Download Costa Rica, Geological Past II

Costa Rica, Geological Past II:
Evolutionary biology in the light of changing landscapes
Stages of “landscapes” of “Costa Rica”:
• Ocean Stage
• Archipelago Stage
• Stage of Subsident Islands
• Island Arc Stage
• Isthmus Stage (land bridge)
When previously separated faunas come in contact this is known as
“biotic interchanges”
One of the most famous is the “Great American Interchange”
The connection between South and North America had dramatic
repercussions for the fauna, most noticeably the mammalian fauna of
the southern continent
Formation of a land bridge and the consequent closing of a seaway
had great biogeographic effects.
• The land bridge permitted the migration and intermingling of North
American and South American land mammals and floras
• It formed a barrier to the dispersal of marine faunas by dividing a
previously homogenous marine fauna.
~3 Mio
~ 130 Mio yrs
~60-6 Mio yrs
Although the current land bridge, which was formed ~3 Mio yrs
ago, is probably the first unbroken land link between the
Americas, there may have been earlier, stepping stone routes
across the gap.
As the Caribbean plate drifted eastward, and the South & North
America plates drifted westward, the first volcanic islands
related to the convergent plate boundary on the eastern side of
the Caribean plate (proto- Greater Antilles) moved relatively
eastward. Afterwards, establishment of a new subduction zone
along the western margin of the Carribean plate caused a new
generation of volcanic islands, the Central American Island Arc.
Marine Faunas:
• A previously homogenous marine fauna has become divided into
Pacific and Caribbean parts, in which “geminate” species, that is,
poorly differentiated pairs of species descended from a common
ancestor, have developed
• Another outcome of the evolutionary divergence of the western
Atlantic and eastern Pacific faunas is that now the fish faunas have
only 1% of their species in common.
• Extinctions have also occurred. After the continuous Central
American sea became separated into two parts 43 genera and
subgenera of mollusks that had belonged to the combined fauna
went extinct on the Atlantic side, and 4 on the Pacific side.
Mammalian Faunas:
• Before the opening of the Atlantic and rifting between North and South
America (200-150 Mio) North and South America were connected
therefore had similar mammalian inhabitants.
• By~100 Mio years North & South American continents had rifted apart
• By late Paleocene (~50 Mio years), the modern orders of mammals –
the groups such as horses, dogs, cats that are still the dominant land
vertebrates – evolved in North America, Africa, and Europe
• But, South America shows no signs of possessing these forms. It instead
evolved its own distinctive mammalian fauna which primarily consisted
of three groups: marsupials, xenathrans (armadillos, sloths, anteaters) and
ungulates. Some still populate South America but many are now extinct
such as: Marsupial saber-tooth carnivors, ground sloths, the most heavily
armored mammal that ever lived – the glyptodonts
Mammalian faunas, continued
• New arrivals came in from outside since ~ early Oligocene ( ~ 30
Mio years) on rare occasions when the first volcanic islands to
become later Central America formed along the western side of the
Carribean plate. Immigration likely occurred by waif dispersal,
hopping from island to island. Rodents are a major group which first
appeard in the Oligocene.
• In the late Miocene (~7-9 Mio years) further small additions to the
fauna arrived (racoons and allies, Procyonids) still entered by waif
dispersal
• At 3 Mio years ago, the modern Panamanian land bridge formed
causing for an extraordinary clash and exchange for faunas (the Great
American Interchange)
Primary invasion were roughly equal in both directions,
Takeover of South America by northern mammals had, among
others, two sources:
1. weight of numbers
2. different rates of speciation after arrival
This especially in the light of climatic changes that took place
form savannah type vegetation on both sides to tropical
rainforest of modern times including along with a rapid uplift of
the isthmus especially of the Cordillera de Talamanca and of the
northern Andes.
Summary:
• At present, about 50% of South American genera are descended
from originally North American mammals; the proportion of southern
mammals in the North is much lower, being less than 2%.
• The pattern of primary immigration is similar in both directions.
Something like 10% of the genera from each side successfully
invaded the other. But the subsequent proliferation of the immigrants
the pattern diverged markedly. Not the original invasion, but their
relative success afterwards let to the superiority of North American
mammals.
Punta Judas