Ichnology, stratigraphy and paleoenvironment of the Boerne Lake spillway dinosaur tracksite, south-central Texas.
Record flooding in south central Texas resulted in four deaths and several million dollars in property damage in late June of 1997. When the flood waters receded, erosion had removed vegetation, soil and bedrock, necessitating repair to the emergency spillway at Boerne Lake near Boerne, Texas. When Natural Resources Conservation Service officials surveyed the damaged area, they discovered newly exposed dinosaur tracks on the floor of the spillway.
Geologists from Baylor University, the University of Texas at San Antonio and Indiana University-Purdue University Fort Wayne investigated the site. In cooperation with city, county and federal officials, they attempted to keep the site location concealed until the tracks could be documented. However, the news media learned of the site and turned the discovery into a TV and newspaper mass media event with national news coverage.
Boerne Lake is located approximately 0.5 miles west of Interstate Highway 10 at the Ranger Creek exit just northwest of Boerne, Texas (Fig. 1). The spillway is located along the southeast corner of this lake, which provides potable water to the City of Boerne. The tracks lie in thin-bedded limestone in the upper third of the spillway.
Following an initial evaluation of the site, flood detritus and remnants of the overlying red clay were removed using small hand tools. Photo-documentation included oblique photos of the three well-preserved trackways from multiple perspectives. Each individual footprint was also photographed from directly overhead to capture the track shape as preserved.
A detailed footprint map was created by laying out a chalk line grid on the track layer at five foot intervals with the axes corresponding to magnetic N/S and E/W directions as determined in the field with a Brunton compass/clinometer. Individual footprint centers were measured relative to this grid, yielding north-south and east-west coordinates for the center of each print. These coordinates were input into a CADD program to develop the base map. Individual footprint bearings as measured with the Brunton were utilized to orient the tracks on the map relative to magnetic north.
Footprint measurements were made in accordance with standard protools (e.g., Leonardi 1987; Thulborn 1990; Lockley 1991; Farlow & Galton 2002). Footprint lengths were measured from the rear margin ("heel" - actually the back of the toe region of these digitigrade animals) of the print to the tip of the middle toe (digit III). Footprint widths were measured across the tips of the inner and outer toes. Maximum footprint depths were measured both in the toe region and in the rear ("heel") portion of the print.
Individual footprint compass bearings were sighted from the rear margin of the print to the tip of digit III. The averages of the bearings of a particular footprint and the print preceding it, and also of the footprint and the print following it, were taken to indicate the overall direction of travel of the dinosaur during the two steps. The bearing of a given footprint could then be compared with these two estimates of the animal's direction of travel. Footprint rotation with respect to the preceding print compares the inward (toward the trackway midline) or outward (away from the trackway midline) orientation of a particular footprint relative to an overall direction of travel estimated as the average of the bearings of the print in question and the preceding footprint. Footprint rotation with respect to the following print uses an overall direction of travel estimated from the print in question and the following footprint. By convention (Leonardi 1987) negative values indicate that the footprint toes inwar d, and positive values indicate outward footprint rotation. A zero value indicates that the footprint bearing coincides with the dinosaur's direction of travel.
Trackway paces denote the distance from a given print to the next print made by the opposite foot, and strides refer to the distance from a given print to the next print made by the same foot. When possible, paces and strides were measured using the tip of a footprint's digit III as the reference point; where this was not possible, the centers of footprints were utilized. From the pace ending in a particular footprint, the pace beginning with that footprint, and the stride opposite the footprint (made as the opposite foot was brought forward), the pace angulation around the footprint was calculated using the law of cosines. This indicates the linearity/narrowness of the trackway; a pace angulation of 180[degrees] means that footprints of the left and right side of the body fall on a single line, a very narrow trackway. With increasingly narrow trackways the value of the pace angulation is sensitive to slight errors of measurement. If the measured stride is greater than the sum of the two paces opposite it, th e pace angulation cannot be calculated. In such cases the pace angulation was estimated as [approximately equal to] 180[degrees].
Casts of selected footprints were made and topographically digitized to evaluate print morphology. Utilizing this information, contour maps of the footprints were created.
Stratigraphic field investigative methods included measuring and describing the section at the site, field correlation with local marker beds of the upper Glen Rose Formation exposed at adjacent cliff outcrops, photo-documenting stratigraphic features of the site and surrounding area, and sampling select horizons for further analysis. Field correlations to adjacent outcrops were performed visually. The flat-lying nature of the beds of the Glen Rose Formation facilitated gross correlations between outcrops, as did the appropriate topographic and geologic maps (U.S.G.S. 1964; Texas Bureau of Economic Geology 1982).
