Miocene Freshwater Dolphins from La Venta, Huila, Colombia Suggest Independent Invasions of Riverine Environments in Tropical South America

ABSTRACT

The two extant genera of strictly freshwater dolphins Inia and Platanista are the result of convergent evolution to freshwater environments with reduced visibility. Characterized by their long snout and small melon, these extant taxa are clustered into two clades, Iniidae in South America and Platanistidae in Southern Asia. Their evolutionary history leading to freshwater environments remains mostly unknown, because many of their related fossil species have been found in marine environments. Here, we report riverine dolphin remains (two rostral fragments and a periotic) from two stratigraphic levels of the late middle Miocene (ca. 12.5 Ma) from La Venta, Colombia. The periotic has a reduced cochlear aqueduct mediodorsally oriented, the anterior process is relatively thin, and the dorsal opening of the facial canal is located lateral to the spiral cribriform tract. The rostral fragments are dorsoventrally flattened; the mandible features two longitudinal ventral grooves, and the premaxilla-maxilla suture of the rostrum is located in a deep lateral groove. These characteristics indicate that the specimens belong to Platanistidae, the lineage of the Ganges river dolphin Platanista. Platanistids had also been recorded on coeval strata from the Fitzcarrald arch, Peru. The occurrence of middle Miocene platanistids in both the La Venta and Fiztcarrald localities suggests that members of this lineage moved into freshwater environments in South America earlier than the ancestors of the modern Amazon river dolphin Inia. The subsequent collapse of the Pebas ecosystem could have played a role in the extinction of non-marine Platanistoidea in South America.

ACKNOWLEDGMENTS

This research was sponsored by the Smithsonian Tropical Research Institute, the Anders Foundation, 1923 Fund and Gregory D. and Jennifer Walston Johnson. J. W. M-B thanks the Canadian Queen Elizabeth II Diamond Jubilee Scholarships (QES-AS), managed through a unique partnership of Universities Canada, the Rideau Hall Foundation (RHF), Community Foundations of Canada (CFC) and Canadian universities. The QES-AS is made possible with financial support from IDRC and SSHRC. We thank M. Sanchez-Villagra (PIM) for helpful comments on the manuscript and South American fauna. We are grateful to reviewers G. Bianucci (U. Pisa), T. Park (U. Oxford) and editor O. Lambert (IRSBN) for their thorough and helpful review. We extend our gratitude to O Lambert (IRSBN) and G Bianucci (U. Pisa) for helpful comments on the record of ‘river dolphins.’ We thank M. Carvalho (STRI), J. Carrillo (PIM), R. Sanchez (Museo Urumaco), E. Cadena (U. Rosario), C. Suarez (IGM), J. Carrillo (PIM), and O. Aguilera (U. Federal Fluminense) for their help during fieldwork. C. Castillo found the partial rostrum fragment described in this work. We extend our gratitude to A. Rincon (MUN) and A. Reyes (MUN), R. Vanegas (VPPLT), R. Salas-Gismondi (MUSM), M. Urbina (MUSM), and M. Gomez (IGM) for providing access to specimens in their care.