THE IMPACT OF SAUROPODS AS ECOSYSTEM ENGINEERS ON FOOD WEB DYNAMICS AND MEGAFAUNAL DIVERSITY AT DRY MESA QUARRY, COLORADO OF THE UPPER JURASSIC MORRISON FORMATION

The Morrison Formation of the western United States is among the best preserved Upper Jurassic dinosaurian ecosystems, and throughout its approximate 7 million year duration was dominated by the largest terrestrial animals ever, the sauropods. The sympatry of these megaherbivores alongside megapredatory theropods differs dramatically from later well-sampled Laramidian dinosaurian ecosystems. This raises questions about the dynamics of ecosystems capable of sustaining such megafaunal diversity. Due to its high megafaunal taxonomic diversity, we selected the Dry Mesa Quarry in Colorado to investigate food web structures. We constrained dietary ecology of the Dry Mesa assemblage using multiple proxies including extant predator-prey relationships, comparative anatomy, dental microwear, phylogenetic bracketing of ecological traits and isotopic data from the literature. This was used to reconstruct a probable food web using ecological network analysis.

Our results find a key relationship between the ontogenetic mass change of dinosaurs and food web dynamics. Sauropods likely played key roles in the Morrison ecosystem. Due to their massive body sizes, adult sauropods were likely ecosystem engineers with a low spatial density whose foraging affected the distribution of vegetation. Their sheer mass made adult sauropods immune to predation, but small juvenile sauropods did not enjoy such safety. Thanks to sauropods’ r-selected reproductive strategy and age-segregated gregarious habits, immature sauropods were an abundant and more easily acquired prey item than contemporary ornithischians, who may have undertaken non-aged segregated gregariousness. This supported high megapredatory and megaherbivore diversity. A key ecosystem dynamic revealed by our data was the critical body mass threshold where sauropods were likely immune from predation but prior to reaching full size.The presence of numerous independent juveniles is a key difference between sauropod-dominated ecosystems and later ornithischian and mammalian ecosystems, where parental care and nursing produce limited ecological ontogenetic shifts in the latter. Future studies will construct additional Morrison Formation localities and succeeding Cretaceous food webs to evaluate their ecological functional diversity and dynamics.