Structurally the site lies south of the Llano Uplift and the associated San Marcos Arch, both of which were positive structural features during the Cretaceous Period (Adkins 1932). These two features appear to have been primary controls of the depositional and structural setting of the site, creating a broad area of shallow-water and shoreline deposition in the encroaching Trinity Sea (Stricklin et al. 1971).
The shallow shelf of central Texas during early Cretaceous time exhibited a cyclically transitional shoreline and shallow marine environment along an epeiric sea. Typical environments included streams, marshes, tidal flats, lagoons and shallow subtidal marine environments. Deposition occurred primarily in shallow water, with frequent subaerial exposure. Isolated rudist and coral patch reefs occurred commonly in the shallow waters, with shelf margin reefal facies occurring farther southeast along the developing Stuart City Reef Trend (Stricklin et al. 1971).
DESCRIPTION OF THE BOERNE LAKE SPILLWAY TRACKSITE
The site occurs on the gently sloping spillway formed of thinly bedded, horizontal limestone layers. Much of the face is covered with flood debris. The active portion of the spillway during the flood was denuded of vegetation.
Dinosaur prints exposed on the spillway of Boerne Lake occur in thinly bedded limestones in the upper Glen Rose Formation of the Lower Cretaceous Trinity Group, about 40 m above the "Corbula bed" that marks the top of the lower Glen Rose (Fig. 2). This stratigraphic position corresponds to the upper part of Unit 3 of the upper Glen Rose (Stricklin et at. 1971). This is considerably higher in the Glen Rose Formation than the common track-bearing layers, which lie a few meters below the Corbula zone (Stricklin et at. 1971; Stricklin & Amsbury 1974).
The Boerne Lake Spillway tracksite is within the upper third of a 4.4-m-thick cyclic interval of thinly bedded wackestone and nodular clayey wackestone, which represents shallow-subtidal to tidal-flat deposition. Fossils in this unit include abundant thin-shelled oysters, other small bivalves, thin, high-spired gastropods, orbitolinids, miliolids and ostracodes. Cyanobacterial stromatolites, mud-cracked layers, and horizontal dissolution cavities (probably dissolved evaporite layers) are associated with the dinosaur prints. The upper meter contains two bored hardground surfaces.
The track-bearing lithologic unit represents a period of high-frequency depositional cyclicity on a very shallow and partly restricted inner shelf. During a period of lowest sea level, when the sabkha-like tidal flats prograded farthest seaward, dinosaurs were able to move through the area. While not necessarily diagnostic of tidal flat deposition, it is nonetheless characteristic of many Lower Cretaceous dinosaur footprints in Texas to occur in such environments.
This upper interval of Unit 3 rests on a well-defined bored and oyster-encrusted hardground at the top of a 2-in-thick burrowed wackestone containing some requinid and monopleurid rudists (probably equivalent to the "Massive" marker bed of Unit 3 of Stricklin et al. 1971). Just below this, the upper Glen Rose is predominantly clay with abundant orbitolinids, probably representing a slightly deeper and open inner shelf.
Immediately overlying the tidal-flat and hardground layers of upper Unit 3 is about a meter's thickness of clay containing echinoids, bivalves, gastropods and green algae, representing a return to more open, normal-marine conditions. This fossiliferous clayey interval is probably the basal layer of Unit 4 as defined by Stricklin et al. (1971).
Although the track-bearing surface is covered with numerous depressions that might have been footprints, most of these are so poorly preserved that nothing can be said about the number and kinds of animals responsible for them. Three sets of footprints can unambiguously be associated in trackways, however (Fig. 3).
A large bipedal dinosaur made Trackway OA. None of the footprints in the trackway is well preserved. The average footprint length in this trail is about 49 cm, and the average width about 43 cm (Table 1). Footprints of greater length than width are usually interpreted as having been made by theropod dinosaurs (Moratalla et al. 1988; Thulborn 1990; Lockley 1991). Although some of the prints have relatively short, thick toes (Figs. 4, 5), a feature typical of prints attributed to ornithopods (Thulborn 1990; Farlow & Chapman 1997), the best preserved footprint in the trackway shows rather longer, narrower toes, giving it a more theropod-like appearance (Farlow 1987; Pittman 1989; Hawthorne 1990). Consequently the ornithopod-like gestalt of most of the prints in the trackway is likely an artifact of preservation, and the trackmaker is identified as a large theropod. The size of the prints is comparable to those of footprints attributed to large theropods at other Glen Rose tracksites (Farlow 1987; 2001; Pittman 1 989).
