CORRELATION BETWEEN CLIMATE CHANGE AND BIODIVERSITY IN THE CONTINENTAL UPPER JURASSIC MORRISON FORMATION

The fauna of the continental Upper Jurassic Morrison Formation is extremely well sampled because of the exceptional nature of preservation of fossils, a large number of quarries, and widespread interest in dinosaurs. Previously published biostratigraphies identified up to six biozones within the Morrison based on first and last appearances of long-ranging taxa of dinosaurs and other vertebrates, ostracodes, and charophytes. Changes in taxonomic diversity and paleocommunity composition have been reported in multiple biozones.

We studied a detailed stratigraphic section through the Morrison Formation in southeastern Utah, USA, to characterize and correlate paleoclimatic and biotic trends. We used lithologies, trace fossils, and pedogenic properties to characterize the paleohydrology and paleoclimate during deposition. We also performed stable isotope analyses of organic carbon at 0.5 m intervals throughout the entire section. We used existing biostratigraphic records and food web software to reconstruct paleocommunity structure for each biozone. Food web reconstructions quantify the connectance i.e., resistance to extinction, of paleocommunities.

The most striking trend in paleoclimate indicators is an upward decline in soil drainage conditions that corresponds to a positive excursion in organic carbon isotopes; this is consistent with other evidence for increasing humidity and decreasing temperatures towards the end of Morrison deposition. The onset of paleoclimate change occurs in a biozone that includes a marked decline in taxonomic diversity, a possible reduction in body size of dominant taxa, and very low paleocommunity connectance values. We postulate that the recorded paleoclimate change resulted in the observed biotic trends.