PALEOECOLOGY OF EARLY PENNSYLVANIAN VEGETATION ON A SEASONALLY DRY LANDSCAPE

Most reconstructions positing the paleoecology of Pennsylvanian vegetation involve wetland ecosystems, particularly peat-forming habitats (now coal) and associated waterlogged clastic substrates, where communities experienced minimal seasonal dryness. In contrast, strata between coal-bearing intervals are typified by calcic and vertic paleosols, incised landscapes, and redbeds that contain the remains of vegetation adapted to seasonal rainfall. These dryland floras, inferred to have grown on moisture-deficient soils, are not as well understood due to preservational bias.

To clarify the paleoecology of vegetation in basinal lowlands during intervals of seasonal precipitation, plant remains were studied from the Lower Pennsylvanian Tynemouth Creek Formation of New Brunswick, Canada, which accumulated on a fluvial megafan adjacent to an elevated margin. The redbed-dominated succession exhibits features consistent with deposition under seasonal conditions, including evidence for episodic discharge due to monsoonal precipitation, degraded interfluves mantled by vertic paleosols, and scattered waterholes in fluvial tracts. By integrating sedimentologic and taphonomic observations with quantitative megafloral analyses in facies context, this research shows how plant communities were distributed in Early Pennsylvanian dryland settings.

Gigantic cordaitalean trees dominated the dryland ecosystem, with dense, monotypic forests blanketing degraded, moisture-stressed interfluve surfaces. Medullosalean pteridosperms were centered on wetter parts of the landscape, particularly near shallow ponds and perennial lakes in interfluve hollows, and aside waterholes that remained in fluvial drainages during the dry season. Dense stands of calamiteans grew alongside fluvial channels, where they were buried by crevasse splay and levee deposits during monsoon floods. Lycopsids and ferns were exceedingly rare and occupied the wettest habitats. Taxa traditionally characterized as ‘upland’ plants were a rare but relatively diverse component of the ecosystem. The presence of these unusual plants in the same beds as cordaitaleans and pteridosperms suggests that they were not washed in from ‘extrabasinal’ communities, but lived together with other clades on basinal lowlands.