Abstract
The Moscovian plant macroflora at Cottage Grove southeastern Illinois, USA, is a key example of Pennsylvanian (323–299 Million years ago) dryland vegetation. There is currently no palynological data from the same stratigraphic horizons as the plant macrofossils, leaves and other vegetative and reproductive structures, at this locality. Consequently, reconstructions of the standing vegetation at Cottage Grove from these sediments lack the complementary information and a more regional perspective that can be provided by sporomorphs (prepollen, pollen, megaspores and spores). In order to provide this, we have analysed the composition of fossil sporomorph assemblages in two rock samples taken from macrofossil-bearing inter-coal shale at Cottage Grove. Our palynological data differ considerably in composition and in the dominance-diversity profile from the macrofossil vegetation at this locality. Walchian conifers and pteridosperms are common elements in the macroflora, but are absent in the sporomorph assemblages. Reversely, the sporomorph assemblages at Cottage Grove comprise 17 spore taxa (∼16% and ∼63% of the total assemblages) that are known from the lycopsid orders Isoetales, Lepidodendrales and Selaginallales, while Cottage Grove’s macrofloral record fails to capture evidence of a considerable population of coal forest lycopsids. We interpret our results as evidence that the Pennsylvanian dryland glacial landscape at Cottage Grove included fragmented populations of wetland plants living in refugia.
Highlights
The Pennsylvanian Subperiod of the Carboniferous (323–299 Ma) was characterized by a series of glacial–interglacial cycles that exerted profound control on the distribution of vegetation at this time (Eros et al, 2012)
We have analyzed the palynological composition of two samples from sediments containing a conifer macrofossil that occurred in a layer sandwiched between channel-bottom siltstones and conglomeratic deposits
Walchian conifers and medullosan pteridosperms comprise ∼7% and 12% of the identifiable macrofossils recovered from sediments at Cottage Grove, but prepollen produced by these two plant groups is completely absent from both palynological samples (Fig. 3; Table 2). This comparison of the macrofossil and sporomorph records at Cottage Grove builds upon previous comparisons of these two fossil groups in the Carboniferous (e.g., Mahaffy, 1988; Willard, 1993; DiMichele & Phillips, 1994), and emphasizes that they provide very different pictures of the standing vegetation at a given locality (e.g., Chaloner, 1968; Gastaldo et al, 1998; Jackson & Booth, 2007; Mander, Kurschner & McElwain, 2010)
Summary
The Pennsylvanian Subperiod of the Carboniferous (323–299 Ma) was characterized by a series of glacial–interglacial cycles that exerted profound control on the distribution of vegetation at this time (Eros et al, 2012) These cyclic climatic changes resulted in the alternating dominance of wetland and dryland vegetation in the Pennsylvanian tropics (DiMichele, 2014). There is evidence for the co-existence of wetland and dryland vegetation in at least regional proximity, reflected by fossil remains of dryland plants preserved alongside fossil wetland vegetation These drier elements are thought to have been transported from upland (extrabasinal sensu Pfefferkorn, 1980) areas into wetland basins (e.g., Gastaldo, 1987; Lyons & Darrah, 1989; Falcon-Lang & Bashforth, 2004; Gastaldo & Degges, 2007)
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