**2. Palaeozoic era**

At the beginning of the Palaeozoic Era, Laurasia was near the equator on the periphery of Pangaea. The latter was a large land mass consisting of the progenitors of the existing continents that were located around the South Pole at that time. In the Cambrian Period (600-500 million years ago), North America was orientated so that the equator ran from western Mexico northwards to the Arctic Ocean along the Yukon-NWT border (Briden & Irving, 1964). During the Ordovician Period (500-425 million years ago), the continent rotated so that the equator lay in a line from just south of Baja California northeastwards to the east coast of Hudson Bay. The rotation of North America continued and by the Carboniferous Period (345-280 million years ago), it passed through San Diego east-north-

© 2012 Hariss, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Hariss, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

eastwards to Cape Breton. Thus the North American plate was rotating in a clock-wise direction as well as moving south relative to the Earth's magnetic poles, which are assumed to approximate the poles of rotation. Throughout this time, it lay in the Tropics.

The Role that Diastrophism and Climatic Change

Have Played in Determining Biodiversity in Continental North America 235

Amongst the new genera and species that evolved on this new landscape were the first Laurasian stoneflies (Plecoptera) and several other groups of insects (Illies, 1965; Zwick and Teslenko, 2000). The best record of these insects comes from the Baltic amber of Eocene age (38-54 million years ago) but by then, the fauna resembled the modern one (Hynes, 1988). The Mecoptera which consist of only about 500 species worldwide today (Penny & Byers, 1979) were particularly conspicuous across Laurasia in the Lower Permian sediments (Carpenter, 1930). In general, the genera and families present at that time are also found in the Mesozoic sediments, but died out during the Tertiary Era as the mean annual air

The jostling of the plates also resulted in tension, causing faulting in many parts of the continents, and this resulted in widespread out-pouring of basaltic magma. This released considerable quantities of carbon dioxide into the atmosphere which would provide sustenance for the tropical forests on the fans below the mountains. In these forests, the first Conifers became abundant, and those insects and amphibians that survived the environmental changes became more widespread. Laurasia began moving northwards away from Gondwanaland just before the end of this period. Since mammals were later to become important species on all the continents, it is probable that the first ancestors of the mammals evolved at this time, prior to the split, although the first known mammalian fossils are dated

The northward movement of the Laurasian plate during the Jurassic period (230-180 million years ago) began about 220 million years before present. The plate consisted of North America, northern Europe and northern Asia. Tropical forests grew in the wetter areas, while sand dunes existed in the drier regions. Laurasia had a varied topography with mountains, plains and shallow seas. The equator now lay across the continent from southern Baja California to New York, so that north-west Alaska was at 50°N (Briden & Irving, 1964, Figure 8). Since the land mass of North America was surrounded by warm oceans, the

South of the land mass, the Tethys Sea was developing between Laurasia and Gondwanaland. It stretched from central and southern China westwards to the Atlantic Ocean and separated Laurasia from South America, Africa and India. The result was a circum-equatorial ocean. This had an enormous effect on the temperature of the Earth since water absorbs approximately five times as much solar radiation as soil. This ocean provided warm surface and thermohaline currents that carried heat northwards to the Arctic Ocean via the North Pacific, as well as hot tropical air masses (Harris, 2002a). The ice cap over Antarctica melted, leaving a series of large islands where a tropical forest evolved (Francis et al., 2008). A mega warm event had begun (Harris, 2012) that was to last from about 200 million years before present until 44 million years ago. Summer temperatures averaged 20o C during this global thermal maximum. The genera present in this Antarctic flora were the

to about 200 million years ago in the early Jurassic Period.

temperatures decreased.

**3. Mesozoic era** 

climate remained hot.

ancestors of the present-day tropical flora.

