The cheilostome bryozoan genus Metrarabdotos occurs abundantly in late Eocene to Recent sediments in the Gulf and Atlantic coastal provinces, the Caribbean region, Europe, and northern and western Africa. The tropical distribution of Recent populations is apparently a consequence of a temperature tolerance of 61° to 82°F. This stenothermy and the distinctive generic morphology of fossil and Recent species provide a basis for identifying tropical faunal provinces on the sublittoral margins of both sides of the Atlantic during late Paleogene and Neogene times.

The northern limit of the genus, congruent in Recent seas with the 70°F. isocryme for surface water, has fluctuated during late Paleogene and Neogene times. The amplitude of latitudinal fluctuation has been 8° in the New World and 34° in Eurafrica. The synchronous latitudinal shifts of this line on both sides of the Atlantic probably reflect variations in Atlantic marine climate. The northern boundary of the genus lay at 31° N. Lat. in the Gulf and Atlantic Coastal Plain and at 40°-48° N. Lat. in Europe from late Paleogene to middle Miocene, at about 26° in America and 28° in Europe in late Miocene, and at its northern maximum of 34° in America and 51° in Europe in the Pliocene; it now lies at 29° in the Gulf of Mexico and at 17 76; in Africa. The greater amplitude in Eurafrica is probably the result of a surface-current gyre similar to the modern one. The pattern of increasing amplitude of fluctuation does not agree with a paleo-climatic model of cooling from early to late Tertiary time.

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