NEW TRACKSITES FROM THE MOENKOPI FORMATION (LOWER-MIDDLE TRIASSIC), GLEN CANYON NATIONAL RECREATION AREA
R. J. Schultz, Martin G. Lockley, and Adrian P. Hunt
Department of Geology
University of Colorado
Denver, CO, 80217-3364
Vertebrate and invertebrate subaerial traces are abundant in the Triassic Moenkopi Formation and have been documented in numerous publications. Subaqueous traces are equally common and widely distributed within the Moenkopi Formation, yet they are not nearly as well documented.
Subaqueous traces, observed on lands administered by the National Park Service from Dinosaur National Monument in the north to Glen Canyon National Recreation Area in the south, are perhaps the most common trace fossils in the Moenkopi Formation. Two new tracksites described herein from the Glen Canyon National Recreation Area are representative of subaqueous traces in the Moenkopi Formation.
Three types of subaqueous traces are found at two tracksites in the Moenkopi Formation in Glen Canyon National Recreation Area: (1) Vertebrate swim traces; (2) invertebrate swim traces; and (3) other enigmatic traces. They represent a trace fossil assemblage, or ichnofauna, unique to the Moenkopi Formation and its equivalents.
The Moenkopi Formation has a wide geographic distribution in the Western Interior of the United States and crops out in numerous localities on lands administered by the National Park Service in the western United States. Terrestrial or subaerial tracks and traces (ichnotaxa) in the Moenkopi include the vertebrate traces Chirotherium, Rhynchosauroides (Fig. 1d), Therapsipus, and Rotodactylus. Five taxa of invertebrate (subaerial and subaqueous) trace fossils are found in the Moenkopi and include subaqueous arthropod trails made by limulids (Peabody, 1956; Morales, 1987; Hunt et al., 1993). Often, vertebrate and invertebrate swim traces are found on the same surface (Peabody, 1956).
The purpose of this paper is to briefly describe vertebrate and invertebrate subaqueous traces found at two new localities in Glen Canyon National Recreation Area (GLCA). One locality is in the Trachyte Point area and was found by the authors. The second is in the Farley Canyon area and was reported to us by Martha Hayden.
VERTEBRATE TRACES OF SUBAQUEOUS ORIGIN
Although the vertebrate ichnofossil record is reported to be dominated by Chirotherium, Isochirotherium, and Synaptichnium, vertebrate swim traces are also very common in the Moenkopi (Peabody, 1956; Morales, 1987; Hunt et al., 1993). This conclusion is consistent with the observation of Frank Peabody, who stated that swim or scrape marks are the "most common impressions occurring in Moenkopi strata" (1956, p. 373). Such traces are also observed in an area covering most of the geographic distribution of the Moenkopi, and so can be found in many of the preserves administered by the National Park Service.
Typical large traces have been described as "straight grooves" up to 10 cm in length and 1.5 cm in width (Hunt and Lucas, 1993, p. 20). The numbers of parallel grooves may vary from 1 to 5 (Hunt and Lucas, 1993; Fig. 1e herein). Peabody (1956) observed that the swim traces are nearly always preserved as casts on the under side of a lens of thin limestone, but we have also observed them on the underside of sandstone. Hunt and Lucas (1993) report traces in association with mudcracks, but Peabody (1956, p. 737) stated "no shrinkage cracks indicative of subaerial desiccation occur anywhere on surfaces bearing 'swim' tracks". He also stated that no salt crystal psuedomorphs or ripple marks are found on surfaces bearing swim tracks (Peabody, 1956). The lenses containing trackbearing layers may exceed a thousand square feet (92.9 square meters), and are roughly circular (Peabody, 1956).
New tracksites at GLCA exhibit three types of subaqueous traces. The same trace fossil assemblages, or ichnocoenoses, recur in the same sedimentary facies throughout the Moenkopi Formation and its equivalents (Boyd & Loope, 1984; Lockley and Hunt, in press a, b).
Vertebrate traces (Fig. 1e) are parallel scrape marks thought to be formed when a buoyant animal touched the substrate (probably with a manus) during a swimming "stroke". Vertebrate swim traces at the GCNRA sites vary in size from approximately 5 to 15 cm in length, and 3 to 6 cm in width. Individual digit impressions, probably representing the width of the trackmakers' digits, range from 1 to 3 cm in width. The digit impressions have rounded ends, with no indication of claws, and therefore probably represent amphibians. Note that Rhyncosauroides was not found on the same surfaces as the swim traces.
