The Morrison Extinct Ecosystem Project is a joint NPS-USGS-funded interdisciplinary study to reconstruct the Late Jurassic predominantly terrestrial ecosystem throughout the Western Interior during deposition of the Morrison Formation. This colorful formation is known worldwide for the skeletons of large dinosaurs, especially the giant sauropods, that have been recovered from it and displayed in many museums throughout the world. The formation is exposed in many NPS units including Arches NP, Bighorn Canyon NRA, Black Canyon of the Gunnison NM, Capitol Reef NP, Colorado NM, Curecanti NRA, Devils Tower NM, Dinosaur NM, Glacier NP, Glen Canyon NRA, Grand Staircase-Escalante NM, Hovenweep NM, Wind Cave NP, and Yellowstone NP. The goal of the project is to improve NPS interpretive programs and resource management strategies by applying modern research approaches that will yield an improved understanding of the habitat that existed when the Late Jurassic dinosaurs roamed the western U.S.
The multidisciplinary approach allowed us to study various aspects of the rock and biostratigraphic record for the Morrison Formation, with various lines of evidence leading to an integrated picture. The investigations included studies of regional tectonics, regional stratigraphic framework, radiometric and paleontologic dating, sedimentology, paleosols (fossil soils), dinosaur biostratigraphy, trace fossils, taphonomy (processes that occur between the death of an organism and discovery as a fossil or trace fossil), microfossils, invertebrates, smaller vertebrates, and isotopic analysis of teeth and paleosol nodules. Integration of data from the various studies should result in one of the most complete understandings of an ancient continental ecosystem. Although not entirely finished, enough research has been completed to give a new and improved interpretation of the Late Jurassic ecosystem.
Isotopic dating shows that the Morrison was deposited 155–148 million years ago. Deposition ceased about 7 million years before the close of the Jurassic Period, which ended approximately 141 million years ago. Paleontologic dating demonstrates that the Morrison was deposited during the Kimmeridgian and early Tithonian Ages in the Late Jurassic Epoch.
Because of continental drift, the Western Interior depositional basin was about 650 km (400 mi) farther south than today, placing the Four Corners near the latitude of the southern border of Arizona. Data from other workers dealing with climate on a global scale suggests the Earth was appreciably warmer than today (for example, polar ice caps were absent). Stable isotopes in carbonate nodules from Morrison paleosols indicate a significantly higher carbon dioxide content in the atmosphere than at present. This suggests that the Late Jurassic climate in the Western Interior was appreciably warmer than today.
During the Late Jurassic, a volcanic mountain chain somewhat similar to the present-day Andes existed along the west coast of North America more or less along the California border with Arizona and Nevada. Another highland lay farther inland roughly along the Nevada–Utah state line. The nature of the terrain between these two areas is unclear but it probably included a small number of scattered volcanoes.
To the east of the highlands lay the vast Western Interior lowland plain on which the Morrison Formation was deposited. The inland plain extended from Arizona and New Mexico northward to Montana and into Alberta and British Columbia, Canada. The lowland may have originally extended much farther east, as some beds of possible Late Jurassic age in Iowa and Michigan suggest. Streams originating in the highlands flowed generally eastward, carrying their bedload of sand and gravel onto the aggrading Morrison alluvial plain.
Westerly to southwesterly winds left much of the inland plain in a rain shadow and were responsible for the dry climate that prevailed in most of the region. For much of the time, the climate was semiarid and perhaps arid, as indicated by deposits of bedded gypsum, which forms under highly evaporative conditions; windblown sandstone deposits; magadi-type chert; and saline, alkaline lake beds similar to strata deposited in present-day playa lakes of southeastern California. Somewhat wetter time intervals occurred, perhaps seasonally or on longer time intervals. We envision an environment where surface water was scarce much of the time and was only abundant seasonally and/or during infrequent storms. Some water entered the Western Interior basin, either as surface runoff from precipitation in the highlands farther west or through underground aquifers that were recharged from precipitation in highland source areas. Perennial streams that drained the highlands to the west traversed the Morrison alluvial plain but most likely many of the streams flowed only intermittently during most of Morrison time.
