Abstract

Devonian marine biogeography changes from a highly provincial Early Devonian (highest in the latest Early Devonian) to a very cosmopolitan Late Devonian level. There is a parallel trend from a globally high climatic gradient in the Early Devonian to a very low gradient in the Late Devonian. Correlations between changing levels of orogeny and regression-transgression with levels of provincialism are poor, especially when considered against similar quantities in other periods. There are a larger number of biogeographic units, of course, in the Early Devonian as contrasted with the Late Devonian. The “cause” or “causes” behind changing levels of Devonian provincialism are poorly understood, but it is inferred that changing oceanic current circulation barrier positions, plus changing global climatic gradients may be the first order controls and that they, in turn, may have been largely controlled by changing global geography.

Combined consideration of changes in global climatic gradients, minor, although widespread extinction events affecting the marine benthos, and the disappearance of the Malvinokaffric Realm near the end of the Eifelian, and of the Eastern Americas Realm in the later, but not latest, Givetian, indicate a possible cause and effect relationship between the two minor extinction events, and possibly abrupt lowerings in the global climatic gradient. The overall importance of possibly changing paleogeography in permitting global changes in climatic gradient remain to be evaluated for these two cases.