Late Carboniferous (Pennsylvanian) fossil floras from the United States are well studied as adpression, permineralization, and palynomorph assemblages throughout the stratigraphic column. These data represent an intrabiomic record that can serve as a proxy for climate change in the Carboniferous tropics. The short-term climatic changes that accompanied the alternations between glacial and interglacial intervals did not alter the persistence of the ecological structure of the landscape. Even after floras had been extirpated over large parts of the North American continent in response to marine transgressions, the same plants and plant communities repeatedly returned when the sea receded. However, at the Westphalian-Stephanian boundary (approximately Desmoinesian-Missourian; Moscovian-Kasimovian boundary), major vegetational changes occurred that suggest a significant environmental threshold had been exceeded. Entire clades (most tree lycopsids and medullosans with very large seeds) became extinct, and tree ferns became dominant, changing the aspect of the ecological landscape. This change reflects the overall warming of Earth’s climate, greater seasonality, and shorter periods of wet conditions in the tropics of the late Pennsylvanian.