Vertebrate Tracks and Their Significance in the Chinle Formation (Late Triassic), Petrified Forest National Park, Arizona

Anthony J. Martin1 and Stephen T. Hasiotis2

1Geosciences Program, Emory University, Atlanta, GA 30322.
2Department of Geological Sciences, University of Colorado, Boulder, CO 80309.


Abstract—Previous research in the Upper Triassic Chinle Formation in Petrified Forest National Park (PEFO) has only yielded two isolated footprints, which is in stark contrast to the comparatively large number of vertebrate body fossils found there. We report here the discovery of a numerous reptilian tracks, including two dinosaur tracks, in the Chinle Formation in PEFO, considerably expanding the paleontological database for vertebrates in PEFO.

Tracks of phytosaurs, small-sized reptiles, and dinosaurs occur in ripple-laminated sandstones of the Monitor Butte Member, whereas other small- and intermediate-sized reptile tracks occur in cross-bedded sandstones of the Petrified Forest Member. A phytosaur trackway is discontinuous but consists of 12 footprints having a total length of 2.2 m and was likely made by a juvenile animal with a walking movement that turned slightly to the right. One small reptile trackway (comprised of Rhynchosauroides isp.) represents lateral movement by the tracemaker, rather than straight, forward locomotion. One isolated print that we are attributing to a large theropod and a smaller partial print formed by either a theropod or ornithopod supplement skeletal evidence of dinosaurs in the area. Tracks in the Petrified Forest Member are numerous in places, most show claw marks, and are smaller than phytosaur tracks, suggesting intermediate-sized reptilian tracemakers. Most of these tracks apparently are not parts of continuous trackways, although some show manus-pes pairings, and they are relatively consistent in size and shape, pointing toward a probable adult animal of the same species as a tracemaker.

Vertebrate tracks and trackways in the Chinle Formation of PEFO affirm that sedimentological conditions were sufficient in this area for footprint preservation, although most tracks likely represent undertracks. Further investigation of Chinle strata should yield more information regarding the identity of tracemakers, further reconciling the vertebrate body fossil and trace fossil records in this area.


Introduction

Vertebrate body fossils are common in the Upper Triassic Chinle Formation of the southwestern U.S. and have been the subject of much research (Camp, 1930; Camp and Welles, 1956; Colbert, 1972; Padian, 1986; Long and Padian, 1986; Lucas and Hunt, 1993). Reports of vertebrate trace fossils of the Chinle, especially tracks and trackways, are also well known from the upper part of the Chinle (Hunt et al., 1989; Lockley et al., 1992; Lockley and Hunt, 1993; Lockley et al., 1993; Lockley and Hunt, 1995, p. 99, 103-104). However, tracks from the lower part of the Chinle throughout the southwestern U.S. are apparently uncommon (Lockley and Hunt, 1995, p. 103-104), and the Chinle of Petrified Forest National Park (PEFO) of northeastern Arizona has had only two isolated footprints reported, both of Rhynchosauroides isp. (Santucci and Hunt, 1993; Santucci et al., 1995). The apparent lack of tracks and trackways in PEFO has been interpreted as a result of wet conditions during deposition of some Chinle sediments in the region (M. G. Lockley and A. P. Hunt, personal communication with Hasiotis, 1994; Lockley and Hunt, 1995, p. 103-104). Phytosaurs, the most common vertebrates represented by body fossils in PEFO, have had none of their tracks reported. Other animals in PEFO, such as metoposaurs, rauisuchians, aetosaurs, theropods, dicynodonts, and various other reptiles and amphibians, also seemingly left no trackway evidence, despite their presence indicated by body fossils in parts of the Chinle (Long and Padian, 1986).

We propose that because invertebrate trace fossils are exceedingly common and diverse in the Chinle of PEFO, despite the lack of corresponding body fossils (Dubiel and Hasiotis, 1995; Hasiotis and Dubiel, 1993a, b, 1995), then preservational conditions conducive for invertebrate traces should also apply to vertebrate traces. This observation, combined with the presence of vertebrate tracks in the Chinle outside of PEFO, implies that tracks should be present. Confirmation of their presence is the purpose of this report, where we document the discovery of tracks and trackways made by unknown reptilian tracemakers, phytosaurs, and dinosaurs in PEFO; the phytosaur and dinosaur tracks are the first ever interpreted from this area.

