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 ( 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