CONTINENTAL TRACE FOSSILS, PETRIFIED FOREST NATIONAL PARK: TOOLS FOR PALEOHYDROLOGIC AND PALEOECOSYSTEM RECONSTRUCTIONS

 

 

Stephen T. Hasiotis

Department of Geological Sciences

University of Colorado

Campus Box 250

Boulder, CO 80309-0250

and

Russell F. Dubiel

U. S. Geological Survey

Denver, CO 80225-0046

 

 

ABSTRACT

Continental trace fossils from the Petrified Forest Member of the Upper Triassic Chinle Formation in Petrified Forest National Park (PEFO), Arizona, were made by beetles, termites, soil arthropods, horseshoe crabs, and crayfish. These and other organisms lived in fluvial, marginal fluvial, lacustrine, and floodplain environments that had distinct paleoecosytem and paleohydrologic characteristics. Each trace fossil and its depositional environment represents individual behaviors that reflect specific organism-substrate relationships not preserved by body fossils. Trace fossils are important for paleohydrologic and paleoecologic reconstructions because they preserve: 1) the physiological characters of the organisms that are regulated by their environments, 2) the biodiversity of invertebrates and vertebrates not typically represented in the fossil record, and 3) the distribution of paleo-water table and soil moisture levels, and the seasonal and annual amounts of precipitation and temperature.

 

INTRODUCTION

Diverse and abundant trace fossils occur in the continental deposits of the Petrified Forest Member of the Upper Triassic Chinle Formation in the Petrified Forest National Park (PEFO), Arizona. The traces represent different behavioral patterns of numerous insects and crustaceans, as well as the interactions of different plant rhizoliths and substrates. Because of the lack of preservation of organic material in many substrates, trace fossils play an important role in the documentation of paleoecology, including the behavior, biodiversity, paleohydrology, and paleoclimate in the paleoecosystems of the Chinle Formation. Paleoecologic information and methodology gathered at PEFO can be applied to other continental deposits in the geologic record.

Various types of Chinle continental trace fossils occur within the boundaries of the Petrified Forest National Park (PEFO), and they record a biodiversity of organisms not preserved in the fossil record of the Park. This diversity is presented below as category types and as preassigned ichnotaxonomic designations. Our objective is to familiarize others with the diversity and distribution of trace fossils in PEFO and present their unequivocal utility as paleohyrodrologic, paleoecologic, and paleoclimatic indicators.

Invertebrate organisms (primarily insects and crustaceans) that inhabit both terrestrial and freshwater aquatic environments represent nearly 80 % of the world's biodiversity and biomass. They are typically under-represented in the fossil record of many geologic deposits because of the common lack of preservation as body fossils (Hasiotis and Bown, 1992). Invertebrates today make up a large part of food webs and ecosystems, but paleoecosystem reconstructions commonly ignore or consider invertebrate fossil components insignificant due to this lack of body fossil preservation. Recognition of invertebrate trace fossils in the geologic record will increase our awareness of ancient biodiversity and inter-relationships between organisms and their environments. Understanding these relationships will allow for better and more concise paleoecosystem reconstructions in the future.

 

GEOLOGIC SETTING

The Upper Triassic Chinle Formation was deposited in an intracontinental basin of the Pangaean supercontinent, which marks an exceptional time in the earth's paleogeographic and paleoclimatic history. The Chinle Formation was deposited between 5 and 15 north paleolatitude in the western equatorial region of Pangaea. Throughout most of the Colorado Plateau, the Chinle unconformably overlies the Lower and Middle Triassic Moenkopi Formation and equivalent rocks. In the Defiance uplift region of western Colorado and northeastern Arizona, the Chinle overlies Permian or older strata. The Chinle reaches thicknesses of 500 m and is unconformably overlain by the Lower Jurassic Wingate Sandstone or Moenave Formation and locally by the Middle Jurassic Entrada Sandstone in central Colorado and northwestern New Mexico.

Depositional systems of the Shinarump Member, and Monitor Butte Member in northern Arizona and in the vicinity of Petrified Forest National Park comprise valley-fill sequences at the base of the Chinle that are overlain by fluvial, floodplain, marsh, delta, and lacustrine strata (Stewart et al., 1972; Dubiel et al., 1991). In Petrified Forest National Park, the Petrified Forest Member consists of siliciclastic and intrabasinal conglomerates and laterally extensive, variegated sandstones and mudstones. These units have been interpreted as a complex succession of evolving fluvial channel systems and associated floodplain mudstones, many of which have endured extensive pedogenic modification (Dubiel et al., 1991).

