An Occurrence of Reptile Subaqueous Traces in
the Moenkopi Formation (Triassic) of Capitol Reef
National Park, South Central Utah, USA.
James McAllister1 and John Kirby2
1Biology Department, Indiana University of Pennsylvania, Indiana, PA 15701
2Biology Department, Mansfield University, Mansfield, PA 16933
Historically fossil tracks have been treated as novelties of passing interest, or as a footnote in the context of a site report. When reported, tracks were typically the best examples found and rarely included mention of average or poor quality tracks. Ambiguous tracks and traces (such as subaqueous forms) were rarely acknowledged. Papers by Peabody (1948, 1956) and Lammers (1964), along with an abstract by Webb (1980) are exceptional. They describe and provide understanding to some poorly preserved traces considered to have been produced by swimming tetrapods. These reports are some of the earliest references of fossil subaqueous traces. These authors found their samples in the Moenkopi Formation within and adjacent to Capitol Reef National Park, south-central Utah.
Previous contributions are important and progressive, but reevaluation is necessary and expected with advances in methods, context, and when new specimens are found. Peabody serves as an excellent example of research which emphasized rigor in understanding locomotory processes applied to descriptive morphology. With that basis, behavioral interpretations now have become the focus of trackway workers, especially since the publication of Ostrom (1972). The emphasis of our paper is to add to the Capitol Reef National Park (CARE) vertebrate trace fossil story in two ways. First we can increase further the criteria to corroborate subaqueous interpretations; second, we can document sites which we do not believe previous authors studied.
Materials and Methods
Localities.Three study sites are within the
boundaries of CARE. The sites are in the Torrey Member of the
Moenkopi Fm. and are estimated to be late Scythian
(Spathian)(Hintze, 1988). All contain examples of subaqueous traces formed
by tetrapods. Traces occur as sandstone casts which filled in
the impressions (molds) found in the underlying mudstone.
The mudstone is friable and crumbles upon exposure. The
traces are not underprints (secondary structures created by
sion of layers deeper than the original substrate surface) as evidenced by primary structures on the trace (features created by direct contact of the sediment by the tracemaker; example: striations) and occurrence at a discontinuous sediment interface.
Site 1 is a low ridge, at approximately the 5560 ft. contour. The site curves from the southeast to the southwest approximately 87 meters and broadens into a wide slope. The ridge is west of the Headquarters/Visitor Center, bounded on the east and south by Sulphur Creek. The trace layer studied at this site covers approximately 297 sq. m. of surface area and has a strike and dip of 30° NW and 12° NE, respectively. At this location four smaller study sites were chosen and these sections of the trace layer were flipped for study. The surface areas of flipped rock at each of these sites are: Site 1a = 3.38 sq. m., Site 1b = 5.0 sq. m., Site 1c = 2.0 sq. m., Site 1d = 1.42 sq. m. Numerous large traces occur at each site, and are abundant over the entire ridge exposure. No individual traces can be assigned to specific traceways due to the trace density. The flipped blocks were arranged downslope in the mirror image of their original orientation and aligned to their original compass bearing. The blocks were cleaned, photographed, and samples of the surface duplicated by latex molds. The blocks were subsequently restored to their original position.
Site 2 is a broad slope about 1/4 mile due west of Site
1 along Route 24. This location includes both large and
small traces. They are plentiful but less numerous than Site 1;
individual traceways are scarce. Site 2 was not as well
documented as the others. Study was limited to exposure of the
undersurface to find local areas of future interest. Pictures and latex
molds were made of selected displaced slabs which could not be
oriented in situ.
Site 3 is located approximately 2.5 miles west of Site
2 and then south of Route 24 near a dry streambed. The
fossil site is at the base of a small cliff where a number of
large blocks have fallen. The traces at this site are on two
large blocks whose original orientation and placement within
nearby outcrop are unknown. There are over 40 traces on the two blocks which include two distinct traceways. These traceways and remaining traces were made by a relatively large tracemaker similar in size to those at Site 1. The two displaced large blocks of Site 3 were gridded for analysis using chalk lines at intervals of 25 by 25 cm. A Plumb BarbaraTM (computer-enhanced plumb bob) was used to create an artificial base line for this grid. No directional orientation is implied by the base line. The individual traces of each traceway were then photographed and described. The traceways on both blocks were duplicated using latex peels.
Figure 1Selected traces from Site 3. Note kick-off scours behind the traces. Scale equals 10 cm.
Figure 2Selected traces from Site 3. Note Z-traces. Scale equals 10 cm.
Site 1A hodgepodge of traces oriented to the southwest: no discernible traceways are apparent. All traces are subaqueous and large. Marks of individual traces range from 8 20 mm at greatest width and 21 90 mm in axis length. Most trace prints range from one to three digit impressions.
Site 2Orientation is not apparent for all traces. Many small traces exhibit subaqueous characteristics. Larger traces are indistinct and lack features that indicate subaqueous formation. Variation in sandstone thickness and block size makes this site less amenable to flipping and reconstruction of lower surface. A few large traces are present but the majority are small. The small traces consist of marks from one, two or three digits. Single digit traces can have a width of 34 mm and a length of 18 mm. The threedigit traces can have greatest width and axis length of 11 mm and 21 mm, respectively.
