CRAYFISH FOSSILS AND BURROWS FROM THE UPPER TRIASSIC CHINLE FORMATION, CANYONLANDS NATIONAL PARK, UTAH
Stephen T. Hasiotis
Department of Geological Sciences
University of Colorado at Boulder
Campus Box 250
Boulder, CO 80309-0250
A large number of fossil freshwater crayfish in the Petrified Forest and Owl Rock Members of the Upper Triassic Chinle Formation, adjacent to Canyonlands National Park in southeastern, Utah are described as three morphotypes of body plan. The specimens are the earliest known true Cambaridae and record the earliest known occurrence of fossil crayfish with their burrow. Five individuals were found within burrows, whereas thousands of others occur within a multi-layer mass-mortality bed in the Owl Rock Member. Twenty specimens form the basis for description of two new genera, and three new species of freshwater crayfish. The taxonomy of the new crayfish is also based on their burrowing ability categorized as Type I, II, and III and correlated with Hobbs' classification of primary, secondary, and tertiary, burrowers, respectively.
The earliest known fossils of freshwater crayfish are found in association with their burrows in the Upper Triassic Chinle Formation, on the boundary of Canyonlands National Park, southeastern Utah. The only other known freshwater crayfish fossils from Triassic continental rocks are the taxonomically undescribed crayfish from the Durham Basin of North Carolina (Olsen, 1977), and a small, single specimen in a mudstone from the Petrified Forest Member of the upper part of the Chinle Formation in Arizona (Miller and Ash, 1988), neither of which are associated with burrows. Previously described fossil crayfish occur in strata of the Early Cretaceous/Late Jurassic of Eurasia (Glaessner, 1969), the Early Eocene of North America (Feldmann et al., 1981), and an astacid (United States west coast crayfish family) crayfish from the Miocene and Pliocene of Oregon (Rathbun, 1929). None of these crayfish fossils have been reported in or associated with burrows.
This paper presents a discussion of two new genera of Triassic freshwater burrowing crayfish from the Canyonlands area of southeastern Utah, as well as a discussion of their evolutionary and paleoecologic significance. The Chinle crayfish document new taxa and represent lineages to the origins of North American crayfish families, the Astacidae and Cambaridae. The evolution of freshwater crayfish is reconsidered, as well as their zoogeographical distribution and paleomigrational patterns.
The Upper Triassic Chinle Formation occurs throughout the Colorado Plateau, ranging in thickness from 0 to 400 meters. It was deposited as layers of continental clastic and carbonate sediments (Stewart et al., 1972). In the Canyonlands study area, the upper part of the Chinle Formation is approximately 100 meters thick comprising the Moss Back, Petrified Forest, Owl Rock, and Church Rock Members in ascending order (Stewart et al., 1972; Blakey and Gubitosa, 1983; Hasiotis and Mitchell, 1993). The Chinle Formation comprises multiple phases of stream, lake, and swamp deposition in the study area (e.g., Blakey and Gubitosa, 1983). The paleolatitude of southeastern Utah is considered to have been sub-tropical to tropical from paleotectonic reconstructions of continents that formed the supercontinent Pangea during the Permian and Triassic (Van der voo et al., 1976; Dubiel et al., 1991).
DESCRIPTION OF CRAYFISH FOSSILS AND BURROWS
The Chinle Formation in the Canyonlands area yielded three morphologically distinct types of crayfish fossils from fluvial and overbank deposits. Some of these fossils were discovered in the bottoms of burrows, which were attributed to the crayfish activity based on the burrow morphology. The burrow morphologies are briefly described first because the morphology of the burrows is related to the morphology of the crayfish and its inferred behavior. The Chinle Formation also contains numerous burrows whose architectural and surficial morphologies indicate these are burrows constructed by crayfish (Hasiotis, 1990; Hasiotis and Mitchell, 1993; Hasiotis et al., 1993). Architectural burrow morphologies and their classification as Types I, II, III (Fig. 1A) (in decreasing order of complexity) suggests the possibility that at least three different burrowing species occurred in this area (Hasiotis, 1990; Hasiotis and Mitchell, 1993). This, in turn, is born-out in the diversity of crayfish fossils recovered from Steven's Canyon and Hart's Point near Canyonlands National Park.