 

ICHNOLOGY

Trace fossils documented in this preliminary report are presented as types (designated 1, 2, 3 etc.) that are based on their architectural and surficial morphology. Each type is followed by either: 1) a preassigned ichnotaxonomic designation minus the ichnospecies assignment; 2) a general designation already in use; or 3) by a descriptive designation reflecting the burrow architecture. Each trace fossil type contains a simple diagnosis and a discussion of its origin related to its morphologic features, as well as its paleohydrologic significance within the paleoecosystem.

TYPE 1: Scoyenia White 1929 (Fig. 1)

Diagnosis: Slender burrows with rope-like surficial morphology. Burrow diameters range from 0.2 cm to 10 cm, and lengths from a few cm to 10 cm. Burrows are unbranched, quasi-horizontal to vertical in orientation, and sometimes exhibits peristalsic thickening. Burrow interior is meniscate, backfilled.

Discussion: This ichnofossil is composed of a small diameter, is sinuous, exhibits a radially scratched surface, and is back-filled. Its orientation in outcrop is quasi-horizontal, and sometimes exhibits quasi-vertical variations. The burrow surficial morphology suggests that the trace was produced by deposit feeding insect larvae probably of beetle or true bug origin. The scratch mark marks appear to be paired with an extension of a third, smaller scratch periodically preserverd in the burrow wall. These scatches suggest a limb morphology and the organism was adapted to burrowing in moist, compact substrates such as the silty clay in which Scoyenia is found. S. gracilis is common in the floodplain mudstones and paleosols of various immature stages of development in the Petrified Forest Member. It is absent from more mature paleosols and coarser-grained deposits. In other Triassic rocks, Scoyenia occurs in marginal-lacustine and lacustrine deposits (i.e., Olsen, 1977). The mode of occurrence would suggest that this ichnofossil is indicative of very high soil and sediment moistures approaching 100% saturation of freshwater. In general, Scoyenia ocurrs only in continental and marginal marine deposits with thin continental interbeds (Hantzschel, 1975).

Scoyenia is an indicator of moist to saturated substrates (100% saturation of the pore space), which include immature paleosols and marginal fluvial and lacustrine strata. The Scoyenia ichnofacies currently embodies all continental deposits regardless of the presence of Scoyenia, its abundance, or the diversity and abundance of other ichnofossils in continentally deposited strata. We suggest that the Scoyenia ichnofacies should be limited to perennially moist to wet depositional environments where Scoyenia is the dominant ichnofossil, and the ichnofacies can be subsequently subdivided based upon the depositional setting and co-occurring faunal, floral, and ichnobiotal elements (Hasiotis, in press). Strata in which Scoyenia occurs as a minor ichnofaunal element should not be included in the Scoyenia ichnofacies because the trace represents either an ecological moisture tier in a sediment profile or a remnant population from wetter syn- or post-depositional periods.

TYPE 2: Koupichnium Nopcsa 1923 (Fig. 2)

Diagnosis: Heteropodous tracks of great variability. Two kinds of track imprints are common: 1) Two cheveron-like series of tracks each of 4 oval to round holes or bifid V-shaped impressions or scatches, and 2) one pair of digitate or flabellar, toe-shaped imprints with or without a medial drag mark.

Discussion: At least two ichnogenera are recognized for horseshoe crab (limulid) traces in the ichnofossil volume from the invertebrate treatise. These and a host of other ichnogenera were named for various limulid traces some of which were originally thought to be tracks of amphibians, birds, and mammals. Koupichnium and Merostromichnites are just two of the many overlapping limulid ichnogenera that create many problems in ichnology.

Freshwater limulid crawling and resting ichnofossils occur in marginal fluvial (point bar) and lacustine deposits in the Petrified Forest Member. Freshwater limulid ichnofossils in continentally deposited strata have been recorded in Paleozoic and Mesozoic rocks throughout the world (Seilacher and Goldring, 1971) and are not unique to continental depositional systems. From the similarity to modern and well documented ancient marine limulid tracks, we assume that the continental limulids were nearly identical in morphology.

Continental freshwater limulids are indicative of environments that contained ample amounts of water in continental depositional systems. Their traces are often found in strata that represent shorelines of lentic and lotic paleoenvironments, and are best preserved when the sediments were moist but not submerged. Experiments with tracks of other arthropods have shown that they are best preserved in moist and saturated conditions (i.e., Caster, 1938; Brady, 1939; McKee, 1947; Seilacher and Goldring, 1971). The limulid ichnofossils represent foraging out of the water onto the shoreline. Since they were morphologically similar, a separate ichnospecies should be created to encompass freshwater continental limulid ichnofossils because they had different biological requirements and lived in different environments then did their marine counterparts.