It was during the Silurian Period (425-405 million years ago), that plants started to move from the sea on to the tropical parts of the land. Centipedes and spiders appeared amongst the vegetation. In the succeeding Devonian Period (405-345 million years ago), forests were spreading across the hot, humid land areas and the first amphibians appeared on land. By the Carboniferous Period (425-380 million years ago), the extensive coal beds in both Europe and North America indicate that parts of the area had a hot, continuously humid climate supporting a dense equatorial-type forest (Schwarzbach, 1961). Absence of evidence of glacial deposits but the presence of sand dune deposits suggests that there were also areas of hot deserts in the southwest of the United States. It is sometimes called the "Age of the Amphibians" since they were most numerous during the early part of this Period. Off-shore, extensive limestone deposits were being formed together with extensive coral reefs. By the end of this era, many of the genera of horsetails, conifers, reptiles and insects had already been evolved and were present on land. The tropical forests that produced the coal beds had a diverse fauna and flora, the remains of which are found entombed as fossils. The animals were of predominantly different species and genera than those present today, though the main groups of primitive plants were already well represented. The ancestral club-mosses (*Lycopodium*) that had first appeared in the Silurian Period in what is now Australia, grew to over 30m in height in these forests, as did the trees of the class *Cordiatales.* The first conifers subsequently evolved from these in the Permian Period. Giant horsetails (*Equisetum*) were also abundant. These plants were, of course, essential to support the terrestrial food chain, and the enormous production of coal indicates that there was a spectacular increase in release of oxygen and an enormous reduction in carbon dioxide in the atmosphere (Harris 2010), though some of the carbon dioxide was replaced by volcanic activity during the Permian Period. The swampy areas in the forests also supported a wide range of primitive animals including small amphibians, eel-like creatures and the first small reptiles. The latter were a reasonable match for the amphibians but were not capable of extirpating them. In the air, giant insects flitted about. However, these animals were adapted to a very specific microenvironment and when that ceased to exist due to erosion in the west and mountain building in the east, most were extirpated, e.g., the enormous flying insects. A few key species were able to adapt to the altered environment, and of these, some genera of mycchoriza fungi, *Lycopodium* and *Equisetum* are still to be found today.

By the Permian Period (280-230 million years ago), the equator lay in a line from the California-Oregon border to just north of Newfoundland. Much of the land area was now reduced to a low plain, although mountain chains were rising in the Appalachian region due to collisions of plates (the Appalachian Orogeny). Present-day North America continued to experience a tropical climate, but the mountain chains cut off the rain-bearing winds coming inland from the east coast. This dry environment was ideal for the evolution of a wide variety of dinosaurs from the survivors of the small Paleozoic reptiles, and extensive sand dunes occurred in the southwest of the United States. In the wet areas, the first true reptiles and the ancestors of the crocodiles, alligators and caimen appeared.

Amongst the new genera and species that evolved on this new landscape were the first Laurasian stoneflies (Plecoptera) and several other groups of insects (Illies, 1965; Zwick and Teslenko, 2000). The best record of these insects comes from the Baltic amber of Eocene age (38-54 million years ago) but by then, the fauna resembled the modern one (Hynes, 1988). The Mecoptera which consist of only about 500 species worldwide today (Penny & Byers, 1979) were particularly conspicuous across Laurasia in the Lower Permian sediments (Carpenter, 1930). In general, the genera and families present at that time are also found in the Mesozoic sediments, but died out during the Tertiary Era as the mean annual air temperatures decreased.

The jostling of the plates also resulted in tension, causing faulting in many parts of the continents, and this resulted in widespread out-pouring of basaltic magma. This released considerable quantities of carbon dioxide into the atmosphere which would provide sustenance for the tropical forests on the fans below the mountains. In these forests, the first Conifers became abundant, and those insects and amphibians that survived the environmental changes became more widespread. Laurasia began moving northwards away from Gondwanaland just before the end of this period. Since mammals were later to become important species on all the continents, it is probable that the first ancestors of the mammals evolved at this time, prior to the split, although the first known mammalian fossils are dated to about 200 million years ago in the early Jurassic Period.