Study Area And Stratigraphy

PEFO, in northeastern Arizona, has one of the best exposures of the Chinle in that region. Sediments composing the Chinle Formation were deposited in fluvial, palustrine, and lacustrine environments of conterminous continental depositional basins and modified by various stages of pedogenesis during the Late Triassic (Dubiel, 1989; Hasiotis and Dubiel, 1993b). Tracks described here occur in the southern part of PEFO in the Monitor Butte and Petrified Forest Members of the Chinle. Tracks in the Monitor Butte occur in the region of "The Tepees" and tracks in the Petrified Forest Member occur in the region of "Rainbow Forest" and "The Flattops." The geology of these areas has been well documented in previous studies (Ash, 1986; Dubiel et al., 1994; Demko, 1995; Hasiotis and Dubiel, 1995).

Tracks and trackways in the Monitor Butte Member primarily occur on the tops of ripple-laminated and plane-bedded, fine-grained sandstones in the upper part of the Newspaper Sandstone fluvial complex in the area of The Tepees, mostly at Lone Tepee. Tracks in the Petrified Forest Member occur in the base of a trough cross-bedded, medium-grained sandstone in a thick sandstone (Flattop #1) above the Sonsela/Rainbow Forest sandstone complex, which is near the base of the Petrified Forest Member, in the south end of PEFO (Rainbow Forest).

Description And Interpretation Of Vertebrate Tracks

Tracks in PEFO represent at least four distinctive groups of tracemakers and can be broadly categorized on the basis of track size and shape. These groups of tracemakers are: (1) small reptiles, possibly rhynchocephalians; (2) intermediate-sized reptiles; (3) phytosaurs; and (4) dinosaurs.

Small Reptiles.—The smallest tracks in PEFO are assignable to the ichnogenus Rhynchosauroides. Specimens of Rhynchosauroides consist of five- or four-toed prints, footprint lengths and widths of about 1 cm, and individual toe widths and lengths of 2 mm and 5-7 mm, respectively. These tracks occur in both the Monitor Butte and Petrified Forest Members, although a trackway is preserved in the Monitor Butte. This trackway is unique in its evidence of lateral movement by the tracemaker. Manus prints, preserved as negative-relief epichnia (molds) are parallel to one another, whereas pes prints are apparently absent from the same bedding plane (Figure 1). Some of the prints overlap one another, showing a sequence of lateral movement of the tracemaker from right to left. Other tracks that more-or-less parallel the lateral trackway may or may not have been made by the same individual tracemaker, although slight differences in size and orientation argue more for multiple individuals. If made by the same individual, different preservation modes and lack of preservation of manus prints are responsible for gaps between tracks.

Figure 1Rhynchosauroides trackway from Monitor Butte Member, Chinle Formation, PEFO, showing lateral movement of tracemaker (from right to left).

Figure 2—Bedding plane exposure of tracks used for track census in Table 1; float block from Petrified Forest Member, Chinle Formation, PEFO.

Claw marks without pedal impressions are clearly undertracks, pointing to subtle differences in substrate conditions in the original preservational medium. Close-up views of some tracks also reveal some evidence of foot rotation where sand was pushed up behind the foot as the tracemaker moved. Such details are indicators of a firm, cohesive substrate in places, sufficient to preserve the effects of movement by a rather small vertebrate.

Rhynchosauroides tracemakers left abundant tracks in some Late Triassic deposits (Lockley and Hunt, 1995, p.95) and the most probable tracemakers for Rhynchosauroides were rhynchocephalians (Lockley and Hunt, 1995, p. 87). A few specimens of Rhynchosauroides were also found in the Petrified Forest Member, although trackways were lacking. We anticipate that the abundance of this ichnogenus in the Chinle of PEFO will be better established through future investigations.

Intermediate-Sized Reptiles.—Numerous tracks in the Petrified Forest Member indicate an abundant and active small- and medium-sized tetrapodal fauna in this region. Tracks are preserved as positive-relief hypichnia (casts); the majority of tracks observed were in float blocks but were easily traceable to adjacent outcrops of Flattop Sandstone Bed #1. Some in-situ tracks were found on the underside of a bed within the middle of this thick sandstone unit, thus supporting our correlation of the tracks with their stratigraphic position.

Most tracks in the Petrified Forest Member were apparently made by similar tracemakers, based on consistencies in size and shape of the prints. One bedding plane yielded 15 identifiable prints that showed little variation in size parameters, although not all toe prints were preserved (Figure 2; Table 1). Some manus-pes pairs are evident on this bedding plane, indicating quadrupedal tracemakers, but we could not discern any continuous trackways. Tracks are not distinctive enough to assign to a specific ichnogenus and their variable preservation, probably as undertracks, argues against applying such designations. Nevertheless, claw impressions are clearly represented by most specimens, thus reflecting reptilian tracemakers.