TYPE 3: Camborygma Hasiotis and Mitchell 1993 (Fig. 3)

Diagnosis: Architectural morphology varies from complex structures with multiple openings, shafts, corridors, and chambers, to simple, quasi-vertical shafts with simple chambers. Burrows sometimes preserve chimney structures at their tops. Burrows diameters in centimeters and lengths from 30 cm to 200+ cm. Surficial burrow morphology includes scrape marks, scratch marks, mud-and lag-liners, knobby and hummocky surfaces, pleopod striae, and body impressions.

Discussion: Large-diameter continental ichnofossils exist in the Petrified Forest Member of PFNP. They also in the members of the Upper Triassic Chinle and Dolores Formations on the Colorado Plateau as well as the Upper Triassic Dockum Formation of northwest Texas and eastern New Mexico. The burrows in PEFO occur in floodplain and paleosols and in marginal fluvial facies. These burrows were originally thought to be the product of spring water action (Gableman, 1955), rhizoliths (Lucas et al., 1985), and aestivating lungfish (Dubiel et al., 1987, 1988, 1989). Recent study has demonstrated that the majority of these large-diameter ichnofossils were created by crayfish (Hasiotis and Mitchell, 1993; Hasiotis et al., 1993).

Comparison of Triassic burrows to modern crayfish burrows concluded that the Triassic burrows were produced by crayfish using identical burrowing methods and exhibiting similar behavior patterns despite 220 million years in time (Hasiotis, 1990b, 1991; Hasiotis and Mitchell, 1993). The ichnogenus Camborygma is defined on the crayfish surficial burrow morphology that includes scratch marks, scrape marks, pleopod striae, knobby-hummocky surfaces, mud- and lag-liners, and body impressions.

Four ichnospecies of crayfish burrows (Camborygma eumekenomos, C. symplokonomos, C. araioklados, and C. litonomos), similar to modern crayfish burrow architecture, reflect the depth and fluctuations of the water table, thus indicating the hydrology of the Chinle in PFNP on the west coast of Late Triassic Pangea (Hasiotis, 1990a, 1991; Hasiotis and Dubiel, 1993). Burrow length and complexity of the architecture reflects the depth and stability of the water table in that area. Extensive, three dimensional outcrops allow for the reconstruction of the paleohydrology in the many areas where burrows occur.

In general, crayfish burrows are useful for paleohydrologic, paleoclimatic, and paleoenvironmental reconstructions of the strata in which they occur. Crayfish burrows and their stratigraphic succession reflect the depths and fluctuations of the water table, as well as the amount of water in a system (overall precipitation), seasonality, and climate.

 

TYPE 4: Cylindricum Link 1949 (Fig. 4)

Diagnosis: tube fillings or tubes shaped like test tubes with rounded terminations; walls smooth; diameter up to 5 cm and lengths up to several centimeters; preserved in groups oriented perpendicular to bedding.

Discussion: The architecture of these burrows are most similar Cylindricum and also resemble the marine trace fossil Skolithos, but differs in many ways. Skolithos is commonly long, slender, and less than 1 cm in diameter and occurs in marine nearshore environments in cohesive sands. Cylindricum occurs in groupings of 10 to 100 individuals in fine- to medium-grained crevasse splay sandstones and levee deposits. Many workers often confuse the usage of these two ichnotaxa due to their lack of familiarity with trace fossil literature and a working knowledge of continental and marine ecosystems.

Cylindricum faintly resembles the workings of solitary bees that are designated to the ichnotaxa Celliforma. Because of nomenclature problems and need for further study, the specimens in PFNP are grouped into TYPE 4 traces. Based on comparisons to modern burrowing and nesting organisms, these structures were probably used as brood nests by an unknown flying insect where eggs were deposited.

Paleoenvironmentally, these burrows represent time intervals of lower water table in crevasse splays and in levees deposits. Lowered water tables would allow many types of flying insects to occupy these types of deposits for weeks or even months; the time necessary for development and growth of larvae and pupae to adulthood.

TYPE 5: Alternating-fill, back-filled burrows (Fig. 5)

Diagnosis: Back-filled burrows that range in diameter from 2 mm to 20 mm; showing alternating fills of fine and coarse material; occur individually or as cross-cutting masses; never branched nor with chambers; entrances and terminations rare.

