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The Tertiary sediments of northern Pakistan are an exceptional record of terrestrial sedimentation and represent most of Neogene time. Foremost is the Siwalik Group of the Potwar Plateau, for which multiple, superposed fossil levels span ∼18–6 Ma. Well-developed magnetostratigraphic control provides secure dating so that Siwalik fossil horizons may be interpolated into a time scale with resolution to 100,000 years. We describe the geographic setting of the Potwar, give an overview of the temporal distribution of faunas, and discuss changes in paleohabitat and paleoecology with coinciding faunal change, as seen from the Siwalik viewpoint. The long Siwalik biostratigraphy of many successive assemblages with its resolved time scale may be compared directly with other well-dated sequences. Immigrant arrival and timing of faunal change may be traced. The basins of the Iberian Peninsula show somewhat different timing of introduction of hipparionine horses, and faunal turnover in the Siwaliks clearly precedes the Vallesian crisis in Spain. In contrast to the increasingly seasonal precipitation of the late Miocene Potwar, the paleohabitat of coeval North China appears to have been moist and equable, with high diversity faunas. Continued development and comparison of resolved Neogene records allow increasing resolution of the patterns of faunal change on regional to global levels.

The Siwalik formations of northern Pakistan consist of deposits of ancient rivers that existed throughout the early Miocene through the late Pliocene. The formations are highly fossil- iferous with a diverse array of terrestrial and freshwater vertebrates, which in combination with exceptional lateral exposure and good chronostratigraphic control allows a more detailed and tem- porally resolved study of the sediments and faunas than is typical in terrestrial deposits. Conse- quently the Siwaliks provide an opportunity to document temporal differences in species richness, turnover, and ecological structure in a terrestrial setting, and to investigate how such differences are related to changes in the fluvial system, vegetation, and climate. Here we focus on the interval between 10.7 and 5.7 Ma, a time of significant local tectonic and global climatic change. It is also the interval with the best temporal calibration of Siwalik faunas and most comprehensive data on species occurrences. A methodological focus of this paper is on controlling sampling biases that confound biological and ecological signals. Such biases include uneven sampling through time, differential preservation of larger animals and more durable skeletal elements, errors in age-dating imposed by uncertainties in correlation and paleomagnetic timescale calibrations, and uneven tax- onomic treatment across groups. We attempt to control for them primarily by using a relative-abun- dance model to estimate limits for the first and last appearances from the occurrence data. This model also incorporates uncertainties in age estimates. Because of sampling limitations inherent in the terrestrial fossil record, our 100-Kyr temporal resolution may approach the finest possible level of resolution for studies of vertebrate faunal changes over periods of millions of years. Approximately 40,000 specimens from surface and screenwash collections made at 555 localities form the basis of our study. Sixty percent of the localities have maximum and minimum age esti- mates differing by 100 Kyr or less, 82% by 200 Kyr or less. The fossils represent 115 mammalian species or lineages of ten orders: Insectivora, Scandentia, Primates, Tubulidentata, Proboscidea, Pholidota, Lagomorpha, Perissodactyla, Artiodactyla, and Rodentia. Important taxa omitted from this study include Carnivora, Elephantoidea, and Rhinocerotidae. Because different collecting methods were used for large and small species, they are treated separately in analyses. Small spe- cies include insectivores, tree shrews, rodents, lagomorphs, and small primates. They generally weigh less than 5 kg. The sediments of the study interval were deposited by coexisting fluvial systems, with the larger emergent Nagri system being displaced between 10.1 and 9.0 Ma by an interfan Dhok Pathan sys- tem. In comparison to Nagri floodplains, Dhok Pathan floodplains were less well drained, with smaller rivers having more seasonally variable flow and more frequent avulsions. Paleosol se- quences indicate reorganization of topography and drainage accompanying a transition to a more seasonal climate. A few paleosols may have formed under waterlogged, grassy woodlands, but most formed under drier conditions and more closed vegetation. The oxygen isotopic record also indicates significant change in the patterns of precipitation be- ginning at 9.2 Ma, in what may have been a shift to a drier and more seasonal climate. The carbon isotope record demonstrates that after 8.1 Ma significant amounts of C 4 grasses began to appear and that by 6.8 Ma floodplain habitats included extensive C4 grasslands. Plant communities with predominantly C3 plants were greatly diminished after 7.0 Ma, and those with predominantly C4 plants, which would have been open woodlands or grassy woodlands, appeared as early as 7.4 Ma. Inferred first and last appearances show a constant, low level of faunal turnover throughout the interval 10.7-5.7-Ma, with three short periods of elevated turnover at 10.3, 7.8, and 7.3-7.0 Ma. The three pulses account for nearly 44% of all turnover. Throughout the late Miocene, species richness declined steadily, and diversity and richness indices together with data on body size imply that community ecological structure changed abruptly just after 10 Ma, and then again at 7.8 Ma. Be- tween 10 and 7.8 Ma the large-mammal assemblages were strongly dominated by equids, with more balanced faunas before and after. The pattern of appearance and disappearance is selective with respect to inferred habits of the animals. Species appearing after 9.0 Ma are grazers or typical of more open habitats, whereas many species that disappear can be linked to more closed vege- tation. We presume exceptions to this pattern were animals of the mixed C

