THE EARLY MIOCENE CULEBRA FORMATION OF PANAMA: BIOGEOGRAPHY AND BIODIVERSITY OF NEOTROPICAL MOLLUSCS BEFORE THE ISTHMUS

Changes in the paleogeography of southern Central America through the Neogene have had important implications on global paleoclimate, regional paleoceanography, and the evolution of both terrestrial and marine faunas. The marine invertebrate fauna of the Culebra Formation in central Panama provides an important window into the paleoenvironments, biogeography, and biodiversity of southern Central America during the early Miocene. This study integrates considerable new material (181 sites, ~4600 specimens) with historic collections (91 sites, ~4700 specimens) from the Culebra Formation to investigate the biogeographic affinities, biodiversity, and paleoenvironmental context of Panama’s early Miocene marine fauna.

Freshly exposed stratigraphy made during excavations associated with recent expansion of the Panama Canal permit high-resolution analysis of environmental changes within the stratigraphy of the Culebra Formation. The Culebra Formation accumulated through a major transgression and regression, during which sediments accumulated lagoonal, fringing reef, and shallow-water deltaic depositional environments. New collections from canal excavations also allow for the analysis of biodiversity in the Culebra Formation, at both the local (community) scale and in the context of other known faunas in the broader Atlantic and eastern Pacific. The Culebra Formation (254 species) is not as diverse as some early Miocene tropical faunas, but its fossil record is clearly skewed by size-related lithification and diagenetic biases. When compared to molluscan faunas in other areas in the tropical and temperate Americas, the Culebra Formation is shown to belong to a biotic province extending from northern Peru through northern South America, and the southern Caribbean. These results clearly indicate unobstructed communication of faunas through the Central American Seaway during the early Miocene.