Somewhat wetter conditions probably developed throughout the region toward the end of Morrison deposition, as suggested by black mudstone beds that occur near the top of the formation at scattered localities from the Colorado Front Range foothills to Montana. Abundant carbonaceous mudstone and coal beds at the top of the Morrison in central Montana also suggest greater precipitation (or at least less evapotranspiration) and a temperate climate, at least in the northern part of the western Interior plain, toward the end of Morrison deposition
Although eleven formally named members are recognized in the Morrison (all but two restricted to the Colorado Plateau), for simplicity the formation is divided into upper and lower parts that are separated by a conspicuous difference in clay mineralogy in the mudstone and claystone beds. Clay minerals in the lower part consist dominantly of non-swelling types whereas clay minerals in the upper part consist dominantly of swelling (smectitic) types. The vertical change in clay mineralogy can be traced as far north as northern Wyoming but is not detected in Montana or the Black Hills of northeastern Wyoming and western South Dakota, where all the clays in the formation are of the non-swelling type. Where present, the change in clay mineralogy constitutes a valuable marker horizon for correlation purposes. About 6-15 m (20-50 ft) below the clay change is a fairly persistent paleosol (or closely spaced series of paleosols) that also is fairly widespread and appears to be another useful marker horizon near the middle of the formation. Another closely related formation that correlates with the lower part of the Morrison on the Colorado Plateau and farther north is the Ralston Creek Formation, which is only recognized in the Front Range foothills west of Denver.
The Morrison yields a large and varied fossil fauna and flora that appears, at first glance, to contradict the rather dry or semiarid environment that is interpreted from the rocks. For the most part, water in stream channels probably only flowed intermittently, or seasonally, but it was sufficient to allow small lakes or ponds to form in many places. The lakes and ponds probably dried up completely from time to time, as suggested by dinosaur tracks in some of the lacustrine mudstone beds and by the presence of numerous bones in mudstone beds that are interpreted as waterholes that went dry. Plant fragments, spores and pollen, charophytes, stromatolites, oncolites, sponge spicules, mollusks, and rare fish remains attest to the variety of life supported by many of these lakes, even though many of them may have been ephemeral.
Abundant trace fossils indicate that many other organisms found homes in the Morrison ecosystem. Termite nests as much as 40 m (130 ft) tall in the lower part of the formation in the southern San Juan Basin of northwestern New Mexico indicate that the water table there was at least that far beneath the surface. In the Colorado Plateau region, crayfish burrows are fairly common; most of the burrows extend less than about 5 m (15 ft) below the paleoground surface. Because crayfish burrow down to the water table, their burrows indicate a fairly shallow water table that probably could have allowed plants to survive in spite of the dry climate.
Dinosaur bones and skeletons have been recovered in many parts of the Western Interior and from much of the vertical thickness of the formation in one place or another. The bones are most commonly found in the upper part of the lower Morrison and throughout all but the uppermost part of the upper Morrison. Changes in the dinosaur fauna occur near the middle of the formation and correspond approximately with the distinct paleosol zone and also with the vertical change in clay mineralogy. This correspondence suggests that the dinosaur faunas changed in response to perturbations in the ecosystem, perhaps climatic and/or tectonic in nature. The paleosol zone indicates a decrease in depositional rate and the clay change reflects increased volcanic activity, both of which signal changes in the ecosystem that the dinosaurs were apparently sensitive to.
During the earliest stages of deposition of the lower Morrison (Windy Hill Member and correlative strata), a seaway that was an arm of the ancestral Pacific Ocean extended east across Wyoming and into adjacent parts of Montana, the Dakotas, Nebraska, northern Colorado and northern Utah. Farther south in southeastern Utah and in western and eastern Colorado, gypsum in the Tidwell Member and correlative Ralston Creek Formation was precipitated as evaporite deposits in hypersaline lagoons at the southern margin of the seaway. Elsewhere on the Colorado Plateau, the Tidwell contains lacustrine limestone and mudstone beds that contain charophytes (green algae) and ostracodes, indicating somewhat equable freshwater environments. Locally, however, these lakes were the sites of magadi-type chert accumulation, which reflects highly concentrated lake waters.
