Theropod dinosaur trackways in the Lower Cretaceous (Albian) Glen Rose Formation, Kinney County, Texas.
Dinosaur trackways are common in Texas, known from at least 50 locations (Pittman 1992). One of the more famous of the Texas trackways is now on display at the American Museum of Natural History. This trackway, recovered from the bed of the Paluxy River in what is now Dinosaur Valley State Park in Somervell County, Texas, has been interpreted to document the attack of a predatory theropod, probably Acrocanthosaurus, on its sauropod prey (Thomas & Farlow 1997). This unique interpretation illustrates how behavior can be inferred from trackway analyses.
Trackways can provide evidence for gregarious behavior in dinosaurs, which is usually inferred from the occurrence of a number of tracks oriented in the same direction. This is commonly seen in sauropod track sites, such as the Davenport Ranch in Bandera County, Texas, that records the movements of a herd of at least twenty-three sauropods (Lockley & Hunt 1995). Similar records of herding behavior are reported for herbivorous bipedal ornithopod dinosaurs (Ostrom 1972). Upper Cretaceous Western Interior Seaway coastal plain deposits of the Dakota Group in New Mexico and Colorado contain trackways of ornithopod herds in such abundance that the region is known as the Dinosaur Freeway (Lockley & Hunt 1995). Trackway evidence for gregarious behavior among theropod dinosaurs is less common, usually inferred from the occurrence of parallel tracks of two or more theropods found oriented in the same general direction as non-theropod (prey) dinosaur tracks. Lockley (1991) reported the occurrence of trackways from the Late Cretaceous of Bolivia that apparently represent a group of theropods following a sauropod herd. Dinosaur Valley State Park in Texas has a series of trackways that have been interpreted as evidence of three theropods following a herd of 12 sauropods (Lockley 1991). Farlow (1987) interpreted these trackways as showing only a single theropod following the sauropods.
Paired theropod dinosaur trackways are exposed near the top of the Glen Rose Formation in an ephemeral branch of Live Oak Creek along the eastern edge of Kinney County in southwest Texas (Fig. 1). The site, SMU loc. 330, was made available for this study by Mr. Tom Masterson. This study documents the trackways, which were produced by two theropod dinosaurs moving in the same direction, maintaining a consistent spatial separation, and taking strides of equal length.
MATERIALS AND METHODS
The site was worked during two visits in 1999-2000. Excavation uncovered additional tracks of the easternmost trackway (2c and 2d). The tracks were individually measured, photographed and mapped in accordance with Lockley & Hunt (1995). Site location data is on file at the Shuler Museum of Paleontology, Southern Methodist University, Dallas, Texas (SMU loc. 330).
The Glen Rose Formation crops out along with other Lower Cretaceous sediments in a sinusoidal northeast to southwest trending band across the center of Texas and into Oklahoma. The Glen Rose Formation consists of a wedge of limestones, dolomites and sandstones, representing a variety of depositional environments ranging from transitional shoreline tidal flats and marshes to open marine. This diversity is the result of the transgressive/regressive nature of encroaching Albian seas that deposited the Trinity Group prior to the initial completion of the Western Interior Seaway (Pittman 1992). The Trinity Group is composed of the Twin Mountains, Glen Rose and Paluxy Formations. The Glen Rose is underlain by the terrigenous clastics of the Twin Mountains, with which it has a gradational contact, and is unconformably overlain by the Paluxy, a package of loosely consolidated sediments that ranges from continental clastics to deltaic and beach deposits. In north central Texas the Glen Rose Formation wedge pinches out and disappears, and the merged Twin Mountains and Paluxy Formations are termed the Antlers Formation (Hayward & Brown 1967).
The Glen Rose is subdivided into three members, the lower, middle (or Thorp Spring) and the upper member (Davis 1974). Many trackways in the Glen Rose Formation are distributed within the top of the upper Glen Rose Formation, extending over a large area representing an ancient coastal plain (Lockley & Hunt 1995), including the trackways at SMU loc. 330 (Pittman 1992). The Albian age of the upper Glen Rose is derived from ammonite biostratigraphic zones (Jacobs & Winkler 1998).
