WHEN INVASION AND RADIATION DO NOT COINCIDE: FRESHWATER CORBULID BIVALVES FROM THE NEOGENE OF WESTERN AMAZONIA AND THE PALEOCENE OF NORTH DAKOTA

The biota of the modern Amazon River system is notable in several ways, including the occurrence of endemic freshwater taxa from typically marine clades. Similarly, freshwater Neogene deposits from western Amazonia, including the Miocene Pebas Formation of northeastern Peru, contain endemic molluscs of typically brackish/marine taxa. Some of these taxa occur within intervals of marine incursion, but corbulid bivalves are represented by several genera (including Pachydon) believed to have invaded and radiated within this large fluvial-lacustrine system. At least one Pebasian corbulid, however, has a much longer geologic history.

Several Paleocene corbulid species from the North American Western Interior were once assigned to Pachydon, although they were subsequently reassigned to the marine genus Bicorbula. One of these species (P.? mactriformis), however, occurs in freshwater molluscan assemblages of the Fort Union Group in North Dakota. We conducted phylogenetic analyses that included P.? mactriformis, several species of Pebasian Pachydon and Pebasia dispar, and representatives of genera reported in Neogene marine deposits of tropical America. Our results indicate that most, but not all, South American Pachydon, form a monophyletic group. The Paleocene species P. mactriformis is nested within this clade, which is a sister taxon to the inequivalved marine Varicorbula clade. In addition, Pebasia dispar appears to be basal in a clade that includes Pachydon s.s. + Varicorbula.

These results indicate that for Pachydon, the most species rich Pebasian corbulid genus, initial invasion and subsequent radiation occurred on two different continents and were separated by 10’s of millions of years. Nonetheless, South American freshwater corbulids include several lineages. In addition to the Pachydon clade and Pebasia, Pachydon erectus appears to be convergent with Pachydon s.s. Two other corbulid genera, Ostomya and Guianadesma, also occur in the Neogene of the western Amazon, although they are not yet included in our analyses. Therefore, although Pachydon did not originate in the Neogene of South America, corbulids did undergo an endemic radiation within freshwater habitats both at and above the species level in western Amazonia at this time.