Preliminary Results of the Dawn of the Dinosaurs Project at Petrified Forest National Park, Arizona

Adrian P. Hunt

Mesalands Dinosaur Museum, Mesa Technical College
911 South Tenth Street, Tucumcari, NM 88401


Abstract—The Dawn of the Dinosaurs Project is a multi-year project to understand the pattern and ecological context of early dinosaur evolution in western North America with a special emphasis on Petrified Forest National Park (PEFO). This work has resulted in the discovery of a new dinosaur locality and numerous specimens at other locations. All dinosaur localities at PEFO are in paleosols. Dinosaur Ridge is in the upper Blue Mesa Member of the Petrified Forest Formation and all other dinosaur localities are in the lower portion of the Painted Desert Member. Late Triassic dinosaurs were dryland facies fossils and their apparent rise to dominance before the end of the Triassic in Europe and South America reflects local drying events.

Introduction

The Mesalands Dinosaur Museum is conducting a multi-year study of the Late Triassic faunas of Petrified Forest National Park (Dawn of the Dinosaurs Project). The goal of this project is to understand the pattern and ecological context of early dinosaur evolution in western North America with a special emphasis on Petrified Forest National Park.

Two hundred and thirty million years ago there were no dinosaurs. By 200 million years ago, every terrestrial animal, larger than a modern house cat, was a dinosaur. How did dinosaurs become the dominant terrestrial vertebrates on Earth? The fossil record in Western Europe and South America suggests that dinosaurs took over rapidly in the middle of the Late Triassic (Hunt, 1991). What does the North American fossil record indicate? Dinosaurs are rare components of Late Triassic vertebrate faunas in western North America and more than 90% of specimens have been recovered from one quarry at Ghost Ranch in north-central New Mexico. Dinosaurs usually constitute less than 5% of vertebrate fossils in any fauna in Upper Triassic strata of this area. Petrified Forest National Park (PEFO) preserves a sequence of highly fossiliferous Upper Triassic strata that include important dinosaur localities. The Dawn of the Dinosaurs Project is a multi-year study of the context and composition of Late Triassic dinosaur faunas at PEFO to help elucidate the rise of the dinosaurs.

The Dawn of the Dinosaurs Project at PEFO has three principal goals: (1) to locate all dinosaur localities in the park; (2) to study taphonomic, paleoecologic and stratigraphic context of all dinosaur localities; and (3) to compare and contrast vertebrate-fossil-bearing localities that produce dinosaur fossils with those that do not. The results of this project will facilitate the understanding of the temporal and ecologic context of early dinosaurs. This paper presents some preliminary results of the project gathered during 1996, 1997 and early 1998.

Geological Setting

Two formations of the Upper Triassic Chinle Group (sensu Lucas, 1993) are present at Petrified Forest National Park (PEFO). These are the lower Petrified Forest Formation, which is divided in ascending order into the Blue Mesa, Sonsela and Painted Desert Members and the upper Owl Rock Formation. Vertebrate fossils at PEFO are restricted to the Petrified Forest Formation. Most fossils occur in the upper portion of the Blue Mesa and the lower portion of the Painted Desert Members (Lucas, 1993; Hunt and Lucas, 1995). These faunas are respectively, Adamanian (late Carnian: late Tuvalian) and early Revueltian (early Norian), in age (Lucas and Hunt, 1993: Hunt and Lucas, 1995).

New Dinosaur Locality

Our surveys have resulted in the discovery of a significant new dinosaur locality named Dinosaur Ridge. This locality lies at approximately the same stratigraphic level as the Dying Grounds area in the upper portion of the Blue Mesa Member of the Petrified Forest Formation (late Carnian). Specifically it is 29.6 m below the Sonsela Sandstone Member in a sequence dominated by mudstone, siltstone and silty- and sandy-mudstone. The vertebrate assemblage derives from a sandy mudstone that is grayish purple (5P 4/2) with yellowish gray (5Y 8/1) mottles and contains thin, interbedded pebble lenses.

