2 LONG GEOLOGICAL RECORDS OF CONTINENTAL ECOSYSTEMS

The early Paleogene continental sequence of northwestern Wyoming and s outh central Montana (USA) and the Neogene Siwalik sequence of norther n Pakistan are exceptionally long, fossiliferous, and well studied in terms of geology, paleontology, mammalian evolution, paleoecology, and paleoclimatology. Each record spans about 15 myr of alluvial depositi on in a foreland basin. The fluvial systems differed in size, drainage of floodplains, and change in alluvial architecture through time. Bot h sequences preserved abundant paleosols, which are a source of paleoc limatic signals in stable isotopes from soil carbonates. Fossil collec tions from both records are dominated by vertebrate remains and especi ally by mammals; the Paleogene sequence also preserved abundant floral remains. Patterns of vertebrate preservation differed markedly betwee n the two sequences, placing inherent limitations on paleoecological r econstruction and on the scope of evolutionary studies of mammalian li neages. In this introductory paper, we summarize the major similaritie s and differences in geologic setting, chronology and time resolution, and mammalian faunal composition of these two long sequences. Finally , we present a brief overview of the organization of papers in this sp ecial issue.