By Dr. Michael Donovan, Senior Collections Manager of Paleobotany and Paleoecology
One of the most iconic images in Earth's history is of the vast swamp forests that thrived in tropical latitudes during the Pennsylvanian (323.2 to 298.9 million years ago). Along the equator, wetland ecosystems were filled with giant lycopsids (relatives of modern quillworts and club mosses), horsetails, tree ferns, and seed plants with fern-like foliage (Figures 1-3). Insects ruled the skies, including massive dragonflies with wingspans up to 2.5 feet and palaeodictyopterans, an extinct insect group that sucked fluids from plants with their beak-like mouthparts. Six-foot-long millipedes and diverse amphibians walked along the forest floors. Plants died and decayed in wetland environments, leading to the formation of peat, which was compressed into coal over millions of years. Fossil evidence of the humid wetlands of the Pennsylvanian is abundant in coal mines and paleobotanists have been studying these fossil floras for over two centuries. However, depictions of wetland environments tell only part of the story of life on land during the Pennsylvanian.
Figure 1. Diorama of a Pennsylvanian swamp forest at the Cleveland Museum of Natural History.
Figure 2. Lepidodendron
, a lycopsid tree common in Pennsylvanian wetlands (P-19156)
Figure 3. Macroneuropteris scheuchzeri
, a seed plant typical of Pennsylvanian wetlands (P-21715, collected in Illinois)
The Pennsylvanian was a time of major climate change, with oscillations between glacial and interglacial phases and overall increased warming and drying over the duration of the subperiod. When rainfall was high during glacial periods, swamp forests thrived in the equatorial regions. As the climate became drier during interglacial periods, wetland plants were confined to the remaining consistently-moist areas, such as along shorelines and riverbanks. Plants preferring seasonally-dry conditions, such as conifers (Figure 4) and extinct seed-producing (peltasperms, taeniopterids) and spore-producing (noeggerthialeans) plant groups, spread across the landscape. Many of the drought-tolerant plants exhibited adaptations to reduce water loss, including thick cuticles (protective covering that stops water from evaporating out of the plant), dense hairs on the leaves, and sunken stomata (pores that allow gas exchange). Some of the plants also had deeply penetrating roots to reach groundwater. Drought-tolerant floras grew in Ohio during seasonally-dry phases of the Pennsylvanian, although evidence so far is rare. For example, E.B. Andrews, a geologist at the Geological Survey of Ohio, reported a fossil flora composed of then-never-before-seen plants in Southeast Ohio in 1875, including the dryland plant Megalopteris (Figure 5).
Figure 4. Walchia, a conifer that grew in seasonally-dry environments in the Pennsylvanian (P-8367, collected in Kansas).
Figure 5. Megalopteris–a probable seed plant that grew in seasonally-dry environments in the Pennsylvanian (figure from Andrews, 1875; collected in Ohio)
If fluctuations between wetland and dryland habitats driven by climate change occurred repeatedly during the Pennsylvanian, why are the swamp forest floras so much better known than their drought-tolerant counterparts? Taphonomy, the study of how fossils are formed, can help us answer this question. Fossils only form under particular conditions, and plants growing in or near swampy environments are more prone to fossilization than plants in dryland ecosystems where& wet environments are less prevalent. Check out our new display on the Carboniferous (Mississippian and Pennsylvanian subperiods) at the Cleveland Museum of Natural History to learn more about life during this time!