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An interchange of ideas... and animals...

One of the best parts about paleontology is that it brings in all sorts of different academic fields under one field of study.  That means that paleontologists often need to be jacks-of-all-trades, regardless of our training.  Someone with a geology background is going to have to learn some biology; someone, like me, with a biology background, is going to have to learn some geology.  And we all dabble our toes into fields like chemistry, physics, climate science, and geography.

Geology and biology are particularly complementary.  Geology has always been a part of biological study since the time of Darwin.  Darwin himself was quite an accomplished geologist, and his first paper was on the geological processes that form atolls.  His 'Origin of Species' has entire chapters about the geologic record and the geographic distribution of animals.  These latter two topics have since been inextricably linked with the discovery of plate tectonics.

When two continents meet, it's a momentous event, geologically and ecologically.  The geologic effects are dramatic and obvious - mountain building, volcanics, and earthquakes.  It's theses sorts of effects that have created some of the most awe-inspiring geologic features on the earth - the Himalayas, the Sierra Nevada, the Alps, and many others.
The ecological effects can be just as momentous although perhaps a bit more subtle in their appearance.

When land masses come together, animals and plants which have never encountered each other will come into contact for the first time.  In addition, the joining of land masses may split populations of marine organisms, leading to speciation (a topic for another time).

One of the best-studied exchanges of organisms due to the joining of land masses was caused by the joining of North and South America via the Panamanian Isthmus.  Called the Great American Biotic Interchange, this geologic and ecological event reshaped the ecosystems on these two continents, leading to introductions, and extinctions of a number of taxa.  Just 80km (50mi) across at it's narrowest point, the isthmus was just wide enough to allow for the passage of animals from north to south, and vice versa.

Northward and Southward Migrants

Some migrants in the Great American Biotic Interchange

The rise of this bridge was due to a combination of subduction of the Pacific plate underneath the Caribbean Plate, as well as encroachment of the South American plate on the North American plate.  The timing of the rise has been the subject of some debate, and some recent paleontological finds have shed some light on the question.

It turns out that the first interchange was aquatic, as might be expected.  A recent paper in the Journal of Vertebrate Paleontology discusses some new crocodilian fossils from Central America that appear to be transitional between ancestral caimans of South America, and their North American alligator descendants.  These animals lived in freshwater, so if they crossed the ever-narrowing, salt-water filled, gap between North and South America, the distance must not have been large.  Interestingly, these early alligators date to 19 million years ago.  Mammals and other groups don't cross until 2 million years ago, so the formation of the isthmus must have been a slow process indeed.

These intrepid pioneers started an exchange that reached it's peak much later.  Most of the terrestrial invasions went from north to south.  A number of North American mammals made the journey southward, and displaced many of the animals in South America, hugely changing the ecosystems there.  Many of the animals in South America were marsupials, or pouched animals.  Although their diversity in South America wasn't as great as in Australia today, they were the dominant mammals, and nearly all of them disappeared after the interchange with North America.

A great example are the carnivores (a group near and dear to my own heart).  There was a diversity of South American marsupial carnivores, including large saber-toothed forms, as well as large, carnivorous birds.  When placental carnivores, including saber-toothed cats and fox-like canids, came to South America the native carnivores, marsupials and birds, disappeared.  A number of herbivores disappeared as well, including entire orders of taxa unique to South America that had been the dominant plant-eaters for millions of years.  These gave way to the herbivores common to North America, including the relatives of horses, elephants and camels.

A saber-toothed marsupial
A South American saber-toothed marsupial

The interchange wasn't completely one way though.  Some South American taxa moved northward, although interestingly, most of the South American migrants didn't last to today.  Many of these were sloths and armadillos, many of which were gigantic forms compared to today's taxa in either North or South America.  The only living relatives of the South American invaders are armadillos, opossums, and porcupines.  The large taxa all went extinct soon after humans showed up in North America (from Eurasia), along with a number of large native taxa, like mastodons, mammoths, and large carnivores (but that also is a story for another time).

The question of why the southward migrants fared far better than the northward ones is still a bit of a mystery.  It may have been partly climatic.  The South American animals may not have been able to adapt as well to the cooler and drier conditions of North America, as the North American animals were able to adapt to the tropical and sub-tropical conditions of South America.  It also may have been a question of land mass.  North America is much larger than South America, and was also periodically connected to Eurasia throughout the late Cenozoic.  A larger land mass meant a wider variety of animals, and animals better adapted to varying conditions or competition with other animals.

Niether explanation is particularly satisfying to me.  That's the nice part about paleontology specifically, and science generally - it brings together different fields (geology and biology in this case), and makes as many questions as it answers.

Posted: 4/5/2013 5:37:54 PM by jhollandadmin | with 0 comments

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