The Lower Siwalik, Chinji Formation (Late Miocene) of the Chinji Village area, northern Pakistan has provided remarkable material for the study of terrestrial fossil faunas, magnetic reversal stratigraphy and fluvial sedimentology. This paper considers patterns in the sedimentary stratigraphy, using magnetic reversals to constrain the time framework, and focusing on an intermediate (kilometre) horizontal length-scale. The project aimed to determine the architecture and time relationships of the channel sandstone bodies in a panel 300 m in stratigraphic thickness, and 11 km in horizontal length (along stratigraphic strike). This panel (the 1990 Fence) trends at a high angle to the flow direction of the Late Miocene river channels, and represents about 2 Ma of sediment accumulation. There is a continuous range in thickness of the sandstone bodies, but they can be usefully classified into (i) microbodies, (ii) minor sheets, (iii) thin mega-sheets and (iv) thick mega-sheets. The microbodies are probably mainly marginal features of the thicker bodies. The minor sheets were formed by small river channels, and the mega-sheets were formed as the deposits of the largest, generally braided, channel belts. Two aspects of the intermediate length-scale architecture of the Chinji Formation are analysed: (1) the presence in the Fence of three thick mega-sheets separated by two mudstone-dominated intervals that lasted for about 0.5 and 1.0 Ma, respectively and (2) the abrupt upwards increase in sandstone/mudstone proportions that defines the upper stratigraphic boundary of the Chinji Formation. We suggest that each of the three thick mega-sheet episodes resulted from avulsions into the area of large-channel belt complexes that formed central features of the Chinji river network and were each constrained by scarps or valley-side slopes during episodes of net deposition that lasted for about 100 ka, and may have resulted from climate and/or sea-level changes. The regional upward change from mudstone- to sandstone-domination at the top of the Chinji Formation resulted from a similar, but one-off, and more widespread, change in plan-view style of the river network, produced either by tectonic change in the mountain source area, or by climatic change.

The fluvial Neogene Siwalik formations of northern Pakistan span long time intervals with only minor hiatuses and, being highly fossiliferous, are uniquely suited for studies of change in mammalian faunas. Magnetostratigraphic correlations of a critical stratigraphic section give dates for 45 middle and late Miocene biostratigraphic events. These mark either first appearances or extinctions in the mammal fauna and show that in the Siwaliks there were major fauna turnovers at between 20 and 16 Ma and at 9.5 and 7.4 Ma. Two minor faunal events are dated at 13.2 and about 12 Ma. Many species making their first appearance were immigrants from Europe or Africa and indicate when connections to those regions existed. Immigration and extinction were the dominant modes of faunal change; in situ evolution was much less important. The Siwalik biostratigraphic record correlates closely to climatic, oceanographic, and tectonic events, which probably controlled immigration into southern Asia. Abiotic events were therefore important factors affecting evolution of the mammal communities.