Dinosaur Ridge represents the only locality at PEFO of late Carnian age to produce a diverse accumulation of terrestrial vertebrates (including dinosaurs), The dinosaur specimens represent at least two species. The larger species is only known at present from a proximal tibia that represents an animal comparable in size to the specimen described from Norian strata in PEFO by Padian (1986). A smaller species is represented by several specimens including phalanges, vertebrae and tooth-bearing cranial and dentary fragments. This site is only the sixth known latest Carnian (Adamanian) dinosaur locality in western North America and only the second to yield two kinds of theropod dinosaurs.

The associated fauna is dominated by small tetrapods including several osteoderms of the small aetosaur Acaenosuchus geoffreyi and sphenosuchian vertebrae. Larger vertebrates are represented by fragmental bones and teeth of phytosaurs. Vertebrate coprolites are common.

Dinosaur Ridge represents a overbank mudstone, subject to a fluctuating watertable, that has been pedogenically modified. Color-mottling, carbonate concretions and permineralized roots attest to the paleosol origin of this bed. This locality is laterally equivalent to channel avulsion facies (Hunt et al., 1996)

Other Dinosaur Localities

Introduction.—Apart from Dinosaur Ridge, all dinosaur localities at PEFO are in the Painted Desert area of the northern portion of the park. The first specimens were collected in the 1980's from the most productive locality that is known as Dinosaur Hill (= Lacey Point = Bolt Quarry of some authors). The second most important locality is Dinosaur Hollow which yielded the holotype of Chindesaurus bryansmalli (Long and Murry, 1995).

These dinosaur localities occur in the lower portion of the Painted Desert Member of the Petrified Forest Formation (Hunt, 1995; Hunt et al., 1996). These localities are from 10-30 m below the Black Forest Bed and occur in mottled mudrocks (Hunt et al., 1995; Hunt, 1995).

The Painted Desert localities are characterized by mottled mudrocks, reduction haloes and calcrete nodules indicative of paleosol formation (Hunt et al., 1995). Dinosaur Hill represents an abandoned channel deposit that was subject to seasonal ponding and pedogenic alteration. The productive interval overlies a fine- to medium-grained, well- sorted channel sandstone.

Dinosaur Hill.—The fauna of the Dinosaur Hill locality is dominated by small, terrestrial tetrapods. Significant specimens include a partial skeleton of a theropod assigned by Padian (1986) to Coelophysis that is reposited at the University of California Museum of Paleontology (UCMP). This specimen (UCMP 129618) differs from the neotype of Coelophysis bauri in having (Hunt and Lucas, 1988): (1) femoral head more offset from the femoral shaft, with a deep groove on the proximal face of the head and a concave distal margin to the head so that the medioventral corner of the head forms a ventrally directed point; (2) a tibia with a more robust cnemial crest and very large crest for attachment of the flexor muscle and ligaments on the anterior face; (3) a larger ascending process of astragalus; (4) an ilium with larger supra-acetabular crest and proportionally shorter posterior blade; and (5) proximal tarsals that are more flared.

Other described specimens from Dinosaur Hill include a partial skeleton of the sphenosuchian ?Hesperosuchus (cf. Sphenosuchus of Parrish [1991]) and teeth of an ornithischian identified by Padian (1990) as Revueltosaurus (Anchisaurid of Murry and Long, 1989). Other specimens included a skull and postcrania of the small metoposaurid Apachesaurus and undescribed specimens of a smaller theropod dinosaur.

Our collecting has yielded specimens of a third, smaller, ?ceratosaurian dinosaur including a complete femur and numerous vertebra as well as several more teeth of Revueltosaurus. In addition, we have collected the most complete known vertebral column of the small amphibian Apachesaurus. Other specimens included vertebrae of a very small archosaur and armor plates of a new aetosaur-like crurotarsan previously only known from New Mexico.

The Dinosaur Hill specimens also include a partial skeleton (vertebrae from all portions of body, limb bones, armor plates and miscellaneous other elements) of a very small (dorsal centrum length 6 mm) terrestrial reptile. This animal is an new species and is characterized by a short, stout neck (and presumably large head) and armor that is the shape of a pitched roof with a pitted ornamentation.

Dinosaur Hollow.—The Dinosaur Hollow locality contains only the holotype of Chindesaurus and fragmentary specimens of a rauisuchian. This locality represents a very small fossiliferous pocket that has only yielded a few fragmentary vertebrae in recent years.

