The basics of biogeography

At the time of the extinction of the dinosaurs the world was already divided into continents.  Antarctica and Australia were still connected.  However, we don’t know much about the organisms of the time in Antarctica.  Nearly all of the continent is nowadays covered in ice.  In contrast we know other continents evolved their own organisms.  It was possible for small animals to float across the seas on rafts of entangled vegetation.  If such rafts had upright trees they could function as sails.  But very few succeeded.

It took only ten million years until Africa got contact with Eurasia.  South Asia got contact with Eurasia 30 million years ago.  North and South America got contact at a relatively late point in time.  This was not until less than 3 million years ago.  Through all this time Eurasia and North America have at times been connected.  However, the connection was always far north.  This may cause problem as the climate is colder there.

The map below shows which latitudes the continents stretch across:

 

 

Originally this map was made by the geologist Steven Dutch.  I have removed the text and changed the coloration.  Moreover, I have marked the Equator (grey line) and the tropical circles (pink lines).  The later are just approximate since they were not shown on the map I stated from.  First I saw the Northern Tropic was closely north of Cuba.  Then I drew the southern one at the same distance from the Equator.

Most of the world’s deserts are around the tropical circles.  I think this is the norm there if there is no monsoon.  Where the monsoon becomes less pronounced it gradually becomes drier.  Now the deserts existing today have not always been there   But within the human era I think they have been scrubland.  Yet this is a hash environment which it is not so easy for organisms to get across.

The result is different organisms being found in different part of the world.  Something found in one part of the world can’t be presupposed to be found in another part.  Australia is rightly known for its distinctive organisms.  This continent has been isolated since it broke lose from Antarctica.  The last 15 million years it has came closer to Eurasia.  Birds and bats have been able to fly across the straits.  Murids have gotten across on rafts of flouting vegetation.  Some marsupials of comparable size have gotten in the opposite direction.  They don’t seem to have spread that far.

Organisms found in South Asia today don’t differ much from those of Southeast Asia.  In the same way Central America’s resemble those of South America.  Further north in North America such organisms becomes rarer.  Those of most of North America are more similar to those of Eurasia.  North of Sahara itself North Africa’s are more like Eurasia’s.  But organisms found in Eurasia one can still not presuppose to be found somewhere else.  The exchange with other continents has had its limitations.

 

Uploaded on the 14th of Mars 2025.