A Holistic Approach to Reconstructing Triassic Paleoecosystems: Using Ichnofossils and Paleosols as a Basic Framework


STEPHEN T. HASIOTIS1, RUSSELL F. DUBIEL2, and TIMOTHY M. DEMKO3

1Department of Geological Sciences, University of Colorado, Campus Box 399, Boulder, CO 80309-0399
2U.S. Geological Survey, Box 25046, Denver, CO 80225
3Department of Earth Resources, Colorado State University, Fort Collins, CO 80523


AbstractIn situ indicators of inhabitants and indicators of environmental settings used for reconstructing paleoecosystems are typically not used, and often are secondarily considered compared to the use of plant and vertebrate fossils to characterize paleoecosystems. Environmental and climatic interpretations are formulated from the gross sedimentology, palynomorph composition, and faunal and floral taxonomic components. Much of the paleontological evidence used for the reconstructions and climatic interpretations are often reworked, time-averaged, and out of ecological context.

We propose that paleosols and ichnofossils, in conjunction with the sedimentology and stratigraphy of a unit, be used as a basic framework from which to build on with other paleontological evidence for reconstructing paleoecosystems. Ichnofossils preserve in situ organism behavior that record interactions with other organisms and their environment. Paleosols record the effects of environment, ecologic interactions, and climate. Ichnofossils record the lower portions of the food web, the infaunal components of ecosystems, and the locations and ranges the mobile herbivores and carnivores. When paleosols and ichnofossils are evaluated together the environmental and climatic trends of paleoecosystems can be reconstructed and detailed by other sedimentological, paleontological, and geochemical evidence.


Introduction

Paleoecosystem reconstructions are typically based on plant and vertebrate body fossils that are often reworked and interpreted out of ecological context. The plant and vertebrate fossils also represent primary producers and secondary consumers, respectively. Many invertebrates and their ecologic roles as secondary and tertiary consumers and detrital recyclers go undetected because they are rarely preserved as body fossils. Trace fossils of invertebrates are frequently preserved in many different environments, occur in situ, and are not subject to reworking. The recent recognition of invertebrate ichnofossils in continental settings and their ecological importance provides information about previously unrecorded biodiversity and ecological interactions (Hasiotis and Dubiel, 1993a,b; Hasiotis, 1997).

Ichnofossils are the result of organism-substrate interactions that record in their structure both morphological and behavioral data. Thus, they are valuable sedimentologic and paleontologic interpretational tools for geologists (Hasiotis and Bown, 1992). Ancient and modern animal burrowing and plant-root penetrations also modify substrates in which they occur. The interaction of different suites of organisms with various types of substrates result in different types and stages of soil (paleosol) formation. Invertebrate and plant traces are sensitive indicators of depositional energy, temperature, precipitation, water chemistry (i.e., salinity and alkalinity), oxygenation, substrate consistency, hydrology, biological competition, and nutrient availability; all organisms are physiologically constrained by these environmental parameters. Together, these physical, biological, and chemical components define an ecosystem and provide insight into the paleogeography and paleoclimate (Jenny, 1941; Aber and Melillo, 1991; Hasiotis
and Bown, 1992; Hasiotis, 1997).

Based on our research in the Upper Triassic Chinle Formation in and around Petrified Forest National Park, Arizona, we are strong proponents for the use of ichnofossils and paleosols as a basic framework for reconstructing paleoecosystems. We feel that this framework, when combined with body fossil evidence, will provide a more detailed, holistic, and accurate description of Late Triassic paleoecosystems.

Ichnofossils And Paleosol Evidence

In modern, as well as in ancient, continental settings, the distribution of vegetation types, biodiversity patterns, and soil types constituting the major terrestrial biomes closely corresponds to latitudinal variation in climatic regimes (Aber and Melillo, 1991). In the Upper Triassic Chinle Formation, trace fossils and paleosols (serving as proxies for organisms and soils, respectively) are preserved in nearly every depositional environment. They are not as subject to dissolution as body fossils, and are rarely reworked by subsequent depositional processes as body fossils. Thus, these ichnologic and pedogenic features provide essential information to fully reconstruct the in-place, original faunal, floral, and edaphic components of Late Triassic Chinle ecosystems.