[FIGURE 1 OMITTED]
RESULTS AND DISCUSSION
Two parallel tridactyl trackways are preserved, indicating a direction of movement of 182[degrees]. The western trackway, designated trackway 1, consists of eight tracks. The centerline of trackway 2 lies 270 centimeters to the east. Trackway 2 consists of four tracks, two of which were uncovered during this study.
[FIGURE 2 OMITTED]
The tracks represent a theropod morphology commonly recorded from the Glen Rose Formation. They are distinguished by long, slender toe marks. Phalangeal pads are not well defined. Distal digit imprints are deeper than the mid-foot print. In well-preserved tracks of both trackways, the middle digit (III) is turned medially toward the opposing foot (Fig. 2). Digits II and IV diverge from the longitudinal axis of the foot at about 25 degrees (Table 1). Distinct claw imprints are evident on tracks 1a, 1c, 1e and 2d. There is no impression of the hallux. A distinct heel imprint is apparent.
Preservation and depth among the tracks varies. Tracks 1a-1d are shallower than the others of that trackway (Table 2). There is no evidence of preserved skin impressions in any track. Prints of trackway 2 are deeper, longer and wider than of trackway 1. Average depth of trackway 2 prints is 7 cm, of trackway 1 prints, 4.4 cm. The deeper prints of trackway 2 preserve greater detail of foot morphology (Fig 3), including footpads that presumably correspond with phalangeal joints. Digit divarication angle is similar to trackway 1, about 25 degrees. There is evidence of upward displacement of sediments between joints and digits not seen in the shallower prints of trackway 1. Trackway 2 prints average 9% wider and 8.7% longer than trackway 1 prints. The two trackmakers were of dissimilar size and weight.
[FIGURE 3 OMITTED]
Langston (1974) referred theropod tracks from the Glen Rose to the ichnotaxa Irenesauripus (Sternberg 1926), and mentions the theropod Acrocanthosaurus as a trackmaker candidate. Acrocanthosaurus body fossils are reported from the Twin Mountains Formation of Texas (Harris 1998) and the Antlers Formation of Oklahoma (Stovall & Langston 1950; Currie & Carpenter 2000). Pittman (1992) referred tridactyl tracks from the Glen Rose to the theropod ichnotaxa Grallator (Hitchcock 1858), and noted that Irenesauripus displays character states diagnostic of Grallator. Grallator is a bird-like print, characterized by a medially turned digit III, which usually exhibits preserved footpad impressions (Pittman 1992). Eubrontes (Hitchcock 1845) a theropod ichnotaxa typically larger in size than Grallotor but exhibiting similar overall morphology, was originally considered by Olsen (1980) to be synonymous with Grallator. Recent morphometric analyses indicate that the two ichnotaxa exhibit proportional differences that allow them to be differentiated (Olsen et al. 1998). The SMU loc. 330 trackways appear to represent both Grallator (trackway 1) and Eubrontes (trackway 2). They exhibit variable preservation of a single body morphology caused by disparities in the size and mass of the trackmakers and differences in the substrate. Following Olsen (1998) and Pittman (1992), the trackways are here referred to Grallator, with Irenesauripus considered a Grallator synonym.
While the individual SMU loc. 330 tracks exhibit commonly observed theropod morphology, the trackways are parallel for the length of their concurrent exposure, and have identical pace and stride lengths (Table 3). Mean pace angulation of trackway 2 is more acute than trackway 1 (155[degrees] as compared to 169.3[degrees]), which indicates a greater displacement between the left and right limbs and provides further evidence of a size differential between the two trackmakers.
Hip height of bipedal trackmakers can be estimated using morphometric ratios derived from measurements of bipedal dinosaur skeletons (Alexander 1976; Thulborn 1989). This hip height estimate can then be combined with stride length to produce a stride length/hip height ratio ([lambda]/h). Alexander (1976) demonstrated that in living terrestrial vertebrates the [lambda]/h ratio for a walking gait is <2.0, for trotting or running > 2.0, and suggested the same was true for dinosaurs. The [lambda]/h ratio for SMU loc. 330 trackways 1 and 2 is 1.34 and 1.19 respectively, indicating both trackmakers were moving at a walk.
