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The Upper Triassic Chinle Formation was deposited at an exceptional time in Earth's paleogeographic and paleoclimatic history. During the Triassic, the supercontinent Pangaea was at its greatest size, in terms of both aggregated continental crust and exposed land area. Moreover, the exposed land was divided symmetrically about the paleoequator between the northern and southern hemispheres. These conditions were ideal for maximizing monsoonal circulation, as predicted from paleoclimate models. The Chinle was deposited between about 5?to 15?N paleolatitude in the western equatorial region of Pangaea, a key area for documenting the effects of the monsoonal climate. This study summarizes sedimentologic and paleontologic data from the Chinle Formation on the Colorado Plateau and integrates that data with paleoclimatic models. The evidence for abundant moisture and seasonality attest to the reversal of equatorial flow and support the hypothesis that the Triassic Pangaean climate was dominated by monsoonal circulation.The Chinle Formation contains continental lithofacies deposited in fluvial channels, crevasse splays, lakes, bogs, marshes, and lacustrine deltas that reflect abundant precipitation and shallow water tables. Paleosols and ichnofossils indicate that water tables and lake levels fluctuated episodically. Interbedded lacustrine carbonates and marginal-lacustrine siltstones and mudstones indicate longer-term but regular, episodic fluctuations in lake level. Fine-scale laminations in lacustrine carbonates suggest a seasonal influx of clastic sediment, and thus precipitation, to the basin. Uppermost Chinle strata consist of lacustrine and marginal-lacustrine mudstones interbedded with minor eolian sand sheets and eolian dunes; thus, the later Triassic reflects continued precipitation, but was marked by more pronounced and extended dry seasons.The PALAIOS, Vol. 6, No. 4 (Aug., 1991), pp. 347-370 This article is a U.S. government work, and is not subject to copyright in the United States. perennial fluvial and/or lacustrine systems. The tetrapod fauna is dominated by aquatic forms, notably metoposaurid amphibians, and phytosaurian archosaurs; terrestrial archosaurs and synapsids are more poorly represented quantitatively. Vertebrate taphonomy and the nature of skeletal preservation also provide evidence of seasonality during Chinle deposition.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. A B S T R A C TThe Jurassic and Cretaceous are considered to have been warmer than today on the basis of various climate data and model studies. Here, we use the available global record of climate-sensitive sediments, plants, and dinosaurs to infer broadscale geographic patterns for the Late Jurassic. These provide a context for our more detailed accounts of the Morrison and Tendaguru Formations in North America and East Africa. At the global scale, evaporites predominated in low latitudes and coals in mid-to high latitudes. We ascribe these variations to a transition from drier to wetter conditions between the equator and poles. Plant diversity was lowest in equatorial regions, increasing to a maximum in midlatitudes and then decreasing toward the poles. Most dinosaur remains are known from low-latitude to marginally midlatitude regions where plant fossils are generally sparse and evaporites common. Conversely, few dinosaur remains are known from mid-to high latitudes, which have higher floral diversities and abundant coals. Hence, there is an obvious geographic mismatch between known dinosaur distributions and their primary food source. This may be due to taphonomic bias, indicating that most dinosaur discoveries provide only a small window on the diversity and lifestyles of this group. On the basis of our global-and local-scale studies, we suggest that dinosaur preservation was favored in environments toward the drier end of the climate spectrum, where savannas rather than forests predominated. A holistic approach, incorporating climate and vegetation as well as geography, is required to better understand patterns of dinosaur ecology and evolution.

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