The considerably larger size of these tracks, in comparison to specimens of Rhynchosauroides, clearly indicates larger tracemakers than rhynchocephalians. However, their considerably smaller dimensions relative to foot sizes of larger quadrupedal tracemakers represented by body fossils in the region, such as aetosaurs, phytosaurs, or dicynodonts (Long and Padian, 1986), precluding adults of these animals as sources of the larger tracks. Although these tracks could be from juvenile animals of larger quadrupeds, the size consistency of most tracks suggests that they are more likely from adult reptiles of an unknown but similar species.

Phytosaurs.—Individual tracks and a trackway in the Monitor Butte Member provide the first compelling trace fossil evidence in PEFO of phytosaurs, commonly represented by body fossils in the Chinle there. One individual track (Figure 3a) displays four well-defined claw marks (6-11 cm long) that curve backward from their initial penetration of the sediment; overall track width is about 14 cm. This same curving of four claw marks is also observable in the clearest print in a trackway from the same unit (Figure 3b). Claw marks are similar in length to the previously described footprint (6-12 cm long), although the track width is slightly less (10 cm). A third footprint (not associated with a trackway) shows five foot pads without claw marks; width is 10.8 cm and length is 15.0 cm. This track, interpreted as a pes print, also has a mediolateral pad impression associated with the fifth metatarsal impression.

 Figure 3—Tracks interpreted as made by phytosaurian tracemakers in Monitor Butte Member, Chinle Formation, PEFO. Top, individual track not associated with a trackway. Bottom, track (presumed pes impression) from beginning of trackway shown in Figure 4.

The trackway (Figure 4) is 2.2 meters long, 37-38 cm wide, and has 12 total prints, although it is missing some impressions in the sequence. The overall trend of the trackway turns slightly to the right of the tracemaker. The trackway shows a similarity in spacing between manus and pes impressions (24-28 cm, out of four manus-pes pairs), although stride measurements are uncertain because of the incompleteness of the trackway and partial preservation of tracks. Nevertheless, right pes-pes stride is about 35 cm at the beginning of the trackway and this measurement served as a predictive indicator for other impressions. Variability in measurements can be attributed to changes in surface topography, the turning motion of the tracemaker, substrate conditions, measurement of only partial prints, and their probable preservation as undertracks.

Based on previous assessments of phytosaur body fossils, their functional morphology, and presumed pedal morphology, our evidence points toward phytosaurian tracemakers for the tracks we have described here. The curving of the claw marks indicate a rotational aspect to the tracemaker's movement, which is consistent with interpreted movement for phytosaurs (Parrish, 1986). The obscured impressions of digits (which are not visible in most tracks of the trackway) are also consistent with this rotation of the pes. Footprint sizes are within the range of known phytosaur sizes, although some tracks described here could be from juvenile animals. The trackway width, manus-pes spacings, and stride length could be attributed to other large vertebrate tracemakers (i.e., rauisuchians, aetosaurs) but when viewed in combination with the preceding evidence are not inconsistent with a juvenile phytosaurian tracemaker. Additionally, the sheer abundance of phytosaur body fossils in PEFO in comparison to other quadrupedal vertebrates argues that any relatively large tracks found in the area showing quadrupedalism likely belong to phytosaurs, if track abundance is at all correlative with body fossil abundance.

Tracks indicative of phytosaurian behavior do not seem as common in the Petrified Forest Member as in the Monitor Butte. However, one large four-toed track found there might have been made by a phytosaur, although the absence of identifiable claw marks allows for other tracemaker interpretations, such as metoposaurs. Nevertheless, this track hints at the presence of a larger tetrapodal fauna at the same time and place as the smaller tracemakers, which may lead to more discoveries of larger tracks in the Petrified Forest Member.

Dinosaurs.—Two individual tracks from the Monitor Butte Member suggest the presence of dinosaurian tracemakers. One print, although incomplete, shows dimensions consistent with large theropod tracks in the Chinle from outside of PEFO (Figure 5a). The track is preserved as a positive-relief hypichnion with well-defined relief in the heel region and parts of two toes proximal to the heel. Track form is tridactyl and a medial toe pad distal from the heel is also evident. Overall track length is 26 cm and outside toes are 19-20 cm long, presumably representing minimum size of the track owing to its incompleteness. Pressure ridges are evident in the heel, lower part of the middle toe, and distal toe pad impressions and all three ridges reflect the animal's shifting of weight and subsequent movement in the same direction. Assuming bipedalism and forward movement, the track is from a left pes. We are attributing the track to a large theropod, although without better preservation we cannot make a more precise designation.


Table 1—Track census and descriptive statistics from bedding plane in Flattop Sandstone Bed #1, Petrified Forest Member (n = 15). Prints are presumed pes impressions.

