Wednesday, 12 October 2011

Five New And Oldest Species Of South American Rodents Which Directly Confirm The African Origins Theory

One of the tiny, 41 million year old, rodent teeth
South America is one of the most fascinating continents in palaeontology. A unique set of habitats created a unique set of species. Isolation from the rest of the world allowed evolution to run wild and its final collision with North America, became the stage for one of the greatest ecological showdowns in prehistory.

Many of its creatures are similar to creatures from Africa and Eurasia. Examples of this are the Hoatzin, a flightless bird with a body shape like the African cockatoos and turacos, and all of the simians. As simians evolved after the continents had taken up their familiar shape, the only way they could have reached the New World was via a sea crossing. Many fossils have similar characteristics to African creatures, but there was no direct evidence to prove these theories.

However a recent set of fossil discoveries yielding developmental results in the field of evolution, have changed this. A team of scientists led by Professor Darin Croft from the Case Western Reserve University School of Medicine and Professor Pierre-Olivier Antoine from the Institute of Evolutionary Sciences at Montpellier University were performing a dig upon poorly described rocks along the Ucayali River in Contamana, Peru, last mentioned in a 1948 description of the area by Harvard scientist Bernhard Kummel.

On a walk through time across the 50 to 40 million year old river sediments the team discovered a series of rodent teeth on three different digs between 2008 and 2010. By performing argon dating upon volcanic ash found in the fossil matrix, they were able to date the teeth to around 41 million years old. By extracting pollen from the sediment also, the team found that the entire area was one of lush tropical forest which fits in with the image of South America during the Eocene Thermal Maximum.

The first surprise came when the team identified the teeth as belonging to five separate and completely unknown species. The first, Cachiyacuy contamanensis, was named after the Contamana Region, the second, Canaanimys maquiensis, after the specific locality and the third, Cachiyacuy kummeli, in honour of Bernhard Kummel. The other two, Eobranisamys and Eospina, were mentioned briefly in their paper. They discovered the remains of numerous other mammals also. However they were too fragmentary to be properly identified.

The second surprise came when they dated the fossils. Until then, the oldest known rodent fossils in South America were 32 million year old specimens from central Chile. This gave rise to the hypothesis that rodents crossed the expanse of Eurasia, crossed the land bridge between Russia and North America, before arriving in South America in a theory known as the Northward Expansion.

These new fossils are 10 million years older, which suggests that the creatures either began their world wide migrations far earlier or came from somewhere much closer. This leads neatly into the third surprise. Bones can only reveal so much about the taxonomic relationships of creatures. However dental records are much more reliable. The dental records of the teeth, apart from showing that the creatures would have fed upon soft fruits and seeds, indicate that their closest living ancestors were the African rodents.

The remaining question is how they could have crossed the Atlantic. The most plausible theory is that large storms caused vast sections of mangrove swamp to break off from the African coast and float across to the Americas, carrying with them a multitude of native species and enough food to keep them alive. Current estimates suggest that these strange biological rafts could have taken as little as three weeks to cross the ocean.

Until we find a fossil raft in the mid Atlantic, we cannot say for certain if the theory is sound, but it currently is the best we have and is still easily within the confines of reality. The finds, now permanently stored at the Museum of Natural History in Lima, a very important. By confirming the African connection, other theories such as the origins of the American simians, properly called the new world monkeys, and the hoatzin offer significant evidence to suggest that they are true.