Subsequently, the seaway retreated to the northwest into Canada and streams that drained upland regions to the west of the depositional basin carried gravel, sand, and mud eastward, building an extensive alluvial plain, represented largely by the Salt Wash Member and correlative rocks. In central Colorado, scattered low hills that were remnants of the ancestral Rockies, were sufficiently high to support small streams that probably were intermittent in nature and unrelated to the Salt Wash fluvial system. Small lakes and ponds also developed locally on the alluvial plain as well as in the most distal regions in eastern Colorado and eastern Wyoming. During times when the streams went dry in the Colorado Plateau region, winds from the west and southwest removed sand from the dry stream beds and deposited it farther downwind in extensive dune fields that covered large parts of the Four Corners area. These deposits are represented today by the Bluff Sandstone Member and Junction Creek Sandstone Member, as well as the eolian sandstone facies of the Recapture Member. Smaller scattered dune fields also developed farther north in northern Utah, northwestern Colorado, Wyoming, and South Dakota (Unkpapa Sandstone Member on the east side of the Black Hills), and probably formed by deflation of previously deposited shallow marine sands.
During deposition of the upper part of the Morrison Formation, a large stream complex in the Colorado Plateau region (Westwater Canyon and Fiftymile Members) gave way to a large shallow saline, alkaline lake called Lake T’oo’dichi’, which covered parts of northwestern New Mexico, northeastern Arizona, southeastern Utah, and southwestern Colorado during deposition of much of the Brushy Basin Member. Although much shallower, Lake T’oo’dichi’ had about the same areal extent as Lake Michigan. Judging from similar modern saline, alkaline lakes, the alkalinity of the pore waters would at times have been high enough to cause alkaline burns to human (and possibly dinosaur?) skin. Development of the lake attests to the aridity of the time, as evaporation must greatly exceed precipitation and runoff to achieve the alkalinities and salinities recorded in the mineralogy of the ancient lake deposits. A small amount of surface water entered the lake by intermittent or perhaps perennial streams, but ground water was also an important component of lake hydrology. At times when the lake dried out to form a large pan or salina, flash floods carried sand well out into the lake basin. During times of high evaporation, the waters were too saline and alkaline for most animals to drink.
Throughout most of Morrison time, somewhat fresher water lakes developed east and north of the present-day Front Range of the Rocky Mountains where lacustrine limestone beds that contain a varied fossil assemblage attest to the availability of more potable waters.
Toward the end of Morrison deposition, large fluvial complexes, including the Jackpile Sandstone Member, were locally established as a result of renewed uplift in the highlands west and southwest of the Western Interior. Increased precipitation, especially in the highlands, probably was responsible for the renewed stream activity at this time.
Morrison deposition came to a close with development of a thick fossil soil, indicating a long cessation of deposition toward the end of the Jurassic. The fossil soil, although a useful marker for the top of the Morrison Formation, is only locally preserved because of erosion during the succeeding depositional hiatus and/or by scouring that accompanied deposition of overlying lowermost Cretaceous fluvial strata.
In summary, the habitat for the Late Jurassic dinosaurs was a broad alluvial plain in the rainshadow of highlands that bordered the plain to the west. The presence of large dunefields, evaporites, intermittent streams, and development of a large alkaline, saline lake at various times during Morrison deposition all indicate that the Morrison experienced times of considerable aridity. In spite of the semi-arid to arid climate, life-giving water was delivered to the plain by several means. These included seasonal or intermittent precipitation, perennial and intermittent streams that drained the western highlands, and shallow groundwater that was delivered by aquifers recharged by infiltration in the highlands.
Riparian vegetation was largely supported by water from perennial streams and substream flow within intermittent streambeds. Vegetation on the floodplain had to depend primarily on direct precipitation onto the floodplain (which may have been largely seasonal) and the ability to tap shallow aquifers. The large herbivorous dinosaurs could range across the plain in search of water and vegetation, whereas many of the smaller animals would have found water and shelter in riparian habitats. Scattered lakes and ponds across the alluvial plain supported a variety of aquatic life. Thus, although the Morrison climate was much drier than originally interpreted, the Morrison ecosystem supported a considerable diversity of life, including the largest herbivores that ever lived.
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