Two long geological records of continental ecosystems
Abstract The early Paleogene continental sequence of northwestern Wyoming and south central Montana (USA) and the Neogene Siwalik sequence of northern 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 deposition in a foreland basin. The fluvial systems differed in size, drainage of floodplains, and change in alluvial architecture through time. Both sequences preserved abundant paleosols, which are a source of paleoclimatic signals in stable isotopes from soil carbonates. Fossil collections from both records are dominated by vertebrate remains and especially by mammals; the Paleogene sequence also preserved abundant floral remains. Patterns of vertebrate preservation differed markedly between the two sequences, placing inherent limitations on paleoecological reconstruction and on the scope of evolutionary studies of mammalian lincages. In this introductory paper, we summarize the major similarities 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 special issue.