|NPS Paleontology Research Abstract Volume|
Land plants formed the trophic foundation for large populations of dinosaurs throughout the Mesozoic. Two Upper Cretaceous formations, the Aguja and Javelina, possess a rich dinosaur fauna. This reptilian fauna has been intensely studied at Big Bend National Park. Different and unique species of hadrosaurs, ceratopsians, ankylosaurs and large sauropods are common to the Aguja Formation; many of the dinosaur associations found here do not occur anywhere else in North America. Additionally, associated plant remains are encountered in carbonaceous shales, lignites and sandstones of equivalent age throughout the park. The current study will focus on the microflora and megaflora of both formations and will highlight how past herbivores exerted different selective pressures on plants. Research objectives for 1991 include finding a prolific leaf locality associated with the dinosaur fauna and processing and analyzing a stratigraphic sequence of 80 pollen samples.
The recent discovery of a nearly complete ceratopsid skull in the Aguja Formation of southwest Texas supports previous conclusions that the Aguja ceratopsid represents a distinct species, Chasmosaurus mariscalensis. The diagnostic features of C. mariscalensis include an extensive anteromedial projection of the nasal between the premaxillae, erect supraorbital horns, and laterally rounded squamosal. Nine cranial features that vary among Chasmosaurus species, Pentaceratops sternbergii, and other chasmosaurines are analyzed. Chasmosaurus mariscalensis appears to be most closely related to northern species of Chasmosaurus (C. belli, C. russelli), which also exhibit a transversely flattened nasal horn and modifications of the anterior margin of the external naris. The genus Chasmosaurus, in turn, appears to be most closely related to the other southern chasmosaurine, Pentaceratops sternbergii. The biogeographic history inferred from these relationships suggest that the biogeographic exchange between northern and southern chasmosaurines that must have occurred cannot be explained by a single dispersal event to the south.
The McKinney Springs tongue of the Pen Formation is the highest Upper Cretaceous marine unit in the Big Bend region and consists of four lithologic facies. These intergrading lithofacies are, in ascending order, the calcareous shale facies, interbedded sandstone and shale facies, the phosphate nodule facies, and the silty shale facies. This series of lithofacies records deposition in a muddy shelf or prodeltaic setting during a marine transgression, high water still stand, and subsequent deltaic progradation. The fauna in this unit is not abundant and exhibits a low species diversity.
The calcareous shale facies contains two different types of trace fossils. These are horizontal, randomly-directed, feeding burrows (fodichnia) and vertically-oriented, spiral burrows (Gyrolithes). Macrofauna consists of large oysters (Exogyra ponderosa) and other bivalves including Inoceramus vanuxemi and Durania sp. Many of the bivalves have been bored and encrusted with oysters (Pseudoperna sp.). Also present are fragments of freshwater turtle and dinosaur bone.
The interbedded sandstone and shale facies contains alternating thin beds of sandstone and shale. The sandstone beds contain poorly preserved specimens of Inoceramus sp. The shale beds contain Exogyra ponderosa that are 15 to 20 centimeters in diameter, oriented in life position, and contain clionid sponge and lithophagid bivalve borings in the shell material.
The phosphate nodule facies is a lag deposit which contains rounded and abraded skeletal material. Shark teeth from three species are common - Scapanorhynchus texanus, Squalicorax kaupi, and Cretolamna sp. Also present are phosphatized small inoceramid bivalves, other small mactrid bivalves, small gastropods, and occasional crabs.
Within the silty shale facies, there are several thin sandstone beds which contain well developed trace fossils (fodichnia). Macrofauna consists of occasional shark teeth and fish vertebrae. Two types of ammonites, Placenticeras sp. and Baculites sp., are present in the upper portion of this facies. Bivalves in this facies are restricted to small clusters of Inoceramus biconstrictus.
The project is nearing conclusion with most of the mapping complete. The deposits being studied are of a geologic age unique to North America. From this study an important gap of information will be filled by increasing our understanding of the progressive change in faunal succession. During our 1990 field season two very important specimens were recovered when we prospected a virgin region never before studied. One was the partial skull of the horse Hipparion forcei, and the other was the lower jaw bone and some rib and vertebral scrap of a long-jawed mastodon.
Big Bend National Park has abundant remains of fossil dicotyledonous woods of Cretaceous and Paleocene age. These woods are significant because worldwide there are fewer than 100 records of Cretaceous dicot woods, and fewer than 40 records of Paleocene dicot woods. In collaboration with Tom Lehman of Texas Tech, work is underway to document the diversity of fossil dicot woods of Big Bend. The Aguja Formation (Campanian, Late Cretaceous) has yielded woods of the platnoid (sycamore-like) and phyllanthoid types, wood types that occur at other Cretaceous localities in the northern hemisphere. In addition, there are at least five other wood types, and these represent new species. All five have characteristics of tropical trees, and, as a group, are relatively unspecialized anatomically. In the Javelina Formation (Maestrichtian, Late Cretaceous), a large log (>1m diameter) has specialized features and resembles tropical members of the Malvales.
Over 30 logs in the Paleocene Black Peaks Formation are of a single type, and have been described as a new species, Paraphyllanthoxylon abbotti. The species name recognizes the late Maxine Abbott who collected this material. These woods and a platanoid type are the only Paleocene dicot woods described from the United States. The Cretaceous and Paleocene dicot woods of Big Bend cannot be assigned with confidence to a single extant plant family.
Several samples of silicified and carbonized woods from the Cretaceous non-marine Gulfian Series of geological formations (Javelina or Aguja formations) are under investigation. Representative specimens were examined by R.V. Gentry of Earth Science Associates to determine if any radiohalos were present in the materials. No radiohalos were detected in the petrographic thin sections. Gentry concluded that there had been no uranium infiltration of the Big Bend fossil woods he examined as was the case in the carbonized woods he studied from the Colorado Plateau and from Chattanooga Shale.
A sample of carbonized wood was found to contain 59.2% carbon and was subjected to a radiocarbon age determination. No C-14 was detected and the radiocarbon age was found to be greater than 42,000 years B.P. Other C-14 age determinations will be done. Several silicified wood samples were analyzed for SiO2 to determine the degree of petrification. The results are as follows:
SAMPLE DESIGNATION (% SiO2)
E 4.3-5.3From the analyses, it appears that all samples except D and E are well-petrified. Also energy dispersive x-ray analyses were conducted on all samples. Further chemical tests are planned. Attempts will be made to identify all wood samples from scanning electron micrographs and petrographic thin sections.
|United States Department of the Interior, National Park Service|