Pennsylvanian ‘fossil forests' in growth position (T ⁰ assemblages): origin, taphonomic bias and palaeoecological insights

Abstract

Fossil forests, buried in growth position in a geological instant (T° 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 T° 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 T° 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 T° 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 T° assemblages offers the potential for a nuanced understanding of these evocative phenomena, and much remains to be learned from these important palaeoecological resources.

T0 peat-forming plant assemblage preserved in growth position by volcanic ash-fall: A case study from the Middle Pennsylvanian of the Czech Republic

“…Most of them are parautochthonous and allochthonous (drifted) in origin, although a number of in situ preserved drowned coal-forming forests (Type A assemblage of Gastaldo et al 1995), described in literature as T 0 assemblages, is reasonably high and sufficient for comparison with our data (e.g. DiMichele et al , 2007Gastaldo et al 2004a, b;DiMichele & Falcon-Lang 2011).…”

“…Fundamental controls on the origin of these phenomena are related to rapid burial in a geological instant, preserving the plant relationships in space and time and removing or reducing many taphonomical biases (Opluštil et al 2009a, DiMichele & Falcon-Lang 2011, Rößler et al 2012. Such conditions are achieved especially by crevasse splay deposition, earthquake-induced subsidence which catastrophically drops the vegetated area beneath the water table, rapid eustatic sea-level rise, permineralization of vegetation by hot springs, or its burial by ash falls proximal to volcanic centres (DiMichele & Falcon-Lang 2011). Particular mechanisms and their "products" differ in the fidelity with which they preserve the ecosystem.…”

“…Limited taphonomic bias makes these assemblages ideal for the analysis of tree density, spatial heterogeneity, assemblage diversity, whole-plant reconstructions, assessment of plant interactions and other ecological "parameters" that can be rarely obtained from any other type of fossil preservation (e.g. Opluštil et al 2009a, b, Wang et al 2009, DiMichele & Falcon-Lang 2011, Pše-nička et al 2005. Careful taphonomical analysis of T 0 assemblages thus significantly contributes to much better understanding of the fossil ecosystems they represent (Johnson 2007).…”

Highly productive polar forests from the Permian of Antarctica

“…In situ stumps representing fossil forests provide information about forest structure, biomass and productivity that is not recorded in plant macrofossils or microfossils (e.g., Francis, 1991;Mossbrugger et al, 1994;Williams et al, 2003;Williams et al, 2009;DiMichele and Falcon-Lang, 2011;Rößler et al, 2012). Because forest characteristics are controlled by environmental factors including temperature, precipitation, soil type, and nutrient availability, they contribute to the reconstruction of paleoclimate, particularly if the paleogeographical settings of the fossil forests are well constrained (e.g., Creber and Chaloner, 1985;Taylor et al, 1992;Francis et al, 1993;Williams et al, 2009).…”

Vertebrate trackways among a stand ofSupaiaWhite plants on an early Permian floodplain, New Mexico

“…R EAL-TIME OR so-called T 0 data (Johnson, 2007;DiMichele and Falcon-Lang, 2011) are among the most sought after in the fossil record. Under ideal circumstances they may reveal spatial patterns, growth conditions, behaviors of animals, and interactions among organisms comparable to those attainable from certain kinds of modern ecological studies.…”