Farlow (1981) estimated speeds for theropod dinosaurs from trackways in the Glen Rose Formation of Kimble County, Texas, using methods from Alexander (1976). Estimates ranged from 1.8 to 11.9 m [s.sup.-1], with 12 of 15 estimates falling within the 1.8 to 3.4 m [s.sup.-1] range. Applying this method to the SMU 330 trackway 1 produced an estimated speed of 2.6 m [s.sup.-1]; trackway 2 speed is estimated at 1.8 m [s.sup.-1]. These speeds fall within the walking speed estimates for bipedal dinosaurs provided by Thulborn (1982).
The SMU loc. 330 trackways were produced by a pair of theropod dinosaurs walking in the same direction. The tracks are referred to the ichnogenus Grallator. The absence of appropriately sized theropod body fossils other than Acrocanthosaurus within the Glen Rose Formation and other Trinity Group sediments suggests that this taxon likely made the trackways at SMU loc. 330, a conclusion in concurrence with Farlow (2001).
Estimated speed of the trackmakers agrees with previous estimates from Glen Rose Formation trackways. Although the consistent direction and equal spacing of the trackways appear to suggest gregarious behavior, speed estimates for the two trackmakers differ, which indicates that if the trackmakers were moving in concert, they were not doing so at the same speed during the interval represented by the tracks. Varying preservation between the two trackways suggests they may have been made at different times.
Table 1. Digit divarication angles. Measurements are in compass degrees. DIGITS II-III III-IV II-IV Track Number Divarication angle 1a 24 24 48 1b 26 28 54 1c 24 25 49 1d 25 25 50 1e 24 26 50 1f 28 25 53 1g 24 26 50 1h 25 28 53 Mean 25.0 25.9 50.9 2a 28 20 48 2b 24 25 49 2c 24 26 50 2d 25 25 50 Mean 25.3 24.0 49.3 Table 2. Individual tracks dimensions: W = width, L = length, D = depth. All measurements are in centimeters. W L D Track Number 1a 32 50 2 1b 31 50 3 1c 29 49 4 1d 32 38 1 1e 33 50 6 1f 34 50 5 1g 34 46 6 1h 33 46 5 Mean 32.3 47.4 4.0 2a 37 55 7 2b 34 60 7 2c 36 55 8 2d 35 55 6 Mean 35.5 56.3 7.0 Table 3. Trackway dimensions. Pace length is the distance between the same point on successive footprints of a trackway (left, right); stride length is the distance between the same point on successive same foot footprints (left, left, or right, right); pace angulation measures the angle formed by drawing a line from the most anterior tip of the middle digit of three successive footprints (left, right, left, or right, left, right). All measurements are in centimeters. Dashes indicate non-measured dimensions. Tracks Pace Tracks Pace Tracks Stride Length Angulation Length 1a, 1b 160 1a, 1b, 1c 169 1a, 1c 321 1b, 1c 161 - - - - 1c, 1d 162 1c, 1d, 1e 170 1c, 1e 323 1d, 1e 161 - - - - 1e, 1f 161 1f, 1g, 1h 169 - - 1f, 1g 161 - - 1f, 1h 322 1g, 1h 161 - - - - Mean 161.0 169.3 322.0 2a, 2b 161 2a, 2b, 2c 155 2a, 2c 322 2b, 2c 161 2b, 2c, 2d 155 2b, 2d 322 2c, 2d 161 - - - - Mean 161.0 155.0 322.0
Grateful acknowledgment is made of the gracious hospitality of Ann and Tom Masterson of Houston, Texas, who provided access to their property as well as food and lodging to the author during this study. Thanks also to Drs. Dale Winkler and Louis Jacobs of Southern Methodist University who reviewed this manuscript and made helpful suggestions.
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Jack V. Rogers, II
Department of Geological Sciences
Southern Methodist University
Dallas, Texas 75275
JVR at: firstname.lastname@example.org
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|Author:||Rogers, Jack V., II|
|Publication:||The Texas Journal of Science|
|Date:||May 1, 2002|
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