RESUMEN
The distribution of trace-making organisms in coastal settings is largely controlled by changes in physicochemical parameters, which in turn are a response to different climatic and oceanographic conditions. The trace fossil Macaronichnus and its modern producers are typical of high-energy, siliciclastic foreshore sands in intermediate- to high-latitude settings characterized by cold-water conditions. However, it has been found in Miocene Caribbean deposits of Venezuela, prompting the hypothesis that upwelling of cold, nutrient-rich waters rather than latitude was the main control of its distribution. To test this hypothesis that was solely based on the fossil record, several trenches and sediment peels were made in two high-energy sand beaches having different oceanographic conditions along the Pacific and Caribbean coasts of the Central American Isthmus. As predicted, the burrows were found only in the highly productive waters of the Pacific coast of Costa Rica in connection with upwelling, while they were absent from the warm, oligotrophic waters of the Caribbean coast of Panama. This finding demonstrates that sometimes the past may be a key to the present, providing one of the few documented examples of reverse uniformitarianism.
RESUMEN
We report two fossil procyonids, Cyonasua sp. and Chapalmalania sp., from the late Pliocene of Venezuela (Vergel Member, San Gregorio Formation) and Colombia (Ware Formation), respectively. The occurrence of these pre-Holocene procyonids outside Argentina and in the north of South America provides further information about the Great American Biotic Interchange (GABI). The new specimens are recognized in the same monophyletic group as procyonids found in the southern part of the continent, the "Cyonasua group," formed by species of Cyonasua and Chapalmalania. The phylogenetic analysis that includes the two new findings support the view that procyonids dispersed from North America in two separate events (initially, previous to the first major migration wave-GABI 1-and then within the last major migration wave-GABI 4-). This involved reciprocal lineage migrations from North to South America, and included the evolution of South American endemic forms.