A calamitalean forest preserved in growth position in the Pennsylvanian coal measures of South Wales: Implications for palaeoecology, ontogeny and taphonomy

Abstract

A newly discovered fossil forest is reported from Lower Pennsylvanian (late Langsettian) strata in the classic Amroth to Wiseman’s Bridge section of Pembrokeshire, South Wales. It comprises nearly two hundred sediment-cast Calamites axes and putative groundcover ferns, preserved in growth position in the deltaic mouth bar deposits of a marine-influenced coastal plain. Vegetation-Induced Sedimentary Structures developed around upright axes imply that dense Calamites thickets promoted localized patterns of sedimentation, and ultimately, stabilized substrates, paving the way for the establishment of mire vegetation. Not long after initial colonization, a short-lived brackish incursion resulted in dieback of the Calamites thickets; however, a few axes survived to recolonise the delta front through adventitious regeneration, implying that calamitaleans were, to a limited degree, saline tolerant. A remarkable feature of the fossils is that many examples preserve an internal pith-cast embedded within a stem-cast. Preservation of both pith- and stem-casts for the same plant allows inference of an ontogenetic growth-series, and shows that these clastic-substrate Calamites were much more substantially ‘woody’ than previously conjectured. This raises the possibility that plastic developmental traits like growth rate and longevity may have been what mainly distinguished these opportunistic Calamites in disturbed clastic settings from their much larger, and even more woody, cousins in adjacent mires. Analysis of the fossils also contributes to taphonomic debates, supporting the hypothesis that upright Calamites axes are mostly preserved as stem-casts (rather than pith-casts, as traditionally believed). As such, the newly discovered fossil forest sheds light on the ecology, ontogeny, and taphonomy of Calamites – an iconic, familiar, yet, insufficiently understood Pennsylvanian plant.