Lower and in the early part of the Middle Ordovician. Camaroids, tuboids, stolonoids, and many dendroids were benthonic, encrusting shells or other firm objects, or attached to the bottom. Graptoloids were planktonic and lived in the surface layers of the water; many floated at some depth beneath the surface. The latter types were not so widely dispersed as the former. Many geographically widespread species had visible float structures, whereas those without such structures were limited in their geographic range. Graptolites are not restricted in general to black shales or to fine-grained or quiet-water deposits. They do occur in many sedimentary rock types and in those that formed under turbulent as well as quiet conditions. Graptolites are not restricted to eugeosynclinal deposits. Many well-preserved specimens and the most nearly complete successions of assemblages are known from miogeosynelinal deposits. A broader knowledge of graptolite occurrence should lead to a better understanding of their mode of life and their usefulness for correlation.

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