The distribution and community ecology of Early Pennsylvanian (middle Bashkirian, Langsettian) vegetation on a seasonally dry fluvial megafan is reconstructed from plant assemblages in the Tynemouth Creek Formation of New Brunswick, Canada. The principal motif of the redbed-dominated succession consists of degraded interfluve surfaces overlain by coarsening-upward aggradational sequences, a pattern that expresses the approach of an
active channel system over a part of the megafan where landscape stasis prevailed. Accrual under a (dry) subhumid tropical climate, typified by a protracted dry season and a short wet season with torrential rainfall, resulted in Vertisol-like paleosols, episodic discharge and sedimentation, shallow channels incised into partially indurated interfluve strata, and scattered ‘waterhole’ deposits. Plant fossils, including many upright stumps, are preferentially preserved above paleosol-mantled interfluve surfaces, recording the inundation of a vegetated landscape. Quantitative analysis of 41 census-sampled megafloral assemblages collected in facies context indicates that a cordaitalean-rich flora dominated the dryland ecosystem. Less common was a wetland flora typical of tropical lowlands at coeval localities, comprising medullosalean pteridosperms and calamitaleans with rare ferns and lycopsids. ‘Enigmatic dryland’ plants, taxa of ambiguous affinity including Megalopteris, Pseudadiantites, and Palaeopteridium, were rare but surprisingly diverse. The taphonomic and sedimentologic context of fossiliferous horizons indicates that low-diversity, old-growth stands of gigantic cordaitaleans blanketed distal interfluves and inactive parts of the megafan, environs marked by limited deposition and extended paleosol development. Small patches of the pteridosperm-dominated wetland flora were interspersed within the dense cordaitalean forest, restricted to landforms that acted as waterholes during the dry season, such as perennial lakes, stagnant ponds, and seasonally active interfluve channels. In contrast, cordaitaleans and wetland plants formed mixed communities in disturbance-prone proximal interfluves and fluvial tracts, where more flooding and sedimentation resulted in less moisture-stressed conditions and a wider range of habitable landforms. Dense calamitalean groves persisted alongside fluvial channels, and an array of wetland plants occupied seasonally active abandoned channels that retained water throughout the year (waterholes). Rare ‘enigmatic dryland’ species were more prevalent in flood-prone fluvial tracts, and were dispersed within cordaitalean-dominated and wetland communities rather than forming discrete, compositionally unique patches. Although frequently characterized as ‘extrabasinal’ or ‘upland’ elements, this study confirms that
these unusual plants occupied Pennsylvanian tropical lowlands during episodes of climatic drying.