29 Recent Advances in Systematics, Evolution, and Paleobiology of Fossil Vertebrates

Friday, 21 April 2006
8:00 AM - 12:20 PM

TAPHONOMY OF A SAUROPOD QUARRY IN THE UPPER MORRISON FORMATION, BIGHORN BASIN, WYOMING

Paper number: 29-3
Program: THEME SESSIONS
Time: 8:40 AM – 9:00 AM

Authors

Abstract

More than 100 sauropod skeletal elements collected at the Red Canyon Ranch Upper Quarry in the eastern Bighorn Basin provide taphonomic, anatomical, and stratigraphic data that give insight into a dinosaur species' paleoenvironment and biogeography. The bones were preserved in a greenish siltstone overlain by trough cross-bedded sandstones. The skeleton was completely disarticulated but is believed to represent one individual because of the relative proportions of the recovered bones and a lack of repeated elements. Identified bones include a large (1.5 m) humerus and a proportionately sized scapula, femur, coracoid, and pubis, plus caudal vertebrae and ribs. Comparisons with literature descriptions and museum collections of caudal vertebrae rule out identification as Diplodocus or Barosaurus, while the gracile nature of the humerus and scapula argues against diagnosis as Apatosaurus or Camarasaurus. The size and shape of the latter two elements support an identification of Brachiosaurus, but overall morphology of all skeletal elements is suggestive of a larger than previously recovered specimen of Haplocanthosaurus. A new locality for either of these two genera would greatly benefit biogeographic studies of these rare taxa.

At the Red Canyon Ranch Upper Quarry, larger bones were found in clusters with similar long-axis orientations while smaller bones were dispersed throughout the area. The larger bones displayed little to no wear or weathering, but smaller elements were frequently broken and/or highly weathered. Vertical stratification of bones shows possible sorting by density. The occurrence at the quarry's perimeter of some smaller elements preliminarily identified as turtle, as well as the presence of smaller, more gracile bones (under 10 cm in length), suggests a mixed accumulation of transported small elements. The excellent condition of most large bone surfaces juxtaposed with highly weathered and fragmentary bones indicates either a different provenance for smaller and larger elements or differential initial exposure and burial. The fine-grained, massive siltstone and nicely preserved bone surfaces hint at nonviolent flooding as the main depositional event with subsequent river channel migration over the site.