Animal’s Distribution and Evolution of Continents – Essay
Pangaea persisted throughout succeeding Triassic period and animals like dinosaurs and early mammals got the chance to spread to all areas.
In Jurassic, Pangaea began to break-up. This resulted in partial fragmentation of land areas and evolutionary divergence among land animals, particularly the dinosaurs.
During Cretaceous, Africa and India separated from Gondwanaland and drifted northwards. Fragmentation of land areas, diversification of terrestrial organisms particularly the vertebrates is reported.
Northern continental mass called Laurasia broke into three portions viz., Europe and Eastern North America, Western North America and Asia. This situation is reflected in dinosaur faunas of three areas, as for example Ceratopsian dinosaurs confined to western North America in upper Cretaceous.
During Paleocene, North America and Europe still remain connected as shown by strong similarity of their mammalian fauna. South America and Australia remained connected via Antarctica, which explains why marsupials nowadays occur in these two continents and nowhere else. In Paleocene, there were partial exchanges of land mammals between Africa and Eurasia indicating that barriers prevented free migration of most animals.
During Oligocene and Miocene, extensive glaciations of continent were probably the cause of extinction of its entire terrestrial fauna. Comparison of Miocene mammal fauna suggests that some interchange occurred between Eurasia and North America across the region of Bering straits, but in general mammals of two regions underwent separate radiations.
During Pliocene, North and Central America were reunited with South America resulting in a spectacular interchange of fauna but immigration was mainly from North to South. During Pleistocene, extensive glaciations resulted in falls of sea level of up to 100 meters or more.
This connected chains of Islands (which are now separated) joined British Isles to European continent and Asia to North America across Bering straits, allowing faunal and floral dispersal. Australasia, however, remained an “Isolated continent” as it is to the present day with many unique faunal elements.
The continental drift theory could explain migration pattern of eels. Eels travel to North Atlantic spawning grounds from both America and European rivers. If North America and Europe actually were part of Holarctic landmass (Laurasia) as postulated, ancestral eels could originally have spawned in a common Central Holaractic river system.
Their present descendents still might travel from both West and East to same ancestral spawning area, which now, however, happens to lie in the middle of an ocean as recorded elsewhere. As per the account given by Young (1981), throughout Devonian period, there were two separate continental masses, North mass is known as Laurasia and Southern mass is called Gondwanaland.
The sea of uncertain extent between two continents is called the Hercynian Ocean. Two continents then converged and met during middle Carboniferous to make a single super continent, horseshoe like in shape, known as Pangaea. In subsequent years, various changes occurred and now the whole outer surface of earth is divided into 8 major lithospheric plates and some minor ones.