The crayfish fossils were collected from the Petrified Forest and Owl Rock Members of the Upper Triassic Chinle Formation in Stevens Canyon and Harts Point, adjacent to Canyonlands National Park, San Juan County, southeastern, Utah (Fig. 1C-F). Fossils that occurred within burrows were collected Stevens Canyon near the intersection with Indian Creek Canyon, in the Owl Rock Member. The mass-mortality bed, where the majority of the specimens were collected, occurs in the Owl Rock Member of Stevens Canyon near the Canyonlands National Park boundary. Three disarticulated specimens were collected from the Petrified Forest Member at Harts Point. Modern and ancient crayfish taxonomy is based on the morphology of the chelae, rostrum, antennual scale, antennae, groove patterns and shape of areola, telson and uropods, and relative length and ornamentation of the cephalothorax and abdomen (Fig. 1B).
Five crayfish fossils were recovered from thousands of burrows occurring in the Canyonlands area of southeastern, Utah, and are the first known fossil freshwater crayfish to be associated with burrows. The crayfish exhibit a triangular rostrum with lateral spines and delicate tuberculated architecture of the cephalothorax. The carapace contains cervical and branchiocardiac grooves with a dorsomedian longitudinal suture. These crayfish may or may not be strongly heterochelous.
This type of crayfish morphology is associated with Type I burrows. Type I burrows exhibit complex architecture with multiple branches and multiple chambers. Because of the morphology exhibited by this morphotype (strong shovel-shaped chelae, rounded terga, small triangular rostrum, lack of spines and coarse ornamentation) this crayfish can be identified as a Type I burrower, equivalent to Hobbs' (1981) primary burrower. Hobbs' (1981) classification of burrowers is based on the amount of time the crayfish spends in the burrow and its connection to open water. Primary burrowers are crayfish that spend the greatest amount of time in the burrows, which have no connections to open water. With only a few exceptions, primary burrowers create relatively complex burrows.
Morphologically, this species is consistent with modern primary burrowers. Compared to individuals of Cambarus diogenes diogenes (Girard) (Hasiotis and Mitchell, 1993), both burrowers exhibit relatively short triangular rostra, weakly tuberculated cephalothorax, lack of many spines on the cephalothorax and pereiopods, small scale of antennae, shovel-shaped broad chelae, and a reduced abdomen that exhibit smooth, rounded pleura and terga.
Two morphotypes (morphotypes 2 and 3) of crayfish fossils make up the majority of body fossils from the mass-mortality beds in the lower part of the Owl Rock Member. One of these morphotypes exhibits a long triangular rostrum, and has a well impressed cervical groove with a well developed areola. The body contains a moderately tuberculated cephalothorax, possesses elongate slender chelae, and have a telson with a transverse suture, and uropods with diaeresis.
The features of this type of crayfish are similar to those characters exhibited by modern secondary and tertiary burrowing crayfish (Hobbs, 1981). These Triassic crayfish possess relatively long rostra, chelae longer than those of primary burrowers and tend to be heavy to slender, exhibit more ornamentation and spines, and have more rounded versus sharply rounded shapes (H. H. Hobbs Jr. personal comm., 1989). The Type II and Type III burrows (Hasiotis and Mitchell, 1993) that occur in the upper Chinle Formation that are similar to modern secondary and tertiary burrows, respectively. Because of the similarity between the anatomy these Triassic crayfish to modern secondary burrowers and the presence of Type II burrows in locales in which these fossils occur, they are equivalent to modern secondary burrowers.
The second crayfish morphotype (3) collected from the mass-mortality beds exhibit a small triangular rostrum with large antennae scales of approximately equal length. The chelae are small and slender, and moderately tuberculated. The abdominal somites bear weak transverse grooves, pleura exhibit a rounded angle with the terga that terminates in a sharp spine directed posteroventrally.
