Upper triassic chinle formation, petrified forest national park, arizona stephen t. Hasiotis

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1Department of Geological Sciences, Campus Box 399, University of Colorado, Boulder, CO 80309-0399

2Geosciences Program, Emory University, Atlanta, GA 30322

Abstract—We report evidence for the earliest known reptilian nests in alluvial deposits from the Petrified Forest Member of the Upper Triassic Chinle Formation (Early Norian), Petrified Forest National Park, Arizona. The Triassic nest ichnofossils are nearly 120 million years older than nests previously described from the Late Cretaceous of the Western Interior of the United States. These structures are found in the first "flattop sandstone #1" above the Sonsela Sandstone/Rainbow Forest Sandstone complex at the south end of the park. The hollow, bowl-shaped pits are present in relatively large numbers and occur in two small areas. The pit openings are sometimes constricted with an expansion below. Inside, the shape is circular to elliptical and forms spherical to elongate pits. Rarely, scratch marks are found across the walls. Internal, partial layering is found at the bottom and along the sides. Some pits are rimmed by elliptical depressions with irregular surfaces that contain a few poorly defined vertebrate footprints.

The bowl-shaped pits are interpreted as nest-holes constructed by vertebrates, possibly phytosaurs, aetosaurs, rauisuchians, or dinosaurs. The nests are very similar to those constructed by Late Cretaceous dinosaurs and sea turtles, extant turtles (Reptilia: Cheloniidae), crocodiles, and alligators (Repitilia: Crocodylidae). Females that congregated in specific areas, which are interpreted as nesting sites, most likely excavated the Chinle nests. The patches of irregular ground around the nests represent trample ground and body pits created by the adult. The layering within the nests may represent active modification of the internal walls and floors and backfilling after eggs were deposited. Possible impressions in the basal portion may represent unhatched or partial eggshells.

These ichnofossils appear to represent the earliest known evidence of vertebrate reproductive behavior. If so, they would also represent one of the earliest forms of parental care such that eggs were placed in specialized structures. This is a major step toward the rearing of offspring. Nesting has likely evolved several times in different groups of primitive vertebrates, but basic nest-hole architecture in extant reptiles with Early Mesozoic ancestry has changed very little in nearly 220 million years.


The fossil record of amniotes begins in the Pennsylvanian Period based on reptilian body fossils and footprints (Carroll, 1964, 1969; Lockley, 1989). Body and trace fossil evidence for reptilian nesting behavior is exceedingly rare in the geologic record. Late Cretaceous ground-nest excavations of non-avian dinosaurs (Horner and Makela, 1979; Horner, 1992; Novell et al., 1995; Varrichio et al., 1997) and sea turtle nests (Bishop et al., 1997) represent in situ evidence of nesting. Fossil eggshell fragments, complete egg clutches, and groups of juvenile remains also provide evidence of nesting (Andrews, 1932; Lapparent and Zybyszewski, 1957; Hirsch et al., 1989; Hirsch, 1994; Kirkland, 1994).

Possible Late Triassic dinosaur eggs (Kitching, 1979; Grine and Kitching, 1987) and Permian eggshell (Hirsch, 1979) have also been reported, but not in nests. Smith (1987) documented the helical burrows of mammal-like reptiles in Upper Permian rocks of South Africa, but made no inferences to brood rearing. No specialized nest structures have been reported from rocks older than the Cretaceous.

We report on evidence for the earliest known reptile nests from the Upper Triassic Chinle Formation, Petrified Forest National Park, Arizona (Fig. 1). Hollow, bowl-shaped pits present in large numbers within limited areas exhibit pronounced similarities to hole-nests excavated by extant turtles (Reptilia: Cheloniidae), crocodiles, and alligators (Repitilia: Crocodylidae). Although no fossil material was found within the nests, the size and location of the nests suggests that the constructors may have been reptiles such as aetosaurs, phytosaurs, rauisuchians, or dinosaurs.

Figure 1—Locality map of Petrified Forest National Park (PEFO), Arizona, and the ichnofossil study area (X).


