“Look! There’s an ‘Aequidens’—and there’s a Tomocichla sieboldii—together in the same river!” Here was something we had wanted to see for a long time. In this one river in the far south
western Costa Rica, the Río Coloradito, we could see the
collision of two great cichlid faunas: the “Aequidens” group
from South America, coming in contact with the cichlids of
Central America. Both of these species were breeding in this
river, literally side by side, oblivious to the immense geological
and biological forces that had shaped their world over the past
several million years or so, making this possible.
Samantha Hilber
Complex Geology
As many people are aware, North America and South America
were not always positioned the way that they now appear on
the globe. Plate tectonics have literally floated these massive
chunks of continent around like slow-moving icebergs over
the past quarter billion years or so. While the general patterns
are now well known (e.g., South America was once attached to
Africa while North America was once attached to Europe), the
minor details at the edges of these giants are much less clear.
It just so happens that many of the fishes that intrigue cichlid
keepers live on those edges, and the complex ancestry of New
World cichlids, particularly those in Central America, is buried
in those details.
The idea of continental drift is not that old, and this is
important to keep in mind. When workers like Regan and others
first described and summarized the fauna of Central America,
they had in their minds the idea that Central America had been
there, as is, for a very long time. To suggest to them that the land
and the rivers on it had literally appeared a scant 15 million years
ago would have been peculiar, to say the least. And yet, we now
know this to be the case.
It was only in 1912 that Alfred Wegener proposed continental
drift, and in fact, it wasn’t until the 1960s that it had become
widely accepted. Some Websites (e.g., www.ucmp.berkeley.edu/
geology/ tectonics.html) now plot the movements of the major
continental plates around the globe in captivating animations
that chart the paths of these giants—but what about the crumbs
at the edges of the plates? Costa Rica and Panama are just such
places, sitting on the western edge of the Caribbean Plate, with
the North American Plate to the north, the South American
Plate to the southeast, the Cocos Plate to the southwest, and the
Nazca Plate to the south. With so many heavyweights coming
in close contact, one would think that the geological history of
this region is complex—and it is.
Coates and Obando (1996) give a detailed account of the drama
that has unfolded in this part of the world. In essence, here is
the storyline: as two large continental masses (North and South
America) moved together, the western edge of the Caribbean
Plate, lying on the eastern edge of the junction of the Americas,
got rammed from the southwest by a fast moving chunk of rock
called the Cocos Ridge (part of the Cocos Plate). As this plate
subducted (moved under) the western edge of the Caribbean
Plate (a region known as the Chorotega block of the Costa Rica-Panama microplate), land was thrust upward out of a shallow
sea. Combined with the worldwide lowering of sea levels, a ridge
of volcanic mountains rose above the sea roughly 16 million
years ago. At first these were just a chain of islands between
North and South America, but eventually they rose enough to
create a complete barrier, cutting off the Pacific Ocean from the
