During the Late Cretaceous, western North America was flooded by an inland sea, the coasts of which were covered by broad, low-relief fluvial/alluvial plains on which a wide variety of dinosaurs and plants lived. Although these “actors” are known, their “actions” are not as clear; in particular, details of surface processes, dinosaur behavior and forest structure are not known for certain. In this study, stable isotopes of fossilized teeth from hadrosaurid dinosaurs collected in two different areas within the Kaiparowits Formation are used to investigate possible surface methane production, dinosaur niche partitioning, and the nature of the forest canopies in southern Utah during the Late Cretaceous to answer some of these outstanding questions. Comparison of carbon isotope ratios of tooth enamel between the two areas reveals significant offsets, which can be explained by differences between the areas in either (1) plant communities, (2) environmental and climatic stressors, or (3) the cycling of carbon within a forest canopy. Regardless of the exact cause, the preservation of these differences in hadrosaurid tooth enamel provides evidence of dietary niche partitioning amongst hadrosaurid sub-families within low-lying fluvial environments in southern Utah during the Late Cretaceous. Significant differences in both means and ranges of carbon isotope ratios of tooth dentine also exist between areas. Unlike enamel, dentine does not preserve primary isotopic information; rather, its chemical composition is strongly influenced by chemical processes taking place in soils near the surface. In this case, unusually high carbon isotope ratios of dentine from some sites within the two areas provide the first direct evidence of CH4 production in coastal floodplains of western North America during the Late Cretaceous. Such production of CH4 likely played an important role as a feedback that helped maintain “hothouse” climate conditions during this time. Since CH4 production has a pronounced impact on carbon isotope ratios of gases being emitted from the soil surface, it is possible to trace the movement of these gases into the biotic reservoir. In particular, the existence of hadrosaurid tooth enamel with high carbon isotope ratios suggests that these gases were incorporated into low-level forest vegetation before being eaten by the animals. In order for such “recycling” of carbon to occur, it is necessary for there to be a closed canopy near the surface, meaning that the forest understory is isolated from the open atmosphere due to dense vegetation cover. Thus, results from this study provide the first direct evidence for dense closed canopy forests in southern Utah during the Late Cretaceous.