The implications of a dry climate for the paleoecology of the fauna of the Upper Jurassic Morrison Formation
Introduction
The Morrison Formation has long been known for fossil vertebrates, especially the dinosaur faunas collected by Cope, Marsh, and others over the past 100 years Ostrom and McIntosh, 1966, Breithaupt, 1998, Monaco, 1998, and, to a lesser extent the fossil mammals (Engelmann and Callison, 1998). However, additional elements of the flora and fauna represent a diverse biota (Chure et al., 1998). Many depictions of the Morrison ecosystem have been produced over the past century. Such reconstructions followed the broad sedimentologic interpretation of a fluvio-lacustrine environment, but were strongly influenced by assumptions about the needs and characteristics of the dinosaurs, especially the sauropods. The sauropods were once regarded as gigantic lizards that spent much of their time in water, and were pictured in wet, swampy environments with deep bodies of water by Knight, Zallinger, and other artists (Colbert, 1961). Bakker (1971), Coombs (1975), and others challenged such prevailing interpretations of the sauropods by pointing out that nothing about them was characteristic of aquatic animals. They argued that elephantine reconstructions of sauropods with graviportal limbs implied that they were adapted for dry land rather than marshy conditions.
Dodson et al. (1980) reviewed the evidence of the dinosaur fauna of the Morrison along with that of associated lithologies and saw indications of a relatively dry climate. They proposed a strongly seasonal climate with periods of water scarcity. Discussions of the Morrison paleoenvironment in recent years have tended to follow this interpretation of relatively dry climatic conditions (Farlow et al., 1995).
More recent studies have found further support from diverse sources for interpretation of a semiarid climate with wet and dry seasons as follows.
Retallack (1997) interpreted the Morrison paleosols as indicating annual precipitation of only 600–900 mm with a dry season. He concluded that the soils probably supported dry, open woodland. Demko and Parrish (1998) interpreted the Morrison soils as having formed in a semiarid climate with some seasonal rainfall.
Isotopic analyses of pedogenic carbonates and other materials by D.D. Ekart (oral communication, 1998) reveal oxygen isotope ratios characteristic of a rain shadow effect or strong continentality in the climate of the Morrison, and high levels of atmospheric CO2 that would have produced warm temperatures in the Late Jurassic.
Pedogenic and lacustrine carbonates of the Morrison were formed in a climate best described as semiarid to transitional (to subhumid), with wetter conditions restricted to the northernmost and latest locations Dunagan, 2000, Dunagan et al., 1996. Features of these carbonates also support the interpretation of seasonal drying.
The presence in the Morrison of eolian sediments and a large alkaline, saline lake provide strong evidence of episodes of at least semiarid conditions Peterson and Turner-Peterson, 1987, Turner and Fishman, 1991, Peterson, 1994, Dunagan and Turner, 2004.
Demko and Parrish (1998) reviewed the results of qualitative conceptual circulation models and numerical General Circulation Models for the Late Jurassic and found that they predicted a rain shadow effect that would create semiarid to arid conditions over the depositional basin of the Morrison. Computer simulations of Kimmeridgian climate reported on by Valdes (1994) indicate a semiarid climate with estimates of precipitation of 1–2 mm/day in the winter and <1 mm/day in the summer. Sellwood et al. (1998) point out that different climate models for the Late Jurassic all indicate that the climate in which the Morrison was deposited was at least seasonally dry. Moore and Ross (1996) compared the geographic distribution of Late Jurassic dinosaur localities with paleoclimatic models for that time and found that they were concentrated where the model predicted that evaporation exceeded precipitation.
The Morrison biota has provided relatively little help in refining interpretations of climate. This has been especially true for the dominant, large vertebrate fauna, the dinosaurs, because they are so different from their closest living relatives and only distantly related to possible modern ecological analogues. Foster (1998) attempted a comprehensive survey of the Morrison fauna that was corrected for taphonomic bias, and concluded that the Morrison paleocommunity was unlike any modern community or most ancient ones, especially in the abundance and diversity of large herbivores. But, if we cannot rely on the dinosaurs and other elements of the fauna to provide a clear climatic and ecological signal for the Morrison Formation, perhaps it would be informative to accept the constraints on environmental interpretation from the geological evidence and consider how the fauna could have adapted to the conditions indicated. This perspective may help to provide insights into the paleobiogeography of the Morrison ecosystem.