                               Mean                      Median

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Track Width                6.3 + 1.6 cm                   5.8 cm

Track Length               4.3 + 1.3 cm                   4.0 cm

Width/Length Ratio         1.5 + 0.5                      1.4

Number of Toes       2 (1 specimen); 3 (9 specimens);     4 (5 specimens)

                             Mode = 3                      3

 Figure 4—Dislodged block in Monitor Butte Member, Chinle Formation, PEFO, showing trackway interpreted as made by phytosaurian tracemaker (chalk outlines around tracks). Snowflakes and S. Hasiotis for scale.

A smaller partial print (Figure 5b), also a positive-relief hypichnion, shows a clear impression of one toe (with accompanying clawmark) and a partial impression of the middle toe; the presumed third toe is missing from the print. This track, similar in impression to the other track, is also interpreted as a left pes print. The size and shape of the partial print is comparable to pes impressions of Aetripus, a dinosaur track described from the Chinle outside of PEFO (Lockley and Hunt, 1995, p. 85). The interpreted tracemaker for Aetripus has been a subject of contention but has been most persuasively attributed to ornithischians (Olsen and Baird, 1986). However, Aetripus is, by definition, a track associated with a quadrupedal trackway (Olsen and Baird, 1986), thus until further evidence of quadrupedalism is seen with similar tracks in the Chinle of PEFO, we hesitate to assign this ichnogenus name (or any other) to this partial track.

Both tracks provide independent evidence of a dinosaurian presence in the region of PEFO at the time of Monitor Butte deposition and support body fossil evidence of dinosaurs, although a large enough tracemaker for the first described dinosaurian track has not yet been found here. Indeed, the size of this track is a notable exception to smaller dinosaur tracks found in age-equivalent strata of the region and a discerned gradual increase in dinosaur footprint size toward the Triassic-Jurassic boundary, as postulated by Lockley and Hunt (1995, p. 104-105).

Summary Of Results And Conclusions

Tracks and trackways discovered in the Chinle Formation of Petrified Forest National Park (PEFO), northeastern Arizona, are locally abundant and reflect at least four distinctive tracemakers. A list of possible tracemakers for any vertebrate tracks found in the Park, based on skeletal data, would include the following animals: metoposaurs, phytosaurs, aetosaurs, rauisuchids, poposaurs, trilophosaurs, rhynchosaurs, sphenodontids, and theropods. Interpretation of tracemakers from tracks reported here are difficult owing to the incomplete preservation of most tracks. Thus, our interpretations are limited to small reptiles, intermediate-sized reptiles, phytosaurs, and dinosaurs. We have excluded rauisuchians and aetosaurs as tracemakers for what we interpret as phytosaur tracks for reasons explained previously, but other tracks made by intermediate-sized reptilian tracemakers may relate to poposaurs, trilophosaurs, and sphenodontids. Rhynchocephalians are most likely represented by the smallest footprints, such as Rhynchosauroides.

 Figure 5—Individual tracks interpreted as made by dinosaurian tracemakers, Monitor Butte Member, Chinle Formation, PEFO. Left, Large partial track, presumed theropod tracemaker. Right, Small partial track showing claw mark on far right toe, presumed theropod or ornithischian tracemaker. Lens cap = 5.5 cm in both pictures.

Significant aspects of the our study include the following:

• Tracks and trackways represent the first substantial record of such vertebrate trace fossils in strata from PEFO.

• Most tracks simply indicate walking behavior, although the Rhynchosauroides trackway in the Monitor Butte apparently represents lateral movement, which is unusual for fossil vertebrate tracks.

• The phytosaur trackway in Monitor Butte is one of the few interpreted from the geologic record; some workers have postulated that their preservation was unlikely because of phytosaur locomotion.

• The dinosaur tracks are the first interpreted from the confines of PEFO and they help to confirm body fossil evidence of dinosaur presence in the area during the Late Triassic.

• The sheer abundance of tracks in the Petrified Forest Member and preservation of other tracks show that substrate conditions were conducive for track preservation within the area represented by PEFO.

• The large theropod track in the Monitor Butte is an apparent exception to stratigraphic trends postulated for theropod footprint size during the Late Triassic.

In conclusion, we anticipate that our findings will encourage future exploration of Chinle strata within PEFO for similar and different tracks, as well as better correlation between the traces and the tracemakers through a combination of ichnology and paleontology of vertebrate skeletal material.

Scknowledgments

Many thanks for the warm hospitality and accommodations provided by the PEFO personnel during our stays at the park. Help in some of the field work was also provided by "paleo-interns" Nicole Bonuso, William Pyle, Todd Shipman, and Peter Rocco, during the summer of 1996 at PEFO. We thank the three anonymous reviewers for their reading the manuscript and their subsequent suggestions, which lead to some clarifications of the work. Lastly, we very much appreciate the patience of Vincent Santucci, who waited for the final version of this manuscript while the two authors unknowingly were gallivanting in places far away from the reviewed copies.

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