COMMENTARY

Permian Coal Forest offers a glimpse of late Paleozoic ecology

Arden R. Bashforth1 and William A. DiMichele Department of Paleobiology, National Museum of Natural History Smithsonian Institution, Washington, DC 20560

ittle evokes a sense of wonder about the past like moments fro- L zen in time: the contorted bodies of Pompeii’s horrified citizens fixed in their final poses, the footprints of an ancient hominin family cemented on an African savanna, the tracks of a Jurassic predator closing in on unsuspecting prey along a muddy riverbank, or tree stumps from an ancient Carboniferous rainforest buried in place by a sudden deluge (Fig. 1). These instances provide us with poi- gnant snapshots of deep time, and just like a photograph, they also may capture some of its dynamism. In PNAS, the work by Wang et al. (1) reconstructs the vegetation of an ∼300 million-year-old early Permian- aged mire that was buried and killed by a volcanic ashfall, one of a rare but in- creasing number of occurrences that ge- ologists term T0 deposits (2). The study represents a high-resolution view of a Permian-aged ‘Coal Forest’, so-called be- cause it accumulated atop peat (now coal), Fig. 1. The most classic of T0 assemblages—Fossil Grove at Victoria Park, Glasgow, Scotland, shortly an extinct ecosystem that persisted in after excavation in 1887. [Courtesy Culture and Sport Glasgow (Museums)]. tropical East Asia long after its better- known Pennsylvanian-aged counterparts had all but dried up in Europe and North mobile, plants shed numerous organs tree trunks, lianas entwined in canopy America (1). By using a quantitative (leaves, seeds, and branches) during their branches that fell under the weight of ash, analysis of plant fossils preserved in tuff lifetimes, and almost invariably, they are an absence of groundcover or understory above the coal seam, the work by Wang fragmented and dispersed after death. In beneath trees with dense canopies, and fi et al. (1) provides a 3D reconstruction of fact, paleobotanists rarely nd entire fossil even the impression of a spider perched on the mire vegetation before the eruption, plants at their original sites of growth, a leaf (5). Preserved articulated remains from which patterns of heterogeneity and and they must deduce the growth habits, in a Pennsylvanian-aged tuff in Spain al- ecological gradients emerge. Such T0 de- physiologies, and habitat preferences from lowed reassembly of a bizarre plant that, posits allow paleoecologists to examine remains that have suffered transport be- before discovery, was almost exclusively the past in much the same way neoecolo- fore burial. A high-fidelity record of extinct known from spores (6). In Chemnitz, gists appraise modern environments, and plant communities can best be achieved Germany, entire Early Permian forests to ask questions about the conformity of if they were catastrophically buried in a were enclosed in volcanic ash during 0 observed patterns with various aspects of geological instant, with the resulting T violent pyroclastic surges similar to the ecological theory. deposit effectively fixing the plant rela- Mount St. Helens blast (7). The superb tionships in time and space (3). Accord- preservation allows a glimpse into unique 0 T Deposits: Snapshots of Deep Time ingly, plant fossil assemblages entombed in synecological relationships in the Permian Although plant remains are well-repre- T0 deposits provide an unrivalled chance to forest, such as tree–fern trunks as nurser- sented in the terrestrial fossil record, the reconstruct not only the taxonomic com- ies for young epiphytes and lianas (8). vagaries of fossilization present numerous position of the ancient ecosystem but also Flooding in coastal regions also has the challenges to creating accurate depictions its structure, density, distribution, and potential to entomb extensive forested 0 of ancient vegetation. An enduring and synecological associations. areas, with T deposits forming because of iconic image reproduced in museum dio- The vegetation described in the work by rapid sea-level rise or coseismic events ramas the world over is of a late Paleozoic Wang et al. (1) reflects the key mechanism that suddenly drop a vegetated land- Coal Forest, with myriad bizarre plants for the formation of T0 deposits—volcanic scape below base level. Late Quaternary jumbled together in a steaming tropical ash falls or surges. These events can bury drowned forests abound in tidal zones jungle. However, portrayals of these landscapes in minutes to days—instan- spectacular ecosystems are actually based taneously from a geological perspective— on numerous lines of independent evi- and can reveal captivating ecological in- Author contributions: A.R.B. and W.A.D. wrote the paper. dence, with individual plants being recon- formation. In the Czech Republic, for The authors declare no conflict of interest. structed from disaggregated remains at example, volcanic tuffs overlying Pennsyl- See companion article on page 4927. different sites before reassembly into a vanian-aged coal seams (4) have yielded 1To whom correspondence