Discussion: These are the backfilled burrows with alternating fine and coarse material, with mainly the coarse furrows preserved. These PFNP burrows are similar to the marine back-filled trace fossils Taenidium and Muensteria, but differ from the marine traces which are often branched in multiple directions and places along their paths.

These continental back-filled burrows were most likely produced by soil-dwelling organisms that preferred moist , but not saturated environments. The Triassic burrows are very similar to modern soil-dwelling organisms that forage in substrates with moistures between 7 and 37 % (Hasiotis and Bown, 1992).

TYPE 6: Archeoentomoichnos Hasiotis and Dubiel 1994 (Fig. 6)

Diagnosis: Multistory ramps, floors, and walls constructed in a cylindrical structure approximately 7 cm in diameter; associated with mm to cm diameter-sized corridors greater than 5 cm in length; walls, ramps, and floors, range in thickness from 2 mm to 5 mm. Combined nests and galleries represent polycalic edifices.

Discussion: These nests represent the earliest known examples of social behavior in insects as well as the oldest evidence of termite activity. The ichnofossil evidence predates the body fossil evidence of termites by 135 million years (early Cretaceous). The nests are composed of calies (nest proper), galleries (runways between nests), and peripheral calies (storage chambers). Nests like these termite edifices are important because they preserve the behavior of a group of organisms that reflect the division of labor amount individuals that allow such colonies of insects to perpetuate themselves.

 

 

SIGNIFICANCE IN PALEOECOSYSTEM RECONSTRUCTIONS

Some of the more interesting and informative trace fossils were briefly described here to demonstrate that a diverse group of organisms exists with little or no fossil record within the Park. These traces, through comparisons to modern burrowing analogs, permit reconstruction of organism behavior with respect to various environments, the mechanisms that operated within environments that physiologically regulated organisms, and the internal and external components that shaped the paleoecosystem.

Triassic invertebrates identified through trace fossils such as larval and adult beetles, solitary bees, termites, horseshoe crabs, and crayfish, lived in environments with distinct depositional energies and hydrologic characteristics. These and other invertebrates were physiologically sensitive to intra- and extra-channel depositional events, as well as substrate moisture regimes, which regulated their occurrence within the paleoecosystem. The lateral and vertical stratification or tiering of the traces (organisms) reflected the zonation of the ancient soil moisture levels and ground water table during and after the deposition of the various beds within Petrified Forest Member during the late Triassic. Modern climate studies indicate that water table and soil moisture levels across the continents are controlled by seasonal and annual amounts of precipitation and temperature, which are in turn controlled by regional and global climatic patterns (e.g., Dubiel et al., 1991; Hasiotis and Bown, 1992). Therefore, recognition of trace fossils within Triassic strata yields better reconstructions for invertebrate biodiversity, and paleohydrologic and paleoclimatic characters that shaped Chinle paleoecosystems.

Future field work will continue to document the occurrence and distribution of trace fossils with respect to their association with bed stratification, sedimentary facies and lateral variations, and vertical distribution within the Park. Information gathered from ongoing studies in PFNP and in other national parks and monuments can greatly enhance our understanding of paleoecosystems and their changes as a result of climate changes during the Late Triassic.

 

ACKNOWLEDGEMENTS

We thank Petrified Forest National Park Superintendents Gary Cummins and Michelle Hellickson, and Park Paleontologist (1990-1993) Vincent Santucci, for allowing us to conduct field work . We also thank the Ted Bolich, Ferral Knight, and Angelo and Alexander Voris for their assistance in the field. Funding for this study was made possible by grants from the PFNP Museum Association, American Association of Petroleum Geologists Grants in Aid, and the Geological Society of America Grants in Aid. Dubiel's research is supported by the Evolution of Sedimentary Basins Program of the U. S. Geological Survey. Funding for research in the Park was generously provided by the Petrified Forest Museum Association, Marion Elson, Director.

 

REFERENCES

Brady, L.F., 1939. Tracks in the Coconino Sandstone compared with those of small, living arthropods: Plateau, v. 12, p. 32-34.

Caster, K.E., 1938. A restudy of the tracks of Paramphibius: Journal of Paleontology, v. 12, p. 3-60.

Dubiel, R.F., Blodgett, R.H. and Bown, T.M., 1987. Lungfish burrows in the Upper Triassic Chinle and Dolores formations, Colorado Plateau: Journ. Sed. Petrol., v. 57, p. 512-521.

Dubiel, R.F., Blodgett, R.H., Bown, T.M., 1988. Lungfish burrows in the Upper Triassic Chinle and Dolores Formations, Colorado Plateau - Reply: Journ. Sed. Petrol., v. 58, p. 367-369.