The evidence cited above seems to most consistently support the interpretation that the overall climate of the Morrison paleoenvironment can best be described as semiarid. There was probably a marked seasonality, with a rainy season of unspecified duration punctuating relatively dry conditions. Semiarid regions can be biologically very productive, with diverse subenvironments, but availability of water is likely to be a limiting factor. Well-adapted communities may not experience severe stress from scarcity of water on an annual basis, but semiarid regions are likely to be vulnerable to severe drought conditions at somewhat longer intervals. It is such physical environmental conditions that we postulate for the Morrison ecosystem.
Section snippets
Plants
Plants are represented in the Morrison by plant macrofossils of logs of large trees, leaves, stems and fruiting bodies of conifers, ginkgoes, cycads, ferns, and horsetails Ash and Tidwell, 1998, Tidwell et al., 1998, Engelmann, 1999, Engelmann and Fiorillo, 2000. There are also charophytes (Schudack et al., 1999), and a diverse palynoflora (Litwin et al., 1998).
Evidence from the plants has been interpreted by some workers as indicating humid or mesic environmental conditions throughout the
Invertebrates
Ostracodes (Schudack et al., 1998) and conchostracans (Lucas and Kirkland, 1998) indicate the presence of ephemeral bodies of freshwater, while gastropods (Evanoff et al., 1998) and bivalve mollusks require that some streams had perennial flows for at least periods of several years, but also show evidence of seasonality (Good, this volume). As with the plants, these invertebrates provide evidence of those times and places where water was available.
Trace fossils provide evidence of a diverse
Lower vertebrates
The fish fauna of the Morrison (Kirkland, 1998) includes ray-finned fish that indicate the presence of stable bodies of freshwater. However, lungfish are perhaps the best represented elements of the fish fauna. Lungfish are adapted to stagnant, restricted bodies of water, and some can aestivate in burrows for long periods of time when the water dries up completely.
Frogs and salamanders occur in the wetland deposits of the Morrison (Henrici, 1998), along with turtles and crocodilians that appear
Sauropods
Sauropod dinosaurs dominated the Morrison ecosystem in many respects. Not only were they the largest animals in the fauna, but at the generic level, sauropods constitute more than half the diversity of herbivorous dinosaurs in the Morrison. Why is there such diversity among the large, sympatric herbivores? Differences in the dentition and body form of the sauropod species suggest the possibility that there may have been some kind of resource partitioning with respect to food and other resources.
Mammals
The mammals of the Morrison Formation (Engelmann and Callison, 1998) are far removed from any living mammals in their history of adaptive modification. They are therefore of little value as environmental indicators. The specific adaptations of the Morrison mammals are not apparent from what is known of their anatomy. It seems likely that at least some, such as the triconodonts and dryolestoids, depended on a diet of small invertebrates and possibly small vertebrates. Whether some of the
Taphonomic considerations
Apparent contradictions in climatic indicators between the physical features of the stratigraphic record and the characteristics of the fossil record, particularly the plants, have led us to follow Demko and Parrish (1998) and Retallack (1997) in postulating an environment in a semiarid climate with a diverse habitat structure. Within that environment, elements characteristic of wetter conditions are considered spatially and temporally local. We have given a brief overview of the entire
Conclusions
The fossil flora and fauna of the Morrison Formation offer little help in constraining the general climatic conditions of the paleoenvironment. Although sedimentologic evidence of diverse types seems to consistently indicate strongly seasonal, semiarid conditions during the time of deposition of the Morrison, the plants and some invertebrates and small vertebrates seem to indicate wet conditions. These conflicting interpretations can be resolved if one assumes an overall semiarid environment
Acknowledgements
Support for this research was provided by the National Park Service through the Morrison Extinct Ecosystem Project. Our work has benefited tremendously from interaction with our colleagues in the project who made it truly a cooperative and interdisciplinary effort. The report has been improved by the comments and suggestions from those colleagues, the reviewers: J.M. Parrish and A.K. Behrensmeyer, and the editors: C.E. Turner, F. Peterson and S.P. Dunagan.
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