TAPHONOMY AND PALEOENVIRONMENTAL ANALYSIS OF THE RED CANYON RANCH DINOSAUR SITE, UPPER JURASSIC MORRISON FORMATION, SHELL, WYOMING

Continuing field studies are refining interpretations of the taphonomy and paleoenvironmental setting of a dinosaur site near Shell, Wyoming. The site has two bone-bearing levels preserved within high-sinuosity fluvial channel and associated overbank deposits. A detailed stratigraphic section and map of the lower of these levels shows the vertebrate and fossil plant taphocoenoses found in the various layers.

A basal unit composed of alternating fossil plant-rich and plant-poor lateral accretion surfaces incorporates scattered but fairly complete dinosaur bones, particularly caudal and dorsal sauropod vertebrae. This unit is overlain and truncated by a mud chip conglomerate that contains abundant, but fragmentary, bones and remains of coalified tree trunks and branches. The mud pebble conglomerate layer occasionally preserves dinosaur teeth, including at least one sauropod and two or more theropod taxa. Fossils recovered from the lower quarry are tentatively identified as Camarasaurus, Diplodocus, Allosaurus, and Ceratosaurus.

Quarrying progress was documented using digital photogrammetry to enable the construction of a 3-dimensional computer model, and locations of all fossils and relevant stratigraphic features were recorded using high-resolution real-time kinematic GPS.

Detailed sedimentary descriptions of the upper bone-bearing level record a succession of fining-upward beds composed of siltstones and sandstones. Vertebrate remains are located at the interface of a sandstone and underlying siltstone. The siltstone beds are structureless, but the sandstone beds preserve trough cross beds, ripple cross-lamination, and thin drapes of siltstone rip up clasts. Both the bone-bearing siltstones and rip-up clast conglomerate were screenwashed to assess the microvertebrate fossil record.

Additional studies place the dinosaur site into a broader context. Petrified wood from the Upper Morrison, Cloverly, and Sykes Mountain Formations was collected to assess the plant taphonomy of the region during the Late Jurassic through Early Cretaceous, and the relative importance of sources of coarse clastic sediment in the hinterland was assessed by petrographic analysis of conglomerates and sandstones from the Morrison and Cloverly Formations in Wyoming and southern Montana.