Late Silurian—early Devonian biogeography, provincialism, evolution and extinction

Abstract

There is increasing marine to continental regression from the latest Silurian until the latter half of the early Devonian, when a major transgressive trend is initiated which achieves its maximum in the later middle Devonian and late Devonian. Data suggest a relatively high climatic gradient, but no evidence favouring continental, sea-level glaciation during the late Silurian-early Devonian. Arid climate evidence (marine evaporites, calcretes) shows a well-developed arid belt. Coal deposits are lacking before the late Devonian. Palaeogeography of the time interval is disputed, largely owing to the use of different classes of data - remanent magnetic, lithological, biogeographical. I employ a pangaeic reconstruction because it fits the available lithological and biogeographical data comfortably, but I am under no illusions about its being the `correct' palaeogeography. Rate of phyletic evolution of marine benthos speeds up during the time interval owing to a steadily increasing level of provincialism, that is, cutting up biogeographical entities into smaller entities with consequent smaller populations. There are no major marine adaptive radiations, nor evidence for any marked extinction events during the interval. Few family level and higher taxa become extinct during this time interval; such units as the halysitid corals, pentamerinid brachiopods, and graptoloid graptolites are exceptional. Few adaptive radiations, such as those of the ammonoids and terebratuloids occur during the interval. The absence of other major biotic events during the interval is consistent with its position well within ecologic-evolutionary unit VI (A. J. Boucot, J. Paleont. 57, 1-30, 1983).