Preliminary Report on a New Sauropod Locality in the Javelina Formation (Late Cretaceous), Big Bend National Park, Texas

Anthony R. Fiorillo
Dallas Museum of Natural History, P.O. Box 150349, Dallas, TX 75315


Abstract—A newly discovered sauropod bonebed, that has yielded several dozen bones, has been discovered in the Upper Cretaceous Javelina Formation of Big Bend National Park. The sauropod is tentatively identified as Alamosaurus sanjuanensis. This bonebed consists of the disarticulated remains of possibly three individuals, one adult and two juveniles that are approximately half the adult size. Excavation of this site has thus far shown this accumulation of material to be monospecific.

The locality is within the floodplain facies of the Javelina Formation. Further, based on the occurrence of some steeply plunging bones, this site may have been a focal point for trampling activity, such as an area around a waterhole.


Introduction

In the spring of 1995, members of a dinosaur class from the
University of Texas at Dallas were engaged in a tour of the Late Cretaceous section in Big Bend National Park. During this tour, the class discovered a new sauropod bonebed in the Javelina Formation of the Park. During the winter of 1996, excavation of this bonebed became a joint Dallas Museum of Natural History-University of Texas at Dallas endeavor. Since the spring of 1997 the bonebed has been excavated by joint field parties from these two institutions. The purpose of this report is to discuss the ongoing work in light of the general depositional setting, the taxonomic makeup of the quarry, and the general taphonomic setting.

Langston et al. (1989) provided a detailed overview of the history of vertebrate fossil collecting in the Big Bend area. Saurian remains from this region have been known since 1907 and subsequently the area has received a good deal of attention from paleontologists (Langston et al., 1989). However, despite this lengthy history, few large concentrations of bones have been found in the Cretaceous section of the Big Bend area. Perhaps the most notable exception to this pattern is the W.P.A. Quarry I which yielded the remains of several ceratopsian dinosaurs from the Aguja Formation (Lehman, 1982; 1989). Therefore, with respect to non-microvertebrate material, this new bonebed represents an unusual concentration of vertebrate fossil material in this region.

Geologic Setting

The Maastrichtian sedimentation pattern for the latest Cretaceous in Big Bend National Park was a general southeastwardly-directed paleoflow direction on an alluvial plain (Lehman, 1986). The terminology for the latest Cretaceous units in Big Bend National Park has been the subject of some controversy. Schiebout et al. (1987, 1988) referred to the Javelina Member of the Tornillo Formation while Lehman (1988) maintained that the Javelina Member should be given the rank of formation. For the purposes of this report, I am following the discussion presented by Lehman (1988) and recognizing the Javelina Formation. Within this context, the Alamosaurus quarry discussed here is located in the Javelina Formation of Big Bend National Park. Precise stratigraphic position is difficult to discern because the outcrop exposure is sporadic in the vicinity of the quarry.

The quarry is comprised of two basic lithologies. The lower unit is a light to medium gray to greenish-gray silstone, generally massive in appearance. Brownish-gray clay clasts, up to 1 cm in diameter (though most are only 1-2 mm in diameter), are locally abundant. Clay slickensides are common. Carbonate nodules are present throughout and many contain bone. This unit is at least 2m thick with bones occurring through the approximately upper 1.5 m. The greenish siltstone has weak, "swirled" bedding occurring irregularly through the unit. The upper contact is sharp and irregular (Figure 1).

The overlying unit is also a siltstone but maroon in color. This unit also contains carbonate nodules, none of which contain bone. There appear to be remnants of horizontal bedding in this siltstone. Based on the fine-grained nature of both of these units, the general depositional setting for this bonebed is taken as part of the floodplain facies.

Taphonomic Setting

All vertebrate fossil material recovered from this site thus far is attributed to Alamosaurus sanjuanens is based on the morphology of the ilium, pubis, and cervical neural spines (Gilmore, 1946; Mateer, 1981; Lucas and Hunt, 1989; Geomani, pers. comm.). These bones appear to belong to 3 individuals, namely 1 adult and 2 juveniles. Based on element length, the juvenile individuals are approximately 50% adult size.



Figure 1—Contact between the maroon siltstone and the bone-bearing gray to greenish siltstone at the Alamosaurus quarry, Big Bend National Park, Texas. The feature shown in the center of this figure is suggestive of a sauropod footprint in cross section. No planar bedding is present directly overlying the downward extension of the maroon siltstone.