Invertebrate traces (Fig. 1a, 1b, and 1c) at the GLCA tracksites are small swim traces attributed to limulids. These traces often occur in large numbers indicating considerable activity by these animals. Crescent shaped traces (Fig. 1a) are very similar to limulid traces described by Wang (1993). The so called "comb-like" traces (Fig. 1b) and Kouphichnium-like traces (Fig. 1c) are also characteristic of other limulid traces known from the Mesozoic of North America and Europe.
Two enigmatic traces (Fig. 1f and 1g) were found at the tracksite in GLCA. These traces are narrow sinuous traces from less than 1 to 3 cm in width. The origin of these traces is unknown.
Observations at GLCA reveal that traces are preserved on the underside of a thin lens of sandstone in alternating sandstone-shale sequences. No evidence of subaerial desiccation is present at the two tracksites at GLCA. No ripple marks were found on the trackbearing surface.
Although the Moenkopi Formation has yielded over 22 named vertebrate ichnotaxa, it is perhaps best known for traces such as Chirotherium, Isochirotherium, and Synaptichnium. However, swimming or "scrape" marks probably formed by subaqueous locomotion of amphibians, are common and widely distributed geographically. These traces, observed from Dinosaur National Monument in the north to GLCA in the south, are perhaps the most common trace fossils in the Moenkopi Formation (Peabody, 1956; Boyd and Loope, 1984; Lockley and Hunt, in press a, b).
Documentation of vertebrate subaqueous traces in previous literature contains little stratigraphic information. Sites from at least 16 geographic localities in the Moenkopi Formation are documented, but little is known of their stratigraphic location. It appears that these traces are found on at least 17 stratigraphic levels in the Moenkopi. Trace fossil sites found on lands administered by the National Park Service are located in the Upper Red Member in Capitol Reef National Park, Utah (possibly two stratigraphic levels); the lower Moenkopi near Vernal, Utah (near Dinosaur National Monument); the Wupatki Member in Wupatki National Monument, Arizona; and the upper Moenkopi at Glen Canyon National Recreation Area, Utah (Peabody, 1956; Morales, 1987; Hunt and Lucas, 1993; Lockley and Hunt, in press b; and this paper).
In general, subaqueous traces have a long history of being misunderstood (see Lockley and Rice, 1990 for review). They have been reported as terrestrial tracks of hopping dinosaurs (Bernier et al., 1984.), and later re-identified as swim traces (Thulborn, 1989). They have been misidentified as driftwood prod marks and bird tracks (McAllister, 1989; Lockley et al., 1992). Crescent-shaped traces in the Moenkopi were reported as tracks, but were later identified as current crescents by Frank Peabody (1947).
Because subaqueous tracks are incompletely preserved and seldom studied in detail, little is known about the tracks and the affinity of the trackmakers (Lockley and Rice, 1990). Several authors (Peabody, 1956; Lockley and Hunt, in press a; Morales, 1987) suggest that these traces could be attributed to amphibians, citing subaqueous origin, abundant body fossils of amphibians in the Moenkopi, and clawless toe impressions as evidence that the trackmakers were amphibians. We agree with this interpretation, but believe that a complete understanding of these traces and the ecology of the Moenkopi Formation requires further systematic study of these and other subaqueous traces. For example we have already pointed out that multiple swim tracksites (ichnocoenoses) indicate a characteristic swim track ichnofacies in parts of the Moenkopi Formation (Lockley et al., 1994; Lockley and Hunt, 1994b). Further analysis may yield a better understanding of the affinity and behavior of vertebrate subaqueous trackmakers and paleoecology of the Moenkopi Formation.
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Figure 1. Tracks from the Moenkopi Formation. A, trace of head shield and B, comb-like traces, both attributed to limulids form the Trachyte Point locality. C-G, traces and markings from the Farley Canyon locality. C, limulid trace (cf. Kouphichnium). D, Rhynchosauroides. E, Elongate swim traces and limulid footprints. F and G, Enigmatic sinuous markings.
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