CONTROLS ON THE STRATIGRAPHIC DISTRIBUTION OF NON-MARINE FOSSILS: A CASE STUDY IN THE JURASSIC MORRISON FORMATION, WESTERN USA

Vertebrate history is largely recorded in terrestrial environments; however, covariation between non-marine fossil occurrence and stratigraphic architecture has received minor attention. Here, the widespread Late Jurassic Morrison Formation is used as a template to understand the stratigraphic paleobiology of non-marine systems for vertebrate fossils.

The Morrison fluvial system was compared with the distribution of fossils using the Paleobiology Database (PBDB), literature, and field data. Fifteen Morrison stratigraphic sections were measured at both vertebrate sites and prospected regions where no or few vertebrates were discovered. This study area includes different fluvial architectures from the Colorado Plateau and the Montana-Wyoming region that capture the range in quality and abundance of fossil preservation. The scale and completeness of this study was increased substantially by inclusion of data from the PBDB, which houses a great record of Morrison vertebrates, including 300 collections of 1416 fossil occurrences.

Fluvial systems prograde from the southwest on the Colorado Plateau and from the west in the Montana-Wyoming region as separate entities making the Morrison spatially and temporally heterogenous. Stratigraphic architecture has profound effects on the presence, abundance, type, and quality of vertebrate fossil preservation. Three general categories are identified relative to the source: proximal, medial, and distal. Proximal sites tend to preserve small collections in amalgamated channelforms (~31% of PBDB collections are channels) with rare dry floodplain collections (~8% PBDB collections). Distal sites tend to preserve diverse and relatively large collections in wet floodplain (~29% PBDB collections) to lacustrine settings (~18% PBDB collections) with some crevasse splay localities (~11% PBDB collections), and rare marginal marine sites (~3% PDBD collections). Typically, medial sites are the most fossil-rich regions and preserve fossils in a range of depositional environments including amalgamated to individual channels, floodplain, and crevasse splays. These results indicate that it is necessary to understand and work within a stratigraphic framework to accurately interpret the vertebrate record in terrestrial settings.