The maker of trackway OA moved in a northerly direction across the site, making sharp changes in direction twice (Fig. 3). Over the interval of prints 0A5 through 0A7 the dinosaur abruptly turned to the right, and after making print 0A17 it turned less sharply to the left. The pace angulation is generally high over the length of the trail (Table 2), as is typical for trackways of bipedal dinosaurs (Farlow & Chapman 1997), but the two abrupt changes of direction resulted in abnormally low values of the pace angulation. The dinosaur's footprints usually toe slightly inward (slightly negative footprint rotation), as is typical of Comanchean tridactyl footprints attributed to theropods (Farlow 1987).
Trackmaker OA moved in a leisurely fashion. The average stride is about 242 cm, roughly five times the length of individual footprints in the trail. This is toward the low end of stride/footprint length ratios observed in Glen Rose theropod trackways (Farlow 1987).
What are interpreted as trackways (OD1, OD2) of two smaller bipedal dinosaurs cut across trail OA at the northern end of the latter trail (Table 2; Figs. 3, 4d). Conceivably OD1 and 0D2 constitute the trackway of a single animal with a rather broad straddle and a very low pace angulation; "wide-gauge' trackways attributed to theropod dinosaurs are known from the Jurassic (Lockley et al. 1996; Day et al. 2002). However, unambiguous wide-gauge trackways of bipedal dinosaurs have yet to be found in the Glen Rose Formation, where narrower trackways are common (Farlow 1987), and so it appears more likely that these are trails made by two animals. Most of the footprints in the two trails are little more than vague holes in the ground, but the better-preserved tracks, such as they are, suggest a tridactyl foot (Fig. 6). These are not well enough preserved, however, for proper identification of the trackmakers any more precisely than as bipedal dinosaurs. Both trails have pace angulations typical of bipedal dinosaurs (Farlow 1987), and in both trails the stride/footprint length ratio is about 7-8, toward the high end for trails that were likely made by walking (as opposed to running) dinosaurs (Farlow & Chapman 1997). The two trackways are close together, and roughly parallel. If the two sets of prints were indeed made by two dinosaurs, and if they were made at the same time, the two animals may have been walking side by side.
Although there is significant variability in footprint depth within each of the three trackways (Table 1), the deepest footprints at the site were made by trackmaker OA, the biggest of the three dinosaurs (Fig. 7). Thus footprint depths at the site may not reflect circumstances of preservation alone, but also differences in body weights of the trackmakers. Prints with deep toe regions also tended to have deep "heels", and so there was no systematic variability in depth along the fore-aft axis of the footprints (fig. 7).
[FIGURE 3 OMITTED]
Table 1 Measurements of individual Boerne Lake Spillway dinosaur footprints. Track- Foot- Symmetry Footprint Footprint Toe "Heel" way print (Left or Length Width Depth Depth Right) (cm) (cm) (cm) (cm) OA OA1 Right 46.3 42.1 13.1 9.1 OA2 Left 56.4 38.7 10.7 8.2 OA3 Right 47.2 50.3 12.2 10.1 OA4 Left 53.9 45.7 10.1 11.0 OA5 Right 43.9 43.0 10.7 10.7 OA6 Left 48.8 40.5 15.2 14.9 OA7 Right 45.7 40.5 11.3 6.7 0A8 Left 43.3 36.0 5.5 2.4 OA9 Right 47.9 40.2 11.6 7.3 OA10 Left 48.2 40.8 8.8 6.4 OA11 Right 46.3 48.8 11.3 9.1 OA12 Left 46.6 49.4 10.7 9.1 OA13 Right 46.3 44.2 6.7 5.5 OA14 Left 51.8 44.2 10.1 8.8 OA15 Right 54.9 42.1 8.5 7.6 OA16 Left 48.8 45.7 12.2 7.9 OA17 Right 57.9 48.8 10.7 4.6 OA18 Left 54.9 39.6 7.0 7.6 OD1 OD8 (1st print) ? ? ? ? ? OD6 (2nd print) ? 