Ichnofossils.—Insects and other arthropods constitute the majority of ichnofossil constructors recovered from the Chinle Formation. Today, insects and other continental arthropods constitute 90% of the biodiversity in the world. Chinle trace fossils of millipedes, horseshoe crabs, crayfish, gastropods, mollusks, nematodes, aquatic and terrestrial earthworms, caddisflies, flies, moths, beetles (semi-aquatic, terrestrial, and wood-boring), termites, soil bugs, bees, and wasps provide evidence for the occurrence and interactions of arthropods in the Triassic paleocommunities (Hasiotis and Bown, 1992; Hasiotis and Dubiel, 1993a,b; Hasiotis, 1997). These ichnofossils are important indicators of the arthropods' ecologic roles as herbivores, carnivores, omnivores, and detritivores within primary, secondary, and tertiary levels of the food web.

The depth, tiering, and distribution of trace fossils illustrate the differences of soil moisture and water table levels in different environments (Hasiotis and Bown, 1992; Hasiotis, 1997). For example, extant crayfish burrow to the depth of the water table, soil bugs prefer the intermediate vadose zone, and bees and wasps prefer the upper vadose zone (Hasiotis and Bown, 1992; Hasiotis and Dubiel, 1993a, b; Hasiotis, 1997). Shallow and surface traces of terrestrial beetles abound in point-bar, levee, and overbank floodplain deposits with high soil moisture and water table levels. Termite nests dominate intermediate depths of distal overbank floodplain deposits with low soil moisture and modified by mature soils. Bees and wasps nests also occur with termite and beetle traces, but dominate shallow and intermediate depths of proximal floodplain deposits with moderate soil moisture levels. Shallow to deep crayfish burrows primarily occur in levee and proximal floodplain deposits that are imperfectly drained with highly fluctuating water tables. Surface and very shallow horseshoe crab crawling trails are found mainly on the wet, firm substrates (bedding planes) of point-bar and levee deposits and feeding traces are just below the surface. Flow regime, turbidity, and substrate consistency controlled the distribution of snail, gastropod, oligochaete, and nematode burrows and trails in lentic and lotic water bodies. The occurrence and distribution of these and other organism-substrate interactions were controlled by 1) the depth and fluctuation of the water table, 2) soil moisture levels, 3) depositional energy, 4) substrate texture and consistency, and 5) food web interactions.

Vertebrate ichnofossils, including tracks and trails of small aquatic reptiles, metaposaurs, phytosaurs, and dicynodonts, complement the invertebrate trace fossils. Metaposaur and phytosaur tracks dominate channel and point-bar deposits. Dicynodont tracks occur in levee deposits. Small reptile tracks are found in point-bar and levee deposits. These tracks demonstrate exactly where these organisms spent their time in the environment and the activities in which they may have been involved (herbivory, carnivory, breeding, feeding, etc.), whereas body fossils mainly demonstrate where they died or were carried away and accumulated after death.

Further ecologic information is obtained from rooting patterns and rhizomes of plants preserved in immature to mature paleosols (discussed below). The depth and configuration of roots and rhizomes reflect the amount of soil moisture and depth of the water table in a particular setting. The size of the roots also reflect the stratification of vegetation above the soil surface; canopy trees have large root systems, ground-cover plants have shallow and fine roots, intermediate plant cover has root dimensions intermediate between the other plants.

Paleosols.—Variations in Chinle alluvial, lacustrine, and eolian paleosols reflect lateral and temporal changes in Trias
sic climate, paleogeography, paleohydrology, infaunal biota, and vegetation. As in modern soil-forming processes, parent material, topography, biota, climate, and time constitute the factors that determine what type of soils develop (Jenny, 1941). Major types of paleosols present in the Chinle include Gleysols, Alfisols, Vertisols, Calcisols, and Aridisols, all of which also range in stages of maturity based on the amount of time in their formation (e.g., Mack et al., 1993).