Dubiel, R.F., Blodgett, R.H., Bown, T.M., 1989. Lungfish burrows in the Upper Triassic Chinle and Dolores Formations, Colorado Plateau - Reply: Journ. Sed. Petrol., v. 59, p. 876-878.

Dubiel, R.F., Parrish, J.T., Parrish, J.M. and Good, S.C., 1991. The Pangean Megamonsoon - evidence from the Upper Triassic Chinle Formation, Colorado Plateau: Palaios, v. 6, p. 347-370.

Gableman, J.W., 1955. Cylindrical structures in the Permian(?) siltstone, Eagle County, Colorado: Journal of Geology, v. 63, p. 214-227.

Hantzschel, W., 1975. Trace fossils and problematica. Part W Miscellanea, Supplement 1, and edition; In Teichert, C., ed., Treatise on Invertebrate Paleontology. Geological Society of America and the University of Kansas, Boulder CO and Lawrence, KS, 269 p.

Hasiotis, S.T., 1990a. Upper Triassic Chinle Formation of southeastern Utah, U.S.A.: Crayfish burrows as floodplain and water table indicators. The Canadian Paleontology and Biostratigraphy Seminar, Queen's University, Ontario, Canada, 1990, p. 26-27.

Hasiotis, S.T., 1990b. Identification of the architectural and surficial burrow morphologies of ancient lungfish and crayfish burrows: Their importance to ichnology. The Australasian Institute of Mining and Metallurgy Pacific Rim Congress 90, v. 3, p. 529-536.

Hasiotis, S.T., 1991. Paleontology, ichnology, and paleoecology of the Upper Triassic Chinle Formation of the Canyonlands, southeastern Utah [M.S. Thesis]: State University of New York at Buffalo, Amherst, New York, 350 p.

Hasiotis, S.T., in press. Cluster analysis of trace fossils and continental depositional systems: Redefining the Scoyenia Ichnofacies. Geological Society of America National Meeting, Seattle, Washington.

Hasiotis, S.T. and Bown, T.M., 1992. Invertebrate trace fossils: The backbone of continental ichnology, p. 64-104; In Maples, C.G. and West, R.R., Trace Fossils. Short Courses in Paleontology no. 5, Paleontological Society, 238 p.

Hasiotis, S.T. and Dubiel, R.F., 1993. Neoichnology and ecological tiering in continental settings: Analogs for interpreting Pangean paleoecology, paleohydrology, and paleoclimate. Canadian Society of Petroleum Geologists, with the Global Sedimentary Geology Program, Carboniferous to Jurassic Pangea Program and Abstracts, p. 133.

Hasiotis, S.T. and Dubiel, R.F., 1994. A possible termite nest from the Upper Triassic Chinle Formation, Petrified Forest National Park, Arizona: Ichnos, 10 p.

Hasiotis, S.T., and Mitchell, C.E., 1993. A comparison of crayfish burrow morphologies: Triassic and Holocene fossil, paleo- and neo-ichnological evidence, and the identification of their burrowing signatures: Ichnos, v. 3, p. 291-314.

Hasiotis, S.T., Mitchell, C.E, and Dubiel, R.F., 1993. Application of burrow morphologic studies to discern burrow architects: Lungfish or crayfish?: Ichnos, v. 3, p. 315-334.

Lucas, S.G., Hunt, A.P., and Morales, M., 1985. Stratigraphic nomenclature and correlation of Triassic rocks of east-central New Mexico: a preliminary report: New Mexico Geological Society, Guidebook 36, p. 171-184.

McKee, E.D., 1947. Experiments on the development of tracks in fine cross-bedded sand: Journal of Sedimentary Petrology, v. 17, p. 23-28.

Olsen, P.E., 1977. Stop 11, Triangle Brick Quarry, p. 59-60; In Bain, G. L., and Harvey, B. W., eds., Field Guide to the Geology of the Durham Basin. Carolina Geological Survey Fortieth Anniversary Meeting, 139 p.

Seilacher, A. and Goldring, R., 1971. Limulid undertracks and their sedimentological implications: Neues Jahrbuch Geologie Palaeontologie, Abhandlungen, v. 37, p. 422-442.

Stewart, J.H., Poole, F.G. and Wilson, R.F., 1972. Stratigraphy and origin of the Chinle Formation and related Upper Triassic strata in the Colorado Plateau region: U. S. Geological Survey Professional Paper 690, 336 p.

 

Figures 1-6. See text for descriptions.

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