South America's Missing Mammalshttp://www.scientificamerican.com/article.cfm?id=south-americas-missing-mammals&page=5
A recent article on finding new fossil records of South American mammals that tend to fill in the gaps where our previous knowledge had holes in it. Two sections are of especial interest:
Seafaring RodentsOne of the most significant of our Tinguiririca discoveries was a fossil of the earliest known South American rodent, a find that lends powerful evidence to a debate over the origins of today's living capybaras and chinchillas. Known as caviomorph rodents, these creatures and their immediate kin make up South America's most ancient rodent lineage (and are distinct from the younger rodent line of rats, mice and related creatures that arrived from the north about 3.5 million years ago, when the Isthmus of Panama first reconnected the two Americas). Paleontologists agreed that the first caviomorph rodents arrived sometime within the broad time span between 55 million and 25 million years ago, while South America was still an island. A few younger caviomorph fossils hinted that the predecessors came from Africa, but many researchers found it easier to imagine that the immigrant rodents made the shorter trip from North America, possibly via a chain of Caribbean islands.To help settle the debate, we compared the anatomical details of the Tinguiririca animal to rodent remains found elsewhere in the world. Most informative was the shape of the tiny teeth still rooted in the lower jaw (the upper jaw and molars have yet to be found). That shape implied that the Tinguiririca animal's upper molars had five distinct crests--as did the upper molars of African rodents from the same period. In contrast, North America's ancient rodent species had only four crests on their upper molars. These comparisons strongly suggest that the Tinguiririca rodent is more closely related to animals in Africa. The absence of plausible caviomorph forebears in older North American fossil beds also supports the out-of-Africa theory.Presumably the original caviomorph colonists traveled from Africa to South America on floating logs or other rafts of vegetation--scientists' best guess for how various unusual plants and animals made their way to many geographically isolated regions [see "Madagascar's Mesozoic Secrets," by John J. Flynn and André R. Wyss; Scientific American, February 2002]. The idea of such an incredible transoceanic journey may seem far-fetched, but it is more plausible in the context of the global environment before about 32 million years ago. At that time, the South Atlantic was only about 1,400 kilometers at its narrowest point--half as wide as it is today--and east-to-west ocean currents in the tropics were strengthening sporadically. These conditions would have permitted a journey in about two weeks, and the animals may have gone into torpor (inactivity and greatly reduced metabolism during times of stress). Moreover, sea level was dropping at the time (because of the formation of ice sheets on and around Antarctica), so one or more volcanic "stepping stone" islands, now submerged, may have made the crossing easier.[Emphasis added-DD]
Our ongoing analyses of the numerous faunas, which range between 10 million and 40 million years in age, are revealing fresh insights into the region's history. One of our most significant newer finds--from a site about 100 kilometers north of Tinguiririca, in the drainage basin of the Cachapoal River--is the most complete skull of an early New World monkey yet discovered. The five-centimeter-long skull, with both eye sockets and every tooth in the upper jaw intact, came from a petite monkey weighing about one kilogram at the most. Named Chilecebus carrascoensis, this creature resembled modern New World monkeys, such as marmosets and tamarins. As with the caviomorph rodents, experts had long debated whether New World monkeys originated in North America or Africa. But anatomical details of the Chilecebus skull and teeth argue for its common heritage with a group of primates originating in Africa. Like the caviomorph rodents, it seems the ancestors of Chilecebus somehow made the Atlantic crossing from Africa.
|Caviomorph Rodent (Guinea pig)|
And so there you have it, two groups of animals long postulated to bear evidence of a transalantic land bridge continue to support the idea of at least "Stepping stone" islands in the past. And those mid-Atlantic temporary volcanic islands have all sunken under the sea now. The story of the sunken island of Atlantis is not alone. This only goes to confirm what Lewis Spence had said, along with those experts which he quoted and those experts which continued to quote him subsequently.
Best Wishes, Dale D.