Site 3Two traceways are recognized. The original orientation of the trace block is unknown but each traceway is oriented at 40 degrees to the apparent direction of the current. Both traceways have evidence of locomotion by all four appendages. One traceway, composed of 13 traces, has three zshaped traces. The individual ztraces, appear to be formed from the action of one digit and have a greatest width and total axis length of 41 mm and 87 mm, respectively. Most associated marks have two digits associated with the trace and have total trace greatest widths and axis lengths of 57 mm and 114 mm, respectively. The second oriented traceway is composed of 20 traces.
Vertebrate subaqueous traces described in the literature occur as sandstone casts that had filled imprints preserved in underlying mudstone. Exposed traces occur on the underside of resistant sandstone ledges where the mudstone eroded away. This typically makes the traces difficult to examine as it requires removal and flipping of the sandstone layer to expose the traces. At CARE the concordance of stratigraphic dip and hillside slope (forming a small hogback) combined with the lateral extensiveness of the trace-bearing sandstones allows sampling along a large uninterrupted surface. Furthermore, the sandstone is jointed and the underside can be exposed by flipping blocks which are neither so small that there is destruction of many traces, nor too large to require excessive physical effort. These factors combine to make these localities extraordinary. A large exposure can be sampled by two people equipped with handtools.
Prior work by Peabody (1956) and Lammars (1964)
CARE were especially important in documenting subaqueous traces. Although there was no specific listing of recognition criteria, the important characteristics they used to justify their interpretations can be interpreted from their writings. Peabody was impressed by the lack of distinctiveness of the "swim" traces. The lack of definitive series (trail continuity), poorly defined imprints that appeared as if digit tips formed the traces, and the corroboration of the physical environment were important to his interpretation. Salt crystal pseudomorphs, shrinkage cracks, and ripple marks occur elsewhere in the park sediments but not near the "swim" traces.
The sedimentary criteria that form the environmental interpretation should agree with the expected environment of the trace fossils. For example, the lack of salt pseudomorphs, shrinkage cracks, and ripple marks were considered important to Peabody (1956). He used these characters to refine his initial subaqueous track paleoenvironmental interpretation as one which did not undergo subaerial desiccation. Peabody (1956, pg. 738) also considered the traces to have been made in a particular environment, " shallow but extensive pools of a floodplain." However, observations of current produced sedimentary features (especially at Site 3), the offset nature of some traceways (Sites 2 and 3), and the presence of ripplemarks (Site 2) indicate a need to reevaluate the specific environmental interpretation as one which was highly influenced by currents. Although the sedimentary evidence indicates a subaquatic environment in general, and is consistent with the evidence of the traces, each individual character can be used in the interpretation of a variety of environments which underwent similar processes. Sometimes the characters may not simply be a checklist but rather may build several lines of independent evidence which together corroborate an interpretation.
In addition to evidence from the tracks and evidence relating to the depositional environment, Peabody also mentioned an association of a limuloid trackway with subaqueous traces. The presence of this traceway enhances the interpretation of the proximity of an aqueous environment for the CARE Moenkopi Formation.
Lammers (1964) provides additional recognition criteria for Capitol Reef subaqueous traces. He noted that individual traces had striations caused by scales and nails which obscured expected track details. Overhangs along the posterior of the traces would be unusual for non-buoyant tetrapods. Further, the general confusion of the traces, lack of full foot impressions, the abnormal elongation and smearing of the traces, lack of consecutive series, randomness, disorder, and overlapping indicated to Lammers random swimming movements.
McAllister (1989) listed subaqueous features which
can be separated into three categories: criteria inherent to
individual traces, criteria inherent to sequential traces, and
corroborative evidence from sediments/paleoenvironment.
The characters used to distinguish subaqueous traces are
considered to be easily made by a buoyant paddler, but unlikely to
be consistently made during normal terrestrial locomotion.
The descriptions of the subaqueous traces by Peabody and
Lammers were examined and interpreted to correspond to our list
Table 1Criteria helpful for subaqueous interpretation.
Criteria inherent to individual traces are: reflecture of digits (retraction mark of digit tips made from posterior of trace anteriorly), depth of the mark corresponding to arc of limb, elongation of traces, posterior overhangs (continuations of the digit tips posteriorly into the sediment, creating a hanging edge in the cast), striations parallel to direction of propulsion, and preferential impression of distal ends of digits. Criteria inherent in multiple trace comparison or traceways are: great variability in pace angulation, trace lengths excessively variable compared to widths, ratio of manus and pes traces unequal (manus typically underrepresented), and unexpected configurations (lack of traces or extra traces in an expected sequence). Sedimentary and environmental criteria are: association with other appropriate fauna/flora, association with expected sedimentary features, and environmental interpretation.
Our examination of the CARE specimens allows us to include additional recognition criteria. They are the presence of kick-off scours, z-traces, and buoyancy/size-mitigated variably-preserved traces.