Other localities.—Fragmentary theropod specimens have been collected from other localities in the Painted Desert. These specimens all come from paleosol sites in the same stratigraphic interval as Dinosaur Hill and Dinosaur Hollow.

Non-Dinosaurian Localities

The Dawn of the Dinosaurs project has also involved examining localities that yield vertebrate fossils but no dinosaur remains. Prospecting for non-dinosaurian sites has yielded some significant specimens. One of the other new localities less than 200 m from Dinosaur Wash contains the partial skeleton of a new aetosaur. This taxon is distinguished by being narrow-bodied with paramedian osteoderms with a weak pattern of random pits and a ventral bar. Recovered specimens include portions of the carapace, vertebral column and limbs. This aetosaur is related to an undescribed taxon represented by a partial skeleton at the Mesalands Dinosaur Museum from the Santa Rosa Formation of Santa Fe County, New Mexico.

Taphonomy of Dinosaur Localities

Taphonomic investigations of Late Triassic vertebrate localities throughout the American Southwest have recognized three principal taphofaies in fluvial environments. Hunt et al. (1995) identified these three taphofacies in the Upper Triassic strata of PEFO: (1) channel-sandbody-hosted assemblages; (2) floodplain mudrock assemblages; and (3) paleosol-hosted assemblages. Vertebrate specimens in channelform sandstones are dominantly the isolated and abraded fragments of phytosaurs, aetosaurs and metoposaurs, which represent channel -lags. Floodplain assemblages represent the majority of fossil vertebrate specimens and include articulated specimens of larger (dorsal centra > 3 cm in length) tetrapods. These specimens occur in mudrocks formed on proximal floodplains. Paleosol assemblages are depauperate in aquatic/semiaquatic taxa and include articulated specimens of small tetrapods (dorsal centra < 6 cm in length). These assemblages occur in mottled mudrocks that contain carbonate (calcrete) nodules.

Dinosaur Ridge and all the other dinosaur localities at PEFO occur in the paleosol taphofacies. This has provided a good search model for identifying additional localities and utilization of this model led to the discovery of Dinosaur Ridge.


Table 1—Some of the principal vertebrate localities at Petrified Forest National Park characterized by taphofacies. Note that none occur in the channel-sandbody-hosted assemblage. Locality names are those used historically, in the paleontological files at PEFO and in recent publications (modified after Hunt et al., 1995).


     Floodplain           Paleosol        Stratgraphic unit

     taphofacies         taphofacies           (Member) 

-------------------------------------------------------------------

     Dying Grounds                           Blue Mesa

     Crocodile Hill                          Blue Mesa

     Jasper Forest                           Blue Mesa

     Crystal Forest                          Blue Mesa

     Devil's Playground                      Blue Mesa

     Phytosaur Basin                         Blue Mesa

                       Dinosaur Ridge        Blue Mesa                                                    

     Flattops                              Painted Desert

     Black Forest                          Painted Desert

     Billings Gap                          Painted Desert

                     Lungfish Locality     Painted Desert

                       Dinosaur Hill       Painted Desert

                      Dinosaur Hollow      Painted Desert


It appears that the stratigraphic distribution of taphofacies is not random (Table 1: Hunt and Lucas, 1993; Hunt et al., 1995). Vertebrate accumulations of the paleosol taphofacies are better known from the Painted Desert Member. However, this may be in part a collecting bias. Previous collectors have tended to concentrate on the collection of larger specimens and the small, fragmentary specimens from paleosols have been overlooked. We predict that future dinosaur localities will be found in the Blue Mesa Member utilizing the paleosol search model .

Implications For Dinosaurian Evolution

Preliminary results from PEFO indicate that Late Triassic dinosaurs inhabited dryland environments where calcretes were forming and that they were absent from wetter ecosystems. In essence, Late Triassic dinosaurs were dryland facies fossils. This suggests that the apparently rapid rise to dominance of dinosaurs in the Norian as evidenced in Western Europe and South America may indicate a localized drying events. In North America, where the climate remained moist, dinosaurs remained a minor part of the overall ecosystem until the end of the Triassic. At the very end of the Triassic in western North America the eolian facies of the Wingate Sandstone indicate a drying trend (Lucas et al., 1997) that coincided with the rise to dominance of the dinosaurs on this continent.

