Pennsylvanian ‘fossil forests' in growth position (T⁰ assemblages): origin, taphonomic bias and palaeoecological insights

Fossil forests, buried in growth position in a geological instant (T⁰ assemblages) are far more abundant in Pennsylvanian successions than in any other part of the geological record. In this review paper, we evaluate the fundamental controls on the origin of these phenomena, investigate the taphonomic biases that influence their composition, and summarize their palaeoecological significance. Following earlier workers, we highlight that high rates of burial and accommodation are essential for the formation and preservation of T⁰ assemblages. Contexts especially favourable for their origin include ashfalls proximal to volcanic centres, coastal plains drowned by relative sea-level rise, and fluvial environments such as channel bars, crevasse splays, and distributary lobes. Long-term preservation requires high rates of subsidence. Consequently, the vast majority of Palaeozoic T⁰ assemblages are confined to wetland settings at, or close to, sea level, whereas drylands are poorly represented and uplands rarely sampled, if ever. However, this is not the only major bias in the fossil record; taphonomic processes selectively preserve plants dependent on their anatomy and stature, and on groundwater chemistry. Thus, although T⁰ assemblages offer unrivalled insights into the nature of ancient forests (whole-plant reconstructions, tree density, canopy height, productivity, plant hydraulics, cohort dynamics, spatial heterogeneity, ecological gradients, tree–sediment interactions, and animal–plant interactions, to name but a few), it is naive to believe they provide ‘photographic snapshots' of palaeoecosystems. None the less, careful taphonomic analysis of T⁰ assemblages offers the potential for a nuanced understanding of these evocative phenomena, and much remains to be learned from these important palaeoecological resources.