Pangaea Study Resources

PANGAEA Between 360 and 270 million years ago, Gondwana and Laurasia converged to become the supercontinent Pangea (Figure 115), whose name comes from the Greek meaning "all land." The vast continent has an area of about 80 million square miles and covers 40 percent of the earth's surface. It straddles the equator and extends practically from pole to pole, with almost the same amount of land in both hemispheres. Today, by contrast, two-thirds of the continent is located north of the equator. A single large ocean called Panthalassa stretches uninterrupted across the planet, with all the continents clustered on one side. Over the following period, smaller patches of land continued to collide with the supercontinent until it reached its peak size at the end of the Triassic about 210 million years ago. Continental collisions squeezed the crust and pushed enormous masses of rock into several mountain belts in many parts of the world (Fig. 116). Volcanic eruptions are extensive due to frequent continental collisions. During times of highly active continental movement, volcanic activity increases. This occurs mainly in mid-ocean spreading mountains where new oceanic crust is created and in subduction zones where old oceanic crust is destroyed. The amount of volcanism also affects the rate of mountain development and the composition of the atmosphere, which greatly affects the climate. When Gondwana connected with Laurasia to form Pangaea, the collision lifted the Appalachian and Ouachita Mountains. At the same time, Laurasia connected with Siberia, thrusting the Ural Mountains. The island's continued arc clash with North America resulted in an episode of mountain-building in Nevada called Sonoma orogeny, which coincided with the assembly of Pangaea 250 million years ago. North America sat straddling the equator, which stretches from northern Mexico to eastern Canada, during the Carboniferous 300 million years ago. It is believed to have a wet, tropical climate. A huge river system, rivaling today's Amazon, flowed down from the ancient Appalachian Mountains. It flows through a 20-mile-wide valley while draining much of the continent. One river system, which drained an area comparable to the modern Mississippi River, had its headwaters in the Canadian Maritime Province. It flowed south for 2,300 miles, eventually emptying near the current coastal plains of Alabama and Mississippi. A second river system begins in south-central Quebec and stretches all the way to Arkansas.The rivers reach their maximum in the early Carboniferous, during times of low sea level. However, as sea levels rise, rivers flood the land and become vast swamps. Large buildups of peat, which later became coal, accounted for extensive coal deposits in many areas of the eastern United States. Sediments in the Tethys Sea separating Gondwana and Laurasia curve and lift into various mountain belts, including the ancestral Hercynian Mountains in the south Europe.As continents rising higher and ocean basins dropping lower, the land becoming dryer and the climate getting colder, especially in the southernmost lands, which are covered with glacial ice. All known episodes of glaciation occurred during times of sea level decline. Changes in the shape of ocean basins greatly affect the course of ocean currents, which in turn has a noticeable effect on climate. The closure of the Tethys Sea removed the main barrier to the migration of species from one continent to another, and they spread to every part of the world (Fig. 117). When all the continents merged into Pangaea, plant and animal life witnessed great diversity in the oceans as well as on land. The formation of Pangea marked a major turning point in the evolution of life, during which reptiles emerged as the dominant species, conquering land, sea and sky. A continuous shallow margin of water flows around the entire Pangaea. As a result, there are no major physical barriers that impede the spread of marine life. In addition, the ocean is largely confined to ocean basins, leaving continental shelves mostly open. The continental margins are less wide and narrow, limiting marine habitats to near-shore areas. As a result, the habitat area for shallow-water marine organisms is very limited, which accounts for low species diversity. As a result, marine life (flora and fauna) is more widespread but contains relatively fewer species. In northern latitudes, dense forests of primitive conifers, horsetails, and club mosses growing as tall as 30 feet dominate the mountainous landscape. Much of the interior may resemble grassless versions of steppes in Central Asia, where temperatures vary from very hot in summer to very cold in winter. Because grass would not appear for more than 100 million years, the scrub landscape was dotted with bamboo-like horsetails and dense groves of extinct seed ferns that resembled present-day tree ferns. Browsing on the seed ferns are herds of moschops, 16-foot reptiles with thick skulls adapted for butting during mating season, a tactic similar to the behavior of modern farm animals. They were probably preyed upon by a herd of lycaenops (Figure 118), which were reptiles with dog-like bodies and long canines protruding from their mouths. Mammal-like reptiles called dicynodonts also have two dog-like tusks and feed on small animals along river banks. Small reptiles probably ate insects similar to the way modern lizards do. Pangea's climate is one of extremes. The northern and southern regions are colder than Siberia. The interior desert is hotter than the Sahara and almost devoid of life. The formation of continents together created an overall climate that was hotter, drier, and more seasonal than any other time in geological history. These conditions may have led to the mass extinction of most land animals about 30 million years after most of Pangaea gathered. Pangea remained intact for 40 million years, after which it split into its current continents. After examining amphibians from the Carboniferous coal forests, the next chapter explores the evolution of their reptilian cousins during the Permian period.