All bones from this site are disarticulated. Most elements are isolated, though there are some exceptions. The most notable exception recovered so far has been an unprepared mass of six ribs and a scapulocoracoid (?). All bones are preserved within carbonate nodules. The thickness of the encasing nodule varies from bone to bone. This carbonate crust is typically 0.5 cm to 2.5 cm thick.

Though most bones are found oriented close to the same plane as the dip plane, a small sample of steeply plunging bones have been found. The most spectacular example was the discovery of an isolated femur of a juvenile in a nearly vertical orientation, an anomalous orientation given the low-energy sedimentological setting of the site. The occurrence of high-angle bones in the fossil record is somewhat problematic, and in the absence of corroborative sedimentological data, such orientations were typically attributed to trampling (Hill and Walker, 1972). In an experiment involving modern bones subjected to trampling by ungulates, it has been demonstrated that trampling is a viable means for introducing high-angle bones into a muddy substrate (Fiorillo, 1989). Another characteristic for identifying trampling is the occurrence of shallow scratch marks on the bone surface (Fiorillo, 1984, 1988, 1989), but not the occurrence of this feature is correlated with the sand content of the surrounding matrix (Fiorillo, 1991). Given the fine-grained nature of the matrix at the Alamosaurus quarry, it is expected that similar scratch marks will not be found there. Therefore, the orientation of these high-angle bones at the quarry, and the lack of corroborative sedimentological evidence of high-energy stream flow with rapid deposition, suggests that the subset of bones is evidence for trampling at this site during the formation of this locality.

Discussion

With respect to the fossil record, it is generally accepted that bonebeds are "snapshots", that is, they represent very short intervals of time. Given the similarities of preservation and the proximity of the various bones at this site, it is realistic to suggest that these three individuals shared some interaction during their life history. Continued excavation of this quarry and its group of sauropods may provide insight into the population dynamics of these animals as well as more detailed information regarding habitat preferences.

As mentioned above, the contact is irregular but has the appearance in some cases of large footprints in cross-section (sensu Loope, 1986). Given the size and shape of the better defined of these features, it seemed reasonable to suggest that these features were made by sauropod dinosaurs. However, careful excavation of this contact revealed no evidence of skin or claw impressions. Thulborn et al. (1996) described sauropod tracks from the Lower Cretaceous Broome Sandstone in Western Australia. In their description they described "transmitted plates", hardened prints resulting from the pressure of a multi-ton creature walking thereby causing differential compaction of the substrate (Foulkes, pers. comm.). This type of sedimentological structure provides a criterion for identifying features. Therefore, the presence of trampling at the Alamosaurus quarry currently cannot be refuted or corroborated by the sedimentology of the site.

Sauropod remains are infrequently recovered from the Upper Cretaceous of North America. The known distribution of these remains has made some workers recognize an "Alamosaurus community" during the latest Cretaceous, a community extending from west Texas up through New Mexico and into Utah, with possible extension into Wyoming (Sloan, 1970; Lehman, 1987). In contrast, no sauropods are known from the latest Cretaceous of more northern regions such as the North Slope of Alaska (Rich, 1996; Rich et al., 1997), tempting one to conclude that environmental factors related to northern regions explain the distribution of sauropods in North America during this time. However, Rich (1996) and Rich et al. (1997) summarized the distribution of polar dinosaurs and show that sauropods have in fact existed in paleo-Arctic climates in the past. Therefore, the explanation for the global distrubution of sauropods is ecologically complex. Continued excavation of the Alamosaurus quarry is likely to contribute to a better understanding of the biogeographic distribution of sauropod dinosaurs.

Conclusions

A bonebed containing several dozen bones has been discovered in the Upper Cretaceous Javelina Formation of Big Bend National Park. This bonebed, thus far, has proven to be monospecific and yielded only the remains of the sauropod dinosaur Alamosaurus sanjuanensis. This quarry consists of the disarticulated remains of possibly three individuals, one adult and two juveniles that are approximately half the adult size.

The locality is within the floodplain facies of the Javelina Formation. Further, based on the occurrence of some steeply plunging bones, this site may have been a focal point for trampling activity, such as an area around a waterhole.

Acknowledgements

Foremost, I thank the personnel at Big Bend National Park for their cooperation and assistance with this project, in particular Vidal Davila and Valerie Naylor. I also thank Dana Biasatti and Dr. Homer Montgomery of the University of Texas at Dallas for their substantial contribution in the excavation of the bonebed. In addition I especially thank Paul Foulkes of Broome, Western Australia for his visit to the site and sharing his thoughts on the possible footprints of the quarry. The Dinosaur Society provided partial funding for this project.

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