30.5 30.5 7.6 7.6 OD2 (4th print?) ? 19.8 18.9 4.6 4.0 OD0 (5th print?) ? 25.9 21.3 4.6 7.9 OD-2 (6th print?) ? 27.4 25.9 7.6 6.7 OD2 OD7 (1st print) Left? ? ? ? ? OD5 (2nd print) Right? 33.5 30.5 9.1 9.1 0D3 (3rd print) Left? 33.5 30.5 6.1 6.1 OD1 (4th print) Right? 27.4 27.4 7.6 7.6 OD-1 (5th print) Left? ? ? ? ? OD-3 (6th print) Right? 24.4 25.9 4.6 2.1 Table 2 Measurements of Boerne Lake Spillway dinosaur trackways. Track- Reference Footprint Footprint Rotation way Footprint Bearing (degrees) with Respect to: (degrees; 0[degrees] Preceding = North Print QA OA1 320 -- OA2 328 -4 OA3 331 3 OA4 334 -1 OA5 321 -6 OA6 324 -1 OA7 320 -2 OA8 346 -13 OA9 348 1 OA10 350 -1 OA11 350 0 OA12 355 -2 OA13 360 3 OA14 354 3 OA15 353 0 OA16 362 -5 OA17 344 -9 OA18 303 20 OD1 OD8 -- -- OD6 -- -- Missing Print -- -- OD2 -- -- OD0 280 -- OD-2 305 -- OD-4 -- -- OD2 OD7 -- -- OD5 -- -- OD3 -- -- OD1 -- -- OD-1 -- -- OD-3 248 -- OD-5 -- -- Track- Reference Footprint Pace Length way Footprint Rotation (degrees) with Respect to: Following Ending in Beginning with Print Print Print QA OA1 -4 -- 133.5 OA2 1 133.5 118.9 OA3 -2 118.9 120.7 OA4 -7 120.7 114.0 OA5 -2 114.0 103.6 OA6 -2 103.6 116.7 OA7 -13 116.7 135.6 OA8 1 135.6 130.1 OA9 -1 130.1 116.7 OA10 0 116.7 139.3 OA11 -3 139.3 135.0 OA12 2 135.0 121.0 OA13 3 121.0 144.5 OA14 -1 144.5 125.0 OA15 -4 125.0 129.5 OA16 -9 129.5 129.5 OA17 21 129.5 131.1 OA18 -- 131.1 -- OD1 OD8 -- -- 104.2 OD6 -- 104.2 -- Missing Print -- -- -- OD2 -- -- 129.5 OD0 -- 129.5 99.1 OD-2 -- 99.1 106.7 OD-4 -- 106.7 -- OD2 OD7 -- -- 109.1 OD5 -- 109.1 109.1 OD3 -- 109.1 113.1 OD1 -- 113.1 132.0 OD-1 -- 132.0 104.2 OD-3 -- 104.2 111.9 OD-5 -- 111.9 -- Track- Reference Stride Opposite Pace way Footprint the Footprint Angulation (degrees) QA OA1 -- -- OA2 248.4 160 OA3 237.7 166 OA4 232.3 163 OA5 207.3 144 OA6 176.8 107 OA7 253.3 [approximately equal to] 180 OA8 258.5 153 OA9 234.7 144 OA10 249.0 153 OA11 261 144 OA12 247.8 151 OA13 257.0 151 OA14 266.1 162 OA15 -- -- OA16 256.0 162 OA17 243.8 139 OA18 -- -- OD1 OD8 -- -- OD6 -- -- Missing Print 214.6 -- OD2 -- -- OD0 231.0 [approximately equal to] 180 OD-2 204.5 167 OD-4 -- -- OD2 OD7 -- -- OD5 217.3 169 OD3 221.0 168 OD1 252.7 [approximately equal to] 180 OD-1 229.8 153 OD-3 215.8 174 OD-5 -- --
We gratefully acknowledge the contributions of William C. Ward, who contributed significantly to preparation of the site stratigraphic section and the depositional environment analysis, and reviewed the text. Jim Whitcraft assisted in the preparation of artwork. Our project was supported by a grant from the National Science Foundation to J. O. Farlow. This paper is dedicated to the memory of two colleagues: William A. S. Sarjeant, whose research on footprints of extinct vertebrates set a high standard for all who follow, and David L. Amsbury, whose studies of the stratigraphy and sedimentology of the Texas Cretaceous played a key role in our understanding of these topics.
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James O. Jones *
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|Author:||Hawthorne, J. Michael; Bonem, Rena M.; Farlow, James O.; Jones, James O.|
|Publication:||The Texas Journal of Science|
|Date:||Nov 1, 2002|
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