Note that the Yucatan Peninsula was close to what is now the coast of Honduras (before
they cleaved apart). The whole of Central America was previously attached to the
north west corner of South America. Similarly the Caribbean Islands were also attached
to South America on its northern coast.
Rifting, as indicated by the yellow arrows could have separated the landmasses from
South America simultaneously. Central America may have retained a tenuous northern
connection with North America.
The Cocos Ridge, which is now a small ridge on the Pacific side of Central America
may have, at one time, reached right across the Caribbean in the way indicated.
Subsequently, North America was pushed southwards again, as the Arctic Ocean bed
formed and a land link with South America was made through Central America.
By studying the mammals of South America we can come to understand which species
are descendents of those which migrated down from North America (very recently, in
geological terms), which ones are decendents of the ‘old timers’ from the Gondwanan
connection with Africa and Oceania, and which ones have ‘evolved on’.
By comparing brain morphologies it is easy to identify the basal mammals which inhabited
both Gondwanan fragments - some 50 million years ago. The web-site brainmuseum.org
catalogues the brain morphologies of many species of mammal and is a useful resource
in this analysis.
1) The Old-timers of Gondwanan South America/Africa
Anteaters, sloths and rodents from South America show a brain architecture in common
with many African species and this suggests a common ancestry from Gondwanan times
when South America was continuous with Africa.
2) The Old-timers of Gondwanan South America/Oceania
There are 93 species of opossum within the Order Didelphimorpha and other marsupials
in the Orders Paucituberculata and Microbiotheria in South America which show biological
similarity with the marsupials of Australia. This suggests that there was a land
link between 3) The less recent newcomers.
The llamas and Alpacas are derived versions of camelids which entered South America
through Central America during the last 2 million years.
4) Recent Newcomers to South America from North America
These are the:-
Mustelidae, weasels and otters
Canidae, Bush dog, Speothos species
Cervidae, deer, Mazarama species
Lagomorphae, rabbits, pikas
Felidae, Jaguars, ocelot etc, Pantera spp
5) Recently evolved types
Surprisingly, what appear to be very advanced species - the New World monkeys have
brain morphologies which are more in common with the basal prosimians such as the
bushbaby, (Otelemur crassicaudatus) of Africa, than the Old World monkeys, despite
their superficial similarity. This may suggest that only bushbaby like species were
common to both Gondwanan fragments at the time of separation, and their evolution
into monkeys took place independently in South America and Africa. The Old World
are much more advanced than the New World monkeys and there is no equivalent of the
African apes, in the South America continent.