<P> The Carboniferous was a time of active mountain building, as the supercontinent Pangea came together . The southern continents remained tied together in the supercontinent Gondwana, which collided with North America - Europe (Laurussia) along the present line of eastern North America . This continental collision resulted in the Hercynian orogeny in Europe, and the Alleghenian orogeny in North America; it also extended the newly uplifted Appalachians southwestward as the Ouachita Mountains . In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural mountains . There were two major oceans in the Carboniferous the Panthalassa and Paleo - Tethys . Other minor oceans were shrinking and eventually closed the Rheic Ocean (closed by the assembly of South and North America), the small, shallow Ural Ocean (which was closed by the collision of Baltica, and Siberia continents, creating the Ural Mountains) and Proto - Tethys Ocean . </P> <P> The Permian extends from about 298.9 ± 0.15 to 252.17 ± 0.06 Ma . </P> <P> During the Permian all the Earth's major land masses, except portions of East Asia, were collected into a single supercontinent known as Pangaea . Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean (Panthalassa, the universal sea), and the Paleo - Tethys Ocean, a large ocean that was between Asia and Gondwana . The Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo - Tethys to shrink . A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic Era . Large continental landmasses create climates with extreme variations of heat and cold ("continental climate") and monsoon conditions with highly seasonal rainfall patterns . Deserts seem to have been widespread on Pangaea . </P> <P> The Mesozoic extended roughly from 252 to 66 million years ago . </P>

When were earth's landmasses first recognizable as the continents we know today