TWO LATE TRIASSIC VERTEBRATE FAUNAS AT PETRIFIED FOREST NATIONAL PARK
Adrian P. Hunt
Department of Geology
University of Colorado at Denver
Denver, Colorado 80217
Spencer G. Lucas
New Mexico Museum of Natural History and Science
1801 Mountain Road N.W.
Albuquerque, New Mexico 87104
Two different vertebrate faunas are present in the Petrified Forest Formation (Upper Triassic) at Petrified Forest National Park, Arizona. The older fauna is Adamanian (late Tuvalian) in age, occurs in the Blue Mesa Member and is characterized by the presence of Rutiodon, Desmatosuchus and Stagonolepis. The younger fauna is Revueltian (Norian) in age, occurs in the Painted Desert Member and is characterized by the presence of Pseudopalatus and Typothorax. These faunas indicate that there was no major extinction at the Carnian\Norian boundary.
Petrified Forest National Park (PEFO) has yielded a very large number of Late Triassic vertebrate fossils dating back to the excavations of Charles Camp in 1921 (Long et al., 1989). Since then, large collections have been amassed which are housed principally at the University of California Museum of Paleontology, Berkeley. Friedrich von Huene recognized in the 1920's the presence of two different faunas in the Upper Triassic of northeastern Arizona (note that Huene considered one fauna to be Middle Triassic in age), but Camp's (1930) influential revision of the phytosaurs only recognized one. Colbert and Gregory, (1957) similarly only recognized one fauna within the Triassic strata which are exposed at PEFO. However, Gregory (1957) resurrected Huene's concept of two faunas within the Chinle of the Colorado Plateau region (Table 1), which has been followed by all later workers. The aforementioned studies were hindered by the poor taxonomic state of the majority of the common families of Late Triassic vertebrates. Notably, the Carnian aetosaur Longosuchus and phytosaur Rutiodon were not distinguished from the Norian taxa Typothorax and Pseudopalatus respectively. However, in the last decade there have been complete or partial revisions of these groups, notably the metoposaurs (Hunt, 1993a, 1994; Hunt and Lucas, 1993b), phytosaurs (Ballew, 1989; Hunt, 1993b, 1994) and aetosaurs (Long and Ballew, 1985; Hunt and Lucas, 1991, 1992b). Utilizing these advances in taxonomy various authors have discussed the twofold division of PEFO vertebrate faunas but none have provided updated faunal lists at the species level (e.g., Long and Ballew, 1985; Long and Padian, 1986, Murry and Long, 1989; Parrish, 1989, 1993; Murry 1990; Hunt and Lucas, 1993b; Lucas, 1993). Lucas (1993) has recently published a revised stratigraphy of PEFO which allows the stratigraphic distribution of these faunas to be reevaluated. The purposes of this paper are threefold: (1) to provide updated faunal lists for both Late Triassic faunas at PEFO; (2) to evaluate their stratigraphic distribution and age; and (3) to comment briefly on the differences between the faunas. UCMP is the University of California Museum of Paleontology, Berkeley.
BLUE MESA LOCAL FAUNA
The older of the two vertebrate faunas from PEFO comes from the Blue Mesa Member of the Petrified Forest Formation, Chinle Group (Table 3). Most fossils from this unit come from the southern end of PEFO (south of Interstate 25) where the Blue Mesa Member has its main exposures within the park. This is the type fauna for the Adamanian land-vertebrate faunachron and is late Tuvalian (late Carnian) in age (Lucas and Hunt, 1993b). This fauna, like most late Carnian vertebrate faunas of Laurasia and northern Gondwanaland, is dominated by parasuchid reptiles (phytosaurs). Second most common in abundance are metoposaurid amphibians followed by aetosaurs. Other taxa together form a small minority of the fauna. The vast majority of specimens of this fauna come from a thin stratigraphic interval in the upper portion of the Blue Mesa Member (Lucas, 1993; Fig. 1).
PAINTED DESERT LOCAL FAUNA
The upper of the two vertebrate faunas at PEFO is from the Painted Desert Member of the Petrified Forest Formation. This unit is mainly exposed in the northern half of the park (north of Interstate 25), and this area has produced the majority of the Painted Desert local fauna. Some specimens come from the southern portion of the park in the Flattops area.