The study area is at the south end of Petrified Forest National Park (PEFO), Arizona (Fig. 1), where the lower part of the Upper Triassic Chinle Formation is exposed in badlands, buttes, and mesas (Fig. 2). The lower part of the Chinle was deposited in a succession of valley-fill sequences (Cooley, 1958, 1959; Repenning et al., 1969; Stewart et al, 1972; Demko, 1995; Demko et al. 1998). The upper part of the Chinle, of which only the lower part of the Owl Rock Member is preserved within park boundaries, was deposited in a regionally dynamic basin complex of alluvial-lacustrine systems (Stewart et al, 1972; Dubiel, 1989, 1994).

Figure 2—Composite measured section of the Upper Triassic Chinle Formation in Petrified Forest National Park, the stratigraphic position of the ichnofossils (asterisk), and relationship of these units to the regional geology. Modified from Dubiel et al. (this volume).

Bowl-shaped pits are present locally in the flattop sandstone #1 (Billingsley 1985) above the Sonsela/Rainbow Forest Sandstone complex in the upper part of the Petrified Forest Member in the Chinle Formation (Norian) (Fig. 2). These ichnofossils are found in the uppermost part of a 1.5-m-thick upper fine- to medium-grained, trough cross-stratified sandstone. At this locality, the unit has a relatively planar base and a slightly undulatory top representative of an exposure surface with pedogenic features. Further north, this unit contains inclined, heterolithic, accreted strata composed of trough cross-bedded and ripple-bedded sandstones interbedded with mudstone and siltstone. Also in this interval are small, silicified trunks with lateral roots, rhizoliths, small-diameter backfilled meniscate burrows, crayfish crawling trails, wasp cocoons, coleopteran or lepidopteran cocoons, and termite nests assigned to Archeoentomichnus isp.

The ichnofossil-bearing rocks are interpreted as deposits from medium- to high-sinuosity meandering river. The floodplain contained immature cumulative paleosols capped by a simple, mature paleosol. Based on the sedimentary structures and the degree of pedogenesis, the bowl-shaped pits are interpreted to have been in areas close to the active channel where paleosols were weakly developed.


Two distinct localities with a combined total of over 100 pits are found along first flattops sandstone #1 (sensu Billingsley, 1985). Many of the pits at the first locality occur in large float blocks weathered from the outcrop. The pits at the second locality are found in situ along the top of the outcrop. The density of pits is approximately 1/m2 based on measurements for blocks with more than one pit. Proximity of pits averaged 64 +/- 38 cm (N=19), although at least one pair of pits show overlap and another pair had a distance of 150 cm between them.

Discrete hollow, bowl-shaped pits characterize over 100 ichnofossils (Fig. 3). The circular to elliptical openings range from 10-20 cm in diameter and average between 15-16 cm. Occasionally they are associated with a constriction at or just below the paleosurface. Below the opening the internal part of the structures range from 11-44 cm in diameter, averaging 30-35 cm. The walls and floors appear compacted with several layers of sediment. Narrow, elongate, shallow furrows, 7-15 cm in length, are rare and preserved in the walls. The bottoms of a few of the structures contain crescentic to oval indentations around 4-5 cm long and 2-4 cm wide. In some cases, shallow, broad depressions from 63-65 cm long and 35-40 cm wide are present above the deeper, larger pits. The surfaces of these depressions are highly irregular with bumpy protrusions and multidirectional elongate furrows found clustered with one another (Fig. 4). Rare individual vertebrate tracks are found with the irregular surfaces.


The pits and depressions are interpreted as vertebrate nest-holes based on our comparisons of the Triassic ichnofossils to modern burrows and nests constructed by invertebrates and vertebrates. The pits represent the nest proper, most likely excavated by females, in which the eggs were laid. These ichnofossils are very similar to the nest-holes excavated by extant sea and terrestrial turtles (Reptilia: Cheloniidae), crocodiles, and alligators (Repitilia: Crocodylidae) (Brannen and Bishop, 1993; Bishop et al., 1997). In the Triassic nests, the elongate furrows and the compacted thin layers of sediment along the walls and floors reflect the excavation and completion of the nest prior to egg-laying. The crescentic to oval patterns seen in one of the nests grossly resemble impressions of eggshells or eggs. Modern turtle and crocodile eggs have a leathery texture and are not highly calcified. Egg characteristics cannot be determined from the Triassic impressions at this time. The large shallow depressions associated with some of the nests are interpreted as body pits made by the female excavating her nest and laying her eggs. The highly irregular bumpy protrusions and multidirectional elongate furrows within these shallow depressions represent trampled ground that sometimes preserve partial footprints of the nest-maker. Smaller, incomplete pits associated with the nests are interpreted as test pits made by females testing the substrate conditions (e.g., texture, consistency, moisture), as observed in extant turtles (Hailman and Elowson, 1992; Brannen and Bishop, 1993). Thus, several incomplete or much smaller pits adjacent to completed pits and other pits and depressions show various stages of completion as compared to those that appear to be completely constructed nests.