Gleysols are abundant in the basal Chinle (Shinarump Member/"mottled strata"), where they are characterized by extensive purple, yellow, and white mottled horizons, contain deep crayfish burrows and roots, and indicate deep though fluctuating water tables. Alfisols are common in floodplain mudrocks in the lower and middle Chinle (Monitor Butte and Petrified Forest Members), consisting of thick red, clay-rich horizons, locally exhibiting small carbonate nodules. These soils contain red-purple mottles, abundant beetle burrows (Scoyenia) and small rhizoliths, and indicate predominantly moist soils and persistently high water tables. Vertisols (Monitor Butte and Petrified Forest Members) are characterized by clay-rich horizons, deep mudcracks, slickensides, gilgai micro-relief, carbonate nodules, crayfish burrows and extensive rhizoliths, and are indicative of periods of wetting and drying. Calcisols (Owl Rock and Church Rock Members) with carbonate accumulations in their upper portions, contain rhizotubules and rhizocretions, and occur predominantly in siltstone deposits. Aridisols (Church Rock Member) contain varying stages of carbonate nodule development, few rhizoliths, rare bioturbation, and indicate decreased precipitation coupled with persistently deeper water tables.

Significance

The integration of ichnologic, sedimentologic, and paleopedologic information allows for a more complete reconstruction of paleoecosystems, including interpretation of their hydrologic and climatic settings. The sequences of sedimentary facies, paleosols, and associated ichnofossils provide hierarchical criteria to interpret long-term and short-term trends in the evolution and succession of Chinle ecosystems and climates. These criteria also comprise the internal framework that plant, invertebrate, and vertebrate fossils can be placed in to reconstruct various paleocommunities with distinct biological, environmental, and climatic attributes.

Based on all the physical, biological, and chemical evidence collected to date, the Petrified Forest monsoonal (wetter periods with higher humidity) climate became increasingly arid during the Late Triassic (Carnian to Norian). During the deposition of the Shinarump and Monitor Butte Members, the early Chinle (Carnian) climate was warm and humid with ample rainfall. During deposition of the Petrified Forest and Owl Rock Members (Norian), climate became strongly monsoonal with strongly seasonal rainfall and high temperatures. Climate during the deposition of the latest Chinle, represented by the Church Rock Member (latest Norian), became increasingly arid with less precipitation and greater temperature extremes due to lower humidity.

Acknowledgments

We thank the park superintendents, rangers, resource managers, paleontologists, administrative and field personal, interns, and colleagues who have assisted our research over the years at Petrified Forest National Park, Arizona. Without their support and foresight, this work would not be possible. We also thank the many people who volunteered their time with us in the field to collect sedimentologic, paleontologic, and paleopedologic data. This work is part of a dissertation conducted by STH at the University of Colorado, Boulder.

References

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———, and T. M. Bown. 1992. Invertebrate trace fossils: The backbone of continental ichnology. In Maples, C. G. and West, R. R. (eds.), Trace Fossils. Short Courses in Paleontology, Number 5, The Paleontological Society, p. 64-104.

———, and R. F. Dubiel. 1993a. Crayfish burrows and their paleohydrologic significance - Upper Triassic Chinle Formation, Ft. Wingate, New Mexico. In Lucas, S. G. and Morales, M., eds., The Nonmarine Triassic Symposium, New Mexico Museum of Natural History, Bulletin 3, p. G24-G26.

———, and ———. 1993b. Trace fossil assemblages in Chinle Formation alluvial deposits at the Tepees, Petrified Forest National Park, Arizona. In Lucas, S. G. and Morales, M., eds., The Nonmarine Triassic Symposium, New Mexico Museum of Natural History, Bulletin 3, p. G42-G43.

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