For much of the past 130 million years South America was an island continent, and on it organisms evolved in “splendid isolation.” Mammals, especially, evolved into forms not seen anywhere else, and while some mammalian immigrants made their way to South America during the past 30 million years it was not until about three million years ago, with the closing of the isthmus of Panama, that large animals from North and South America began to wander across the new landbridge and mix with the endemic faunas. This is why there were elephants in South America and giant ground sloths in North America, but a new study just published in the Journal of the South America Earth Sciences suggests that the timing of this “Great American Interchange” was a little different.
As stated by scientists Kenneth Campbell, Donald Prothero, Lidia Romero-Pittman, and Nadia Rivera, in order to properly understand earth history you need to know when events occurred, but for years the ages of recent geological deposits in the Amazon Basin have remained contentious. If paleontologists are going to understand when the earliest North American mammals arrived in South America, the ages of the deposits in Amazonia have to be ascertained. To this end they investigated the magnetostratigraphy of a site along the Madre de Dios River in Peru, a site of particular importance given a controversial fossil found there.
In 1996 Romero-Pittman described the prehistoric elephant Amahuacatherium peruvium from the bottom of the outcrop at the Madre de Dios River site, and later studies of the locality assigned it to a Late Miocene age (about 9.5 million years old). If correct, this would place it millions of years before the major pulse of the Great American Interchange, and this led other authorities to challenge the dating of the site and suggest that Amahuacatherium was really a specimen of the more recent genus Haplomastodon. The age of the deposits from which this animal was removed make all the difference in settling this debate.
After testing samples obtained from almost the entire Madre de Dios Formation, the scientists found that the results closely matched those reported from the same site nine years ago. Deposits at the bottom of the formation dated to about 9 million years ago, while those at the top were about 3 million years old. This confirmed the antiquity of the Amahuacatherium bones from the underlying Ipururo Formation, estimated to be about 9.5 million years old. The remains certainly did not come from the more recent Haplomastodon, especially since the minerals in the Ipururo Formation show a different paleomagnetic direction than those above. As the authors state, since the polarity of the earth’s magnetic field has been normal for the past 800,000 years or so, this difference rules out the idea that these bones are from a more recent kind of fossil elephant.
What this means for the history of the Great American Interchange is that some of the larger animals began to make their way south much earlier than previously thought. Clearly the ancestors of Amahuacatherium did not wait for the Panama land bridge to close before shuffling across, nor did other prehistoric mammals such as tapirs and peccaries which are also found in these Late Miocene deposits. Together they may represent an initial pulse of dispersal from North America into South America millions of years before the continents became connected. The great exchange which occurred after the isthmus of Panama formed cannot be denied, but it appears that the pattern of dispersal and evolution of mammals in the western hemisphere during the last 10 million years was more complicated that previously understood.
For more on controversial fossils from South America, see my post on Ameghino’s “elephants”
Campbell Jr., K., Prothero, D., Romero-Pittman, L., Hertel, F., & Rivera, N. (2010). Amazonian magnetostratigraphy: Dating the first pulse of the Great American Faunal Interchange Journal of South American Earth Sciences, 29 (3), 619-626 DOI: 10.1016/j.jsames.2009.11.007