THE GEOLOGICAL EVOLUTION OF THE ANDES AND AMAZON: CONSTRAINING PHYLOGENETIC ANALYSES

Understanding the origins of species and their present-day distribution in tropical South America (TSA) requires complete and accurate knowledge of the past evolution of the physical environment of TSA. Although many paleogeographic scenarios of TSA throughout the Cenozoic have been published, there has been little effort to estimate the uncertainty associated with these reconstructions. Knowledge of uncertainty is essential, for example, to biologists attempting to relate their phylogenetic analyses to scenarios of landscape evolution. Examples of the problem abound, and we will present some recently published phylogenetic studies to illustrate this point.

The main difficulty faced by geologists undertaking paleogeographic reconstruction of the region is that few outcrops exist across all of TSA. Most of the geologic history is buried deep beneath the modern forest. However, it is also true that geologists have made unfounded assumptions in interpreting the geologic and geochemical data from scattered outcrops and drill cores. We will briefly review and provide our own evaluation of prior interpretations of key elements of the history of TSA, such as the Cenozoic history of the uplift of the Andes, marine incursions in the western Amazon, the so-called Pebas mega-lake or mega-wetland, the role of “tectonic arches” in affecting Cenozoic drainages, the eastward flow of the Amazon to the Atlantic, and Quaternary or earlier climate variation. The best future path to reconcile these issues and to accurately constrain the nature and timing of vicariant events is to obtain much more substantial geologic data, and the only effective way to do that is by deep drilling across the entire Amazon.