TERTIARY LAND BRIDGES AND THEIR CLIMATE: BACKDROP FOR DEVELOPMENT OF THE PRESENT CANADIAN INSECT FAUNA

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Abstract
Studies of Quaternary age Coleoptera fossils from North America and Europe plus recent analyses of insect fossils from late Tertiary (Miocene and Pliocene) sites in northern Canada, Alaska, and eastern Siberia show that many of the species composing the present Canadian insect fauna probably evolved during the Tertiary. Thus the history of Tertiary land bridge links between North American and adjacent continents is pertinent for a discussion of the present Canadian insect fauna.Movement of lithosphere plates has contributed directly to the emergence and foundering of Tertiary land bridges in three key areas: Beringia, the North Atlantic, and Middle America.A land bridge probably connected Siberia and Alaska in the Beringian region for most of the Tertiary. During the Eocene its climate was such as to allow exchange of tropical to subtropical adapted plants and animals. Paleobotanical evidence suggests that the Beringian area never supported the type of mixed-mesophytic forests called for by the Arcto-Tertiary concept. By the mid-Miocene much of the region north of 65° was probably a rich coniferous forest. Lowland tundra undoubtedly exited on the fringes of Beringia as early as the Pliocene.Two and possibly three land bridges connected Greenland and Northern Europe during the Tertiary. The DeGeer bridge joined northern Greenland and Norway prior to the opening of the Greenland Sea in the late Eocene – Early Oligocene. The Greenland-Faeroes bridge was never more than a causeway along the route of the submerged ridge now occupied by Iceland. However it persisted longer, possibly existing as a chain of islands as late as the Miocene. A third land bridge may have connected Labrador and Rockall Bank near England during the Paleocene and early Eocene. The climatic history of the North Atlantic region is poorly known, but apparently sites as far north as Ellesmere Island were warm enough during the Paleocene to support subtropical to warm temperate mammalian faunas.For most of the Tertiary the middle American region remained isolated from South America. Changes in movements of several lithosphere plates created conditions appropriate for north-south island dispersal as early as the Miocene but a true land connection, the existing isthmus, did not form until the Pliocene. Paleontological evidence suggests that tropical conditions were not prevalent there until after that event.