The Villavieja Formation is uppermost portion of the Honda Group, a 1250 meter thick collection of Miocene rocks located along the Magdalena River in the badlands of central Colombia. The lower portion of the Villavieja is richest in fossils, being a collection of sandstones and gray and red mudstones. The upper layers contain volcanic deposits, and fewer fossils.
Though the region today is semi-arid, dominated by thorn-scrub acacia and woody cactus, Villavieja was a lowland tropical forest in the Miocene. Fossilized trunks of Goupioxylon have been found which resemble wood of the living Goupia, a lowland forest tree that may grow to 30 or 40 meters tall.
The Villavieja Formation has yielded large numbers of vertebrate fossils, many of them nearly complete skeletons and many of them large and well preserved. This locality has been tremendously important for the study of vertebrate evolution, particularly large mammals. Firstly, Villavieja is uniquely situated in space. Most of South America is tropical, but few fossil sites have been found there. The tropical soils and climates are usually poor for preserving fossils, since decomposition is often too rapid. Villavieja thus shows us life is a tropical region, while most fossil sites in South America come from further south in the cooler temperate zone.
Secondly, Villavieja is situated uniquely in time. In the Late Miocene and Early Pliocene, South America came into contact with Panama and the North American continent. When that happened, many species of plants and animals migrated north or south and a vast mixing of previously isolated species occurred. The migration of animals is called the "Great Faunal Interchange" and is a major event in the history of the Neotropics. Villavieja animals show us what life was like in the path of this interchange just before the event occurred, giving us an idea of what survived, and what did not.
Most of the species from Villavieja have been found nowhere else. Because of this, it has often been difficult to correlate rock strata with layers from other localities. In the past, it was assumed that the diversity of South American vertebrates was rather uniform across the continent at any one time, recognizable by the presence of certain key species which could be found in large numbers. Work at Villavieja has shown that this is not the case, but that the northern tropical faunas are quite different from those of more temperate Patagonia.
The first significant discoveries of fossil vertebrates in this area was made by Brother Ariste Joseph in 1923. During the 20s and 30s, oil survey expeditions encouraged by the Colombian goverment turned up a number of additional new and unusual finds. In 1944, R. A. Stirton of UCMP took special interest in the region and obtained funding for an expedition. Though his ship caught fire and sank near Acapulco, he managed to reach Colombia and gather financial support from several oil companies for his research.
It was on this expedition that the partial skull and skeleton of the fossil monkey Cebupithecia was discovered, a fossil that immediately turned these badlands into a respected and important fossil locality. Fossil primates are exceedingly rare, and even when fossil monkeys are found, it is often merely teeth or a jaw. The La Venta site has yielded fossils assignable to as many as ten genera, whereas most localities will have no more than three. As a result of the richness of vertebrate fossils here, the Villavoeja Formation is one of the best-known and best-studied ancient faunas in South America, of any age.