should be addressed. E-mail: popular family portrait. Although im- unequivocal evidence of vines encircling [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1203261109 PNAS | March 27, 2012 | vol. 109 | no. 13 | 4717–4718 Downloaded by guest on September 29, 2021 today (9) and may become increasingly only a scattering of isolated frames from the final criterion, biotic interaction of common as global warming hastens de- a motion picture. This distinction is not to a species with other site occupants. Can glaciation. In Pennsylvanian-aged strata denigrate these time capsules—they are the organisms compete for resources in 0 in Alabama (10) and Illinois (11), T de- one of paleobiology’s greatest resources— the area? posits formed when earthquakes rapidly but it is critical to keep the matter How do these matters of scale and dropped forested surfaces, bringing in of temporal and spatial scale in mind community assembly apply to Pennsylva- fl oodwaters and sediment that entombed when using them to make ecological nian and Permian Coal Forests? In many the drowned vegetation in growth position. interpretations. parts of the late Paleozoic tropics, these Whereas many trees remain rooted in peat-forming environments covered vast the underlying coal, the overlying mud- A Matter of Scale 0 areas of coastal or alluvial plain with stones contain largely complete fronds and T plant assemblages invite comment on negligible topographic gradient. There enormous prone trees up to 33 m long. matters of ancient ecosystem assembly: are were imperceptible changes in precipi- There also is good evidence for vertical there recognizable gradients, can niche tation and microelevation, and nutrient tiering and even open light gaps in densely partitioning be detected, etc.? Any plant input and disturbance levels were consis- forested mires (12). Rapid sea-level rise (or animal) community within a given area and high sedimentation rates buried a tently low. Although the vegetation was Pennsylvanian-aged swamp in Indiana 0 clearly distributed along landscape gra- (13), preserving a standing vegetation of T deposits allow dients, distinct plant assemblages occurred tall pole-like plants, an assemblage with paleoecologists to over large areas, and ecotones were very no modern analogs. Although almost all diffuse. Plant distribution in these kinds of Pennsylvanian-aged T0 deposits comprise examine the past in wetland is detectably niche-controlled at wetland vegetation growing under humid large spatial scales (11), but it seems to be climates, fortuitous preservation of upright much the same way highly variable and heterogeneous if only conifers in coastal sabkha plains in New one or several disconnected relatively Mexico provided a rare glimpse of semi- neoecologists appraise small areas are examined (4, 12). Thus, arid coastal vegetation (14). where dispersal limitation and a large Pennsylvanian-aged strata also contain modern environments. degree of species substitutability play a innumerable examples of in situ riparian role in community assembly, a small and fl vegetation buried in major oods. For randomly selected T0 deposit may not 0 will be assembled according to three basic example, T deposits show that several provide an accurate portrayal of the entire rules (17). The first rule is a dispersal Pennsylvanian-aged plants had risen to the ecosystem. However, when larger scales of constraint: can a given organism reach the challenge of living in disturbance-prone sampling are possible or when replicates wetland habitats by repeated regenerative area? Together with incumbency (18), this capture identifiable environmental changes growth after burial (15). Additionally, the rule alone may be sufficient to explain the or gradients, such as those changes dis- upright remains of wetland forests with composition of a community if the com- trees up to 6 m high are exposed at several ponent species have similar habitat re- cussed in the work by Wang et al. (1), it levels along the famous Joggins Cliff, Nova quirements and competitive abilities (i.e., becomes clear that late Paleozoic plants, Scotia (16), which was recently declared neutral assembly) (19). However, if the unsurprisingly, record the same kinds of a United Nations Educational, Scientific, species are not uniform in their physical controls on ecological distribution that are and Cultural Organization World Heritage tolerances or substitutability, environ- seen in modern landscapes. They add to Site based on its historic importance. mental selection plays a fundamental role. the growing awareness that processes such Although T0 deposits afford high- Can the organisms live in the resource as neutral dynamics and niche segrega- resolution glimpses into long extinct com- space? Thus, community composition be- tion both operate and are strongly scale- munities, using them to characterize an gins to incorporate niche assembly dy- dependent (21), as much in the past as entire ecosystem would be like viewing namics (20), which are completed with they are today.

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