**3. Mesozoic era**

234 Biodiversity Conservation and Utilization in a Diverse World

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

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

to approximate the poles of rotation. Throughout this time, it lay in the Tropics.

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.

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 climate remained hot.

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 ancestors of the present-day tropical flora.

In the Northern Hemisphere, a tropical biota evolved that was adapted to much higher temperatures than the present-day tropical flora. This extended north to the polar areas, except perhaps on the highest mountains. A tremendous variety of dinosaurs evolved, some reaching gigantic proportions. The first mammals consisted of small rat-like creatures that managed to survive running around the feet of the large, dominant dinosaurs. The oldest known specimen of the Asilidae (robber flies) dates to about 110 million years ago (Grimaldi, 1990), though that author put the evolution of this group at about 144 million years before present. However, they could have originated before Laurasia parted from the southern continents (Yeates & Grimaldi, 1993, Yeates & Irwin, 1996). By Late-Cretaceous times, small primitive marsupials and insectivores similar to shrews and hedgehogs were fairly abundant, and would survive the Cretaceous/Tertiary die-off of the previously dominant Dinosaurs. Cycads and conifers were the main components of the forests, while crocodiles, turtles and lamellibranchs, e.g., oysters, were common in freshwater lakes and swamps. It was during this time that the first birds appeared, apparently evolving from certain groups of dinosaurs.

The Role that Diastrophism and Climatic Change

Have Played in Determining Biodiversity in Continental North America 237

About 80 million years before present, the Laurasian plate no longer had room to continue moving northwards. Eurasia became stationary, but North America started its western movement that continues until today. This gradually extended the Atlantic Ocean northwards as two separate plates were formed from the former Laurasian plate. It was also during the Mesozoic Era that South America, Africa and India separated from the land masses around the South Pole. Africa and India headed towards Eurasia, but South America

At the beginning of the Cenozoic era (63 million years ago), the climate was still tropical, though North America was moving to much higher latitudes. The Pacific Plate was moving north-northwest, but about 43 million years before present, its direction changed to westnorthwest, as indicated by the change in direction of the Emperor and Hawaiian sea-mount chains (Clague & Jarrard, 1973). At the same time, the Aleutian Trench and Chain started to form (Worrall, 1991). This altered the geometry of the Beringian Gateway for warm currents carrying heat to the Arctic Ocean. About 38 million years ago, the Turgai Strait Gateway to the Arctic Ocean closed (Marincovich et al., 1990, McKenna, 1975). At the end of the Paleocene (c. 60 million years ago) in the Southern Hemisphere, the tropical forests were gradually displaced by floras dominated by the *Araucaria* conifers and the southern beech *Notofagus* which could survive freezing winter temperatures. Australia and Antarctica were still joined, but Australia started moving north around 55 million years ago [Kamarovitch & Geoph, 2009). Oxygen isotope data from the Atlantic seas of South America show that there was a marked cooling of the ocean where the planktonic foraminifera lived from about 19o C during the late Paleocene, resulting in sea temperatures of about 6o C by the early Oligocene (Shackleton & Kennett, 1995). A shallow water connection developed between the southern Indian and Pacific Oceans over the South Tasman Rise by about 39 million years ago, and at 38 million years before present, a large area of sea ice had developed over Antarctica. Bottom ocean temperatures plummeted and the thermohaline ocean circulation was initiated. The Drake Passage between Patagonia and Antarctica probably opened up about this time. Meanwhile in the Arctic Ocean, ice-rafted sediments were deposited on the sea floor from about 44 million years ago (Tripati et al., 2008). The mega cold event that we live

Soon after (before 29 million years ago), the Tethys Sea became fragmented into the Mediterranean Sea and an eastern portion including the Indian Ocean, as the Arabian plate collided with the almost stationary Eurasian plate. This resulted in the crumpling of the marine sediments in the former Tethys, which were uplifted into the mountains ranges of Iraq, Persia and Afghanistan (Nomura et al., 1997). Gone was a substantial part of the heat source for the Arctic, and the climate of the northern land areas was cooling (Harris, 2002a). The gradual closing of the gap between Asia and North America reduced the flow of warm ocean currents into the Arctic Basin by about 23.5 million years before present, though this was ultimately offset by currents flowing along the sea connection by the opening of the

moved northwestwards towards the eastern part of Asia-America.

**4. Cenozoic era** 

in was beginning.

North Atlantic Ocean about 20 million years ago.

Eurasia was not a single land mass (Cox, 1974). Shallow epicontinental seas covered part of the plate, so that the Turgai Straits separated Scandinavia and Britain (which was part of North America at that time) from the main Asian land mass. Thus although Britain and Scandinavia were part of the same plate, movement of the terrestrial biota across the land areas to Asia was restricted in time and space.

About 150 Ma before present, North America and Eurasia started to separate at their southern margin, and the North Atlantic Ocean began to form. Initially, the biota of both continents remained the same and the opening proceeded slowly, but as the plates continued to move northwestwards, the biota that required very hot tropical conditions could no longer pass back and forth between Eurasia and North America. The first angiosperms appeared at 144 million years ago.

In early Cretaceous times (135-63 million years ago), an epicontinental sea (the Mid-Continental Seaway) encroached onto the land that is now along the Mackenzie valley, and gradually extended south until it reached the ocean in what is now the Gulf of Mexico. This divided the floras and faunas into eastern and western populations, but by Late Cretaceous times the sea had largely dried up. During this time, the two sides had developed different insect faunas since they could not cross the wide expanse of water, but these became homogenized when the sea no longer acted as a barrier. Combined with the Turgai Straits, the Mid-Continental Seaway also resulted in there being two distinct faunas of land animals including the dinosaurs (Cox, 1974, Noonan, 1988, Wolfe, 1975). One was called Asia-America (Asia plus western North America) with a connection between them at high latitude across present-day Alaska and Siberia, and the other, Euroamerica (eastern North America plus northwest Europe). These faunas remained distinct after the disappearance of the Mid-Continental Seaway. During the early phases of the opening of the North Atlantic in the Middle Cretaceous (90-95 million years ago), a rift is believed to have formed between Greenland and Labrador, resulting in Greenland, the Rockall Plateau and Europe being a single land mass separate from North America (Heirtzler, 1973).

About 80 million years before present, the Laurasian plate no longer had room to continue moving northwards. Eurasia became stationary, but North America started its western movement that continues until today. This gradually extended the Atlantic Ocean northwards as two separate plates were formed from the former Laurasian plate. It was also during the Mesozoic Era that South America, Africa and India separated from the land masses around the South Pole. Africa and India headed towards Eurasia, but South America moved northwestwards towards the eastern part of Asia-America.
