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Document Type

Grand Teton National Park Report

First Page

84

Last Page

91

Abstract

The Morrison Extinct Ecosystem project is generating interesting results that, although preliminary in nature, are starting to converge on an integrated reconstruction of the Late Jurassic habitat in the Western Interior of the United States. A workshop in April, 1996, helped the participants learn where their data are in agreement and where remaining inconsistencies in the data exist. The workshop thus helped focus collective and individual efforts for the remaining field season so that additional field research and sample collection could be directed toward addressing the unresolved questions. A much more complete picture of the Morrison ecosystem is emerging, with such lines of evidence as the isotopic data confirming regional tectonic interpretations, and trace fossils adding subtle but significant clues to the interpretations of the ancient climate and water-table position and fluctuations. A mesic climate interpretation with a moderate amount of available moisture is derived from some of the plant fossils and, at first glance, contrasts with evidence of an arid to semi-arid climate based on the presence of saline, alkaline lake deposits; eolian strata; and evaporite deposits. Thus, the climate may have changed periodically or episodically, leaving evidence in the geologic record of the contrasting climate regimes-not all of which existed at the same time. A possible resolution of these discrepancies is that there was marked seasonality in which part of the year was relatively dry, as reflected by the arid to semiarid environmental indicators, whereas there may have been more precipitation during other parts of the year, as reflected by the more mesic environmental indicators. An additional factor that will need to be evaluated further is the role of the water table. The trace fossils suggest a fairly shallow water table that many of the plants, especially the larger ones, could tap throughout the year and therefore not be unduly affected by seasonal dryness. Also to be evaluated is the possibility that plants grew near stream courses or in overbank ponds in areas having a high water table, and that areas away from the streams were drier. These issues will be the focus of the remaining work, as outlined at the conclusion of the workshop.

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