Fossil passerines from the early Pliocene of Kansas and the evolution of songbirds in North America

Seven living species of passerines are identified or tentatively identified from the early Pliocene Fox Canyon locality (4.3-4.8 mya), Rexroad Formation, Meade Basin, southwestern Kansas. All seven species occur in Kansas today, prirnarily as winter residents or transients, and their migratory behavior may extend to the early Pliocene as well. A review of all known passerines from the Neogene of North America indicates that many modern taxa began to appear in the early Pliocene, much earlier than previously believed. This fossil evidence agrees with some mitochondrial DNA analyses for the estimated age of divergence for living species of longspurs (Calcarius lapponicus and C. niccownii), but similar estimates for the divergence of C. ornatus using cytochrome-b analyses is at least 3 my younger than indicated by the fossil evidence. The fossil songbirds, as well as other vertebrate taxa from the Meade Basin, also indicate a paleoenvironment of short-grass prairie and shrubland with wetlands and ponds nearby, similar to that found in the area today. This prairie-steppe habitat first appeared in the Meade Basin by the early Pliocene and remained relatively stable until the Pleistocene, when it periodically appeared and disappeared with climate change. High seasonal productivity associated with this habitat in the early Pliocene may have promoted migratory behavior and speciation events in songbirds during a period of relative climatic stasis. Received 2 January 2005, accepted 5 January 2006.