Kick-off scours (Fig. 1) occur immediately posterior
to the traces. The sandstone cast infilled the scour and is seen
as the irregular positive relief behind the digit scrapes.
They represent the action of the water eddies created behind
the digits as they pass close over the sediment. At the end of
the propulsive phase (kick-off phase of Thulborn and Wade,
the eddies created by the tips of the digits scour out the area immediately behind the trace. Most of the excavated material disperses into the water column. In a terrestrial case, much of the propulsive phase has the force of the weight-bearing subphase directed downward compressing the sediment. For a fully buoyant tracemaker, the touchdown and weight bearing phases are less well defined, and include pushing sediment (typically fine-grained mud) out of the trace to scatter posteriorly. In the terrestrial trace Thulborn and Wade indicate the manner in which a tracemaker can create striations (retro-scratches) along the imprinted track with a continuation of a backsliding kick-off phase. In this terrestrial situation sediment which is scooped or squeezed out of the track will be deposited on the substrate. If a cast were made of the terrestrial track, the squeezed out or scooped out material would create negative relief. This is an important difference in the modes of formation between buoyant and non-buoyant traces and leads to a fundamental difference in the disposition of the displaced sediment. The difference in mode of formation also helps understand why underprints are not likely to be created by a buoyant tracemaker. Under expected buoyant conditions there will be an extremely small component of substrate compression (downward), compared to a non-buoyant locomotion.
Z-traces (Fig. 2) are interpreted as little double kicks of the tracemaker as the tips of the toes graze the substrate. These traces are made by feet on the side of the tracemaker opposite of the striking current. The trace begins with the typical imprint of one digit entering the sediment in an arc during protraction. The entrance is slightly wider than the rest of the trace (as seen in the entry of other traces), and the depth of the arc does not progress all the way through the sediment as do the two digits on the current side. The digit then retracts anterolaterally a short distance leaving a continuous striated path and trace. The digit continues protracting posteriorly, entering deeper into the substrate, leaving an overhang in the trace cast.
Initial interpretation of the Z-trace establishes the basic mode of formation. The shallow entry and progressive arc-depth indicate that this part of the trace was made first. The continuity of the trace (especially continuity of depth and striations) through the middle of the Z connecting the sides, indicates the creation by one digit. The overhang indicates that this is the end of propulsion; the part of the trace made last. The overhang indicates the direction of the tracemaker movement (toward top of page in Fig. 2) as opposite of the direction the overhang points.
Continued interpretation becomes more speculative, but also more interesting. The initial protraction, quick retraction, and continued final protraction, is interpreted as the tracemaker being at the limits of limb extension (barely touching the substrate) while in an offsetting current. Seemingly these Z-traces indicate an extra little attempt to gain additional grip on the substrate with an immediate second try or extra little flip of the distal limb. For the tracemaker, the sense of one digit barely touching substrate on this side (possibly because the animal is leaning into the current, away from this side) may have caused the tracemaker to try and dig in a bit deeper. Additionally, the side leading into the current may emphasize propulsion, while the side opposite the current may emphasize prevention of current offset. Most importantly, the movement of the limb as described would require a degree of freedom which would only be consistently provided in a buoyant state.
Buoyancy/size-mitigated variably-preserved traces are present at Site 2. Details of some traces are better defined than in other traces (variably preserved). For example, at Site 2 small traces with subaqueous characteristics are well preserved, while large traces are most often poorly preserved with no recognizable subaqueous characteristics. The large ones appear to be full prints of tracks which are deformed and amorphous. Sediment may have been compressed, squeezed, and stuck to the foot as it was removed from the track. The significance of the variably preserved traces is interpreted as relating to the size of the tracemaker verses the water depth and opportunity to become buoyant (buoyancy/size-mitigated). Essentially small tetrapods floated in shallow water while large ones waded. Other variables need to be taken into consideration for a complete interpretation, (ability of small tetrapods to walk along the substrate bottom (Brand 1979), variation in water level over time, and non-contemporaneous trackways). However, the presence of these disparately preserved traces will contribute to a more complete understanding of the subaqueous environment at CARE.
The importance of the Capitol Reef National Park Moenkopi Formation vertebrate traces is clearly evidenced by early references in the literature. Some of the first recognized subaqueous traceways came from the park. Today the extensive deposits and accessibility of the sediments allows continued advancement in traceway interpretations. Previous recognition of subaqueous vertebrate traces include criteria inherent to individual traces, inherent to sequential traces, and correlative criteria from sediments/paleoenvironment. Many of these criteria were originally recognized from CARE specimens. We have further documented criteria not described for this material. Most notable of these are kick-off scours (individual traces), z-traces (individual traces), and buoyancy/size-mitigated variably-preserved traces (sediments/paleoenvironment). These three criteria all rely on buoyancy of the tracemaker to make their characteristic mark of subaqueous formation.
We would like to especially thank Tom Clark, Chief of Resource Management at Capitol Reef National Park for permission to study the traces within the park, the use of facilities during the fieldwork season, and of course his encouragement. Allyson Mathis, now a Park Ranger at Capulin Volcano National Monument, provided us with assistance, enthusiasm, and brought us to the spectacular traceways at Site 3.
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