The highly ornate and spiny characteristics exhibited by morphotype 3 crayfish are those shared by most modern crayfish that are classified as tertiary burrowers (Hobbs, 1981). The morphology of tertiary burrows is simple, and are only constructed for mating or during periods of water deprivation. Type IIIA and Type IIIB burrows of the upper part of the Chinle Formation are those that exhibit the most simple burrow architecture (Hasiotis and Mitchell, 1993). Even though no crayfish were discovered within any Type III burrows, the crayfish anatomy suggests these crayfish were Type IIIA burrowers and most likely spent most of their life outside the burrow, mainly in streams, rivers, and lakes. Type IIIB burrows are greater than 1 m in length are most similar to modern primary burrowers and would have a body plan similar to morphotype 1 Triassic crayfish. These Triassic crayfish lived away from open bodies of water in areas such as proximal and distal floodplains and burrowed to the water table to depths up to 4 m (Hobbs, 1981; Hasiotis and Mitchell, 1993).
EVOLUTIONARY AND PALEOECOLOGICAL IMPLICATIONS
The discovery of burrowing crayfish fossils in late Triassic strata of the Colorado Plateau has significant implications to the evolution of freshwater crayfish, as well as the paleohydrologic structure and dynamics of depositional systems of Triassic paleoecosystems. Paleohydrologic and paleoecologic information obtained from this study can be applied to crayfish burrow-bearing rocks elsewhere in the geologic record to reconstruct ancient water table levels and estimates of seasonal and annual precipitation.
Previous to the Chinle discovery, crayfish were thought to have evolved early in the Cretaceous (130 million years ago) from minor invasions into brackish water by marine lobsters from the late Triassic (230 million years) to the late Jurassic (145 million years) (Glaessner, 1969; Feldmann et al., 1981; Hobbs personal comm., 1989). Burrowing crayfish were thought to have evolved during the early to mid Tertiary (55 to 35 million years ago) after they had become established in freshwater aquatic ecosystems (Hobbs, 1976).
The diversity and distribution of North American Triassic terrestrial burrowing and aquatic freshwater crayfish fossils demonstrates that their evolution began earlier and may reciprocate our views of the lobster-crayfish relationship. Crayfish fossils and the burrows attributed to their activity in the Upper Triassic Chinle Formation date to about 225 million years in age during the presence of the Pangean supercontinent. Trace and body fossil evidence confirms that crayfish were established across ecological settings ranging from fully terrestrial to fully aquatic. The distribution of Triassic aquatic crayfish also included areas in Arizona (Miller and Ash, 1988) and in North Carolina (Olsen, 1977). The diversity, paleogeographic distribution, and ecological specialization of the Triassic crayfish implies that the group evolved possibly as early as or earlier than the Permian (286 million years ago). The crayfish body and trace fossil evidence may suggest that lobsters, thought to have evolved early in the Triassic (245 million years ago), evolved from aquatic freshwater crayfish that inhabited coastal streams, rivers, and lakes. The majority of lobster body fossils occur in the Jurassic and Cretaceous with only a hand full from the Triassic (e.g., Glaessner, 1969).
Paleoecologically, Triassic crayfish exhibited nearly identical behavior with respect to burrow architecture, depth, and the seasonal and annual distribution and fluctuation of the water table (hasiotis and Mitchell, 1993). Triassic crayfish burrows that exhibit short lengths and complex architectures reflect a shallow water table with dampened fluctuations (usually due to a nearby body of water). Crayfish burrows that exhibit long lengths and simple architectures reflect a deep and fluctuating water table (Hasiotis and Mitchell, 1993). The architecture, depth, and distribution of Triassic crayfish burrows can be used to reconstruct local and regional water table depths and fluctuations that could be related to seasonal and annual precipitation during the Triassic monsoonal climate.
I extend my thanks to Russell Dubiel, Lance Grande, James Kirkland, Mike Parrish, and Roy Plotnick for their comments and insights to this research. I especially thank Rodney Feldmann and Horton Hobbs Jr. for their helpful discussions concerning freshwater and marine Decapoda. This research was part of a Master's thesis conducted at the State University of New York at Buffalo. The study was funded in part by the American Association of Petroleum Geologist, the Sigma Xi Foundation, and the Graduate Student Association of the University at Buffalo.
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Fig. 1A-F. A. Triassic crayfish burrow architecture based on burrow complexity and overall depth. B. Anatomical parts used in taxonomic descriptions of modern and ancient crayfish. C-F. Examples of crayfish fossils recovered from mass-mortality beds in the Owl Rock Member, upper Chinle Formation.
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