Acknowledgments

I thank Petrified Forest Museum Association and the National Park Service for supporting this project and Micky Helickson and Mark DePoy and many of the staff of PEFO for diverse help and encouragement. Andrew Heckert, Tom Olson, Phil Huber, Phil Bircheff and many students from Mesa Technical College have assisted with fieldwork.

References

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———. 1995. Stratigraphy and taphonomy of Late Triassic dinosaur localities, Petrified Forest National Park, northeastern Arizona. Program and Abstracts of Presented Papers and Posters of Third Biennial Conference of Research on the Colorado Plateau, 26.

———. and Lucas, S. G. 1993. Sequence stratigraphy and a tetrapod acme zone during the early Revueltian (late Triassic: early Norian) of western North America. New Mexico Museum of Natural History and Science Bulletin, 3: G46.

———. and———. 1995. Two Late Triassic vertebrate faunas at Petrified Forest National Park, p. 89-93. In Santucci, V. L. (ed.), National Park Service Paleontological Research. National Park Service Technical Report NPS/NRPO/NRTR-95/16.

———, and ———. 1998, A new theropod dinosaur from the Norian (Late Triassic) of Petrified Forest National Park, AZ and east-central New Mexico: Geological Society of America, Abstracts with programs, in press.

———, V. L. Santucci, and A. J. Newell. 1995. Late Triassic vertebrate taphonomy at Petrified Forest National Park , p. 97-100. In Santucci, V. L. (ed.), National Park Service Paleontological Research. National Park Service Technical Report NPS/NRPO/NRTR-95/16.

Hunt, A. P., T. J. Olson, P. Huber, T. Shipman, P. Bircheff, and J. E. Frost. 1996. A new theropod locality at Petrified Forest National Park with a review of Late Triassic dinosaur localities in the park; In Boaz, D., Dierking, P., Dornan, M., McGeorge, R. and Tegowski, B. J., (eds.), Proceedings of the Fourth Annual Fossils of Arizona Symposium: Mesa, Mesa Southwest Museum and Southwest Paleontological Society, p. 55-61.

Long, R. A. and P. A. Murry. 1995. Late Triassic (Carnian and Norian) tetrapods from the Southwestern United States. New Mexico Museum of Natural History and Science Bulletin 4, 254 p.

Lucas, S. G. 1993. The Chinle Group: revised stratigraphy and chronology of Upper Triassic nonmarine strata in the western United States: Museum of Northern Arizona Bulletin, 59: 27-50.

———. and A. P. Hunt. 1993. Tetrapod biochronology of the Chinle Group (Upper Triassic), western United States: New Mexico Museum of Natural History and Science Bulletin, 3: 327-329.

———, O. J. Anderson, A. B. Heckert, and J. W. Ester. 1997. Phytosaur from the Wingate Sandstone in southeastern Utah and the Triassic-Jurassic boundary on the Colorado Plateau; In Anderson, B, Boaz and McCord, R. D., (eds.), Proceedings of the Southwest Paleontological Symposium, Mesa Southwest Museum and Southwest Paleontological Society, p. 49-58.

Murry, P. A. and R. A. Long. 1989. Geology and paleontology of the Chinle Formation, Petrified Forest National Park and vicinity, Arizona and a discussion of vertebrate fossils of the southwestern Upper Triassic, p. 29-64. In Lucas, S. G. and Hunt, A. P. (eds.), Dawn of the age of dinosaurs in the American Southwest. New Mexico Museum of Natural History, Albuquerque.

Padian, K. 1986. On the type material of Coelophysis (Saurischia: Theropoda) and a new specimen from the Petrified Forest of Arizona (Late Triassic: Chinle Formation), 45-60. In Padian, K. (ed.), The beginning of the age of dinosaurs: faunal change across the Triassic-Jurassic boundary. Cambridge University Press, Cambridge.

———. 1990 The ornithischian form genus Revueltosaurus from the Petrified Forest of Arizona (Late Triassic: Norian: Chinle Formation). Journal of Vertebrate Paleontology, 10: 268-269.

Parrish, J. M. 1991. A new specimen of an early crocodylomorph (cf. Sphenosuchus sp.) from the Upper Triassic Chinle Formation of Petrified Forest National Park, Arizona. Journal of Vertebrate Paleontology, 11: 393-396.