The Painted Desert fauna is Revueltian (Norian) in age on the basis of the presence of the phytosaur Pseudopalatus and the aetosaur Typothorax (Lucas and Hunt, 1993b). This fauna is also dominated by parasuchid specimens, but metoposaurs are rare and mainly restricted to localities dominated by terrestrial reptiles (Hunt and Lucas, 1993b; Hunt et al., 1993). Specimens of the aetosaur Typothorax are the second most abundant vertebrate fossils. Note that previous reports of a traversodontid from the Painted Desert Member (e.g., Murry and Long, 1989) are in error (Hunt and Lucas, 1993c). We identify the partial skeleton of a sphenosuchian (UCMP 129740), previously assigned to cf. Sphenosuchus sp. (Parrish, 1991), as cf. Hesperosuchus sp. and consider Padian's (1986) specimen of Coelophysis (UCMP 129618) to represent a new genus (Hunt and Santucci, 1993). Like the Adamanian fauna, the Revueltian fauna at PEFO is stratigraphically restricted and the majority of specimens occur in the lower part of the Painted Desert Member (Lucas, 1993; Fig. 1).
FAUNAL EVOLUTION ACROSS THE CARNIAN/NORIAN BOUNDARY
The Adamanian and Revueltian faunas at PEFO straddle the Carnian/Norian boundary, which has been postulated as a time of a major tetrapod extinction (e.g., Benton, 1994), but this has been disputed recently (Hunt and Lucas, 1992a; Lucas, 1994). Comparison between the two faunas at PEFO should provide a test of extinction theories. Even a cursory examination of Table 3 reveals that there is no major extinction at the Carnian/Norian boundary in the America Southwest, which is typified by the faunas at PEFO. The major groups of tetrapods (phytosaurs, aetosaurs, metoposaurs) exhibit generic level turnover at most. Terrestrial tetrapods appear more common in the Painted Desert fauna (Parrish, 1993), but this is biased by the fact that the older portion of the Revueltian is an acme zone for terrestrial tetrapods across the southwestern United States (Hunt and Lucas, 1993a). Aquatic/semiaquatic elements of the fauna (fish, metoposaurs), with the notable exception of phytosaurs, are less numerous in the Revueltian fauna (Hunt and Lucas, 1993a; Parrish, 1993). The only extinction at the Carnian/Norian boundary which has any global significance is the demise of the last dicynodont Placerias which disappears globally at this time though its disappearance precedes the end of the Carnian (Lucas and Hunt, 1993a). The replacement of xenacanth sharks by hybodonts at the Carnian/Norian boundary is consistent with a pattern throughout the Chinle Group (Huber et al., 1993). Some other rarer taxa may have become extinct at this time for example the enigmatic reptile Trilophosaurus. The apparent extinction of the prosauropods between the two faunas is an example of the Lazarus effect because these dinosaurs are known from Norian and Rhaetian (by footprints) portions of the Chinle Group elsewhere.
We thank the Petrified Forest Museum Association for supporting various projects at PEFO and Vincent Santucci for diverse help.
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Lower portion of formation Upper portion of formation
Eupelor Rarer Eupelor
Primitive species of Phytosaurus Primitive species of Phytosaurus
Advanced species of Phytosaurus
Typothorax Rarer Typothorax
Table 1. Gregory's (1957) twofold division of the Chinle vertebrate faunas in the Petrified Forest Formation (sensu Lucas, 1993) in the region of what is now Petrified Forest National Park. Note that Hesperosuchus was found outside the park.
Blue Mesa Member local fauna Painted Desert Member local fauna
undescribed shark hybodont
Arganodus dorotheae Arganodus dorotheae
indeterminate coelacanthid indeterminate coelacanthid
cf. Turseodus sp. cf. Turseodus sp.
indeterminate redfieldiid indeterminate redfieldiid
indeterminate colobodontid indeterminate colobodontid
Buettneria perfecta Buettneria sp.
Apachesaurus sp. Apachesaurus gregorii
Postosuchus kirkpatricki Postosuchus kirkpatricki
cf. Hesperosuchus sp.
Vertebrate trace fossils:
Table 2. Vertebrate faunas of Petrified Forest National Park (Long and Ballew, 1985; Murry, 1989, 1990; Murry and Long, 1989; Huber et al., 1993; Hunt, 1993a, b, 1994; Hunt and Lucas, 1993b; Lucas and Hunt, 1993b; Parrish, 1993; Santucci and Hunt, 1993; PEFO archives; original identifications). * indicates nomina nuda in Murry and Long (1989).
Figure 1. Stratigraphic distribution of the Blue Mesa and Painted Desert local faunas at Petrified Forest National Park, Arizona. FS is Flattops sandstone and PDS is Painted Desert sandstone.