Figure 3 —An example of a block of sandstone with the nest ichnofossils, with a schematic diagram of the block.

Figure 4 — A. Example of trampled ground around some of the nest ichnofossils. B. Close-up of a partial footprint within the trampled area.
The distribution of the Chinle nest-holes are also similar to the nest distribution of extant sea and terrestrial turtles (Reptilia: Cheloniidae), hole-nesting crocodiles, and hole-nesting alligators (Repitilia: Crocodylidae), as opposed to mound-nesters (Cott, 1961; Webb et al., 1983; Woodward et al., 1984; Thorbjarnarson, 1996). Today, the females of these reptiles congregate along rivers, swamps, and beaches to construct their nests and lay their eggs. The females and their offspring return to the same areas to nest for many consecutive years, reflecting nesting site-fidelity (Cott, 1961; Carr, 1967; Mazzotti, 1989; Leslie, 1997). Similar patterns of nest site-fidelity have also been observed with nests constructed by Late Cretaceous dinosaurs (Horner, 1982). Nests interpreted as having been constructed during the same reproductive season are separated by a mean distance analogous to the body-length of adult hadrosaurs. For the Chinle, the spacing between the nests is similar to the length of the interpreted body pits. A few examples of nest construction overlap likely reflect recolonization of the nesting site. This interpretation is supported by observed nest-site reuse by extant crocodilians and alligatorids (Cott, 1961; Webb et al., 1983; Thorbjarnson and Hernandez, 1993). However, in some cases, nesting sites can be also occupied by more than one species of reptile, such as crocodilians and iguanas (Dugan et al., 1981; Bock and Rand, 1989). This implies that the nests may not necessarily be indicative of monospecific nest makers. At this point, the morphology of the Chinle nests suggests only one type of nest-maker.


Based on the size, spacing, and distribution of the nest ichnofossils, the constructors could have been aetosaurs, small phytosaurs, rauisuchians, or dinosaurs. Within 1500 m of the nesting sites is fossil evidence of reptilians that lived within the same stratigraphic interval, which include a partial skull, teeth, and armor plates of phytosaurs, armor plates of aetosaurs, and tracks and trackways of swimming reptiles (Martin and Hasiotis, 1998). Evidence of theropod dinosaurs are also found in both the lower and higher stratigraphic levels (Padian, 1986; Hunt, 1995; Long and Murray, 1995; Hunt et al., 1996; Martin and Hasiotis, 1998). The Triassic nest ichnofossils are nearly 120 million years older than nests previously described from the Late Cretaceous of the Western Interior of the United States. Whoever the nest-makers were, their ichnofossils suggest that they were gregarious, lived and bred along or in perennial watercourses, and exhibited basic parental instincts. There is no way to tell if these reptiles tended to their nests, eggs, or young after hatching. However, the pattern of nests is analogous to that observed for dinosaurs in the Late Cretaceous purported to have cared for their eggs/offspring (Horner and Makela, 1979; Horner 1982; Varricchio et al., 1997) and hole-nesting crocodilians and alligatorids (e.g., Kushlan and Simon, 1981; Mazzotti, 1989; Thorbjarnson, 1996).

The morphology of the Triassic nest ichnofossils and the pattern of their occurrence suggest that basic nest construction and architecture has remained relatively unchanged for over 200 million years. The ichnofossils imply that eggs were cared for through their deposition in excavated nests, rather then simply laid on the ground or in vegetation. This observation indicates that rudimentary parental care may have begun at least in the Triassic and may be even older, and is a major step toward the rearing of offspring and advanced parental care.

We thank Timothy M. Demko and Howard Feldman for comments and suggestions to the manuscript. We also thank Jordi De Gilbert, Nicole Bonuso, Pat Quinn, and Todd Shipman for assistance in the field. The Petrified Forest Museum Association generously supported this research. This research is part of a Ph.D. dissertation by Stephen T. Hasiotis, at the University of Colorado, Boulder.


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