The Iowa Mangrove Forest?

Destination Iowa (320 – 300 million years ago) – enjoy a lush marine environment where you are not yet threatened by dinosaurs and need not cope with allergies induced by flowering plants. Be sure to visit the thriving mangrove forests that straddle the border between land and sea.

The image of Pennsylvanian Iowa does not resonate with the state’s flat, corn-dominated landscape. However, evidence from petrified plants suggests that mangroves temporarily dominated the flora of central Iowa. These anatomically preserved plant fossils are present in coal balls, spherically shaped coal concretions. They formed through the process of permineralization, when water rich in the mineral calcium carbonate (CaCO₃) trickled into plant cells, resulting in structural preservation. Subsequent debris accumulated on top of the plants, freezing them in time.

Anne Raymond, a paleoecologist at Texas A&M, has spent her academic career documenting prehistoric plant life. With the knowledge that many plants disappeared abruptly in the late Pennsylvanian, she seeks to relate paleoclimate changes to the fossil record. In 1988, she published a study in which she convincingly argued the existence of fossilized mangroves in a collection of 78 coal balls retained from Iowa’s Urbandale mine. Known as cordaitaleans, these ancient mangroves composed 56 percent of identifiable coal ball debris. Conversely, freshwater tree ferns, including the seed plants Medullosa and Psaronius, represented 31 percent of debris, and lycopods composed five percent of debris (Raymond 1988). Raymond not only proposed the mangrove lifestyle of cordaitaleans but also developed a hypothesis for early plant succession in the Urbandale swamp.

Urbandale Cordaitaleans: Saltwater Inhabitants

To persist in their saltwater environments, mangroves have evolved root structures that limit salt intake and enable subsistence at tide’s edge. In many species, roots are coated with suberin, a rubbery material that prevents water from penetrating tissue (Werner and Stelzer 1990). Abundant prop roots systems keep mangrove trunks upright in soft marine sediments. Some mangroves even have pneumatophores, roots that extend out of the mud to take in air (Warne 2007). Likewise, cordaitalean fossils indicate evidence of massive root systems that served as a salt filtration system. Raymond found two indicators of swamp salinity in Urbandale coal ball cordaitalean fossils – the distribution of pyrite crystals and the ratio of shoot to root debris.

Present in sea water, sulfur-reducing bacteria allow for formation of iron-sulfide complexes, including pyrite (FeS₂). Raymond uncovered an abundance of this mineral in the Urbandale coal balls. Disseminated pyrite that occured both inside and outside roots indicated freshwater species that were unable to exclude saltwater. Pyrite that rimmed fossilized roots evidenced saltwater exclusion. Among the Urbandale coal balls, Raymond found that cordaitalean roots lacked internal pyrite. Thus, they remained alive throughout marine inundations. Conversely, pyrite saturated tree fern and lycopod roots, indicating their vulnerability to marine infiltration (Figure 1).

                                                   (a)                               (b)


Figure 1. (after Raymond 1988). (a) Salt water kills the roots of fresh water plants. Sulfate-reducing bacteria inside and outside the root produce sulfide, which combines with iron to form pyrite. (b) Salt-resistant plant roots in salt water exclude the salt. No pyrite forms inside these living roots.

 

Low cordaitalean shoot to root ratios further supported Raymond’s theory. Like modern mangroves, cordaitaleans living in a marine environment would have benefitted from extensive root tissue. To determine shoot to root ratios, Raymond superimposed a grid over peels from fifty Urbandale coal balls (a total sample surface area of 4299 cm²). After identifying the organ type (i.e. shoot or root) and plant form for the largest piece of debris in each square, she calculated the percentage of shoot debris for each coal ball. Interestingly, nearly twenty percent of Urbandale coal balls featured debris composed of only zero to ten percent shoot debris. As expected, these low-shoot coal balls contained primarily cordaitalean fossils. Conversely, coal balls with higher shoot percentages contained evidence of Medullosa, Psaronius, and lycopods – the freshwater plants. Thus, the taxonomic composition of low-shoot coal balls aligns with Raymond’s pyrite inferences, implying that Pennsylvanian cordaitaleans were mangroves.

The Urbandale Swamp Flora: A Freshwater-Saltwater Gradient?

If cordaitaleans were indeed ancient mangroves, did they grow in a different subenvironment from their freshwater counterparts? Did freshwater and saltwater plants live at the same time? In 1983, Raymond T.L. Phillips preliminarily approached this question when they analyzed the paleoecology of Iowa coal balls from three mines, including the Urbandale Mine. They discovered a gradient between cordaitalean debris and tree fern debris – within each coal ball, the freshwater plants’ roots were oriented closer to the cordaitalean debris than were their shoots. Based on this relationship, Raymond and Phillips suggested that tree ferns succeeded cordaitaleans in early Iowa swamp forests. However, their gradient analysis was largely quantitative and excluded lycopod fossils (Raymond and Phillips 1983).

In her 1988 study, Raymond completed a root-penetration analysis of the Urbandale coal balls that confirmed her earlier gradient theory. She tabulated the number of instances in which roots belonging to the four major taxa (cordaitaleans, Medullosa, Psaronius, and lycopods) penetrated debris of each of the other taxa. Consistent with the gradient analysis, root penetration data suggested that cordaitaleans colonized the swamp first and were replaced by Medullosa and Psaronius – in terms of root contact, the freshwater tree ferns grew through the cordaitalean debris ninety percent of the time (Figure 2a). Raymond further postulated that the two tree fern species coexisted; the percentages of Medullosa – Psaronius penetrations and Psaronius – Medullosa penetrations were both approximately fifty percent. Although lycopod fossils composed five percent of the Urbandale coal ball debris, Raymond found only root debris. This suggested that lycopods colonized Iowan landscapes following accumulation of cordaitalean and tree fern peat (Figure 2b).
    
 (a)                                                                     (b)


Figure 2. (after Raymond 1988). Peat accumulation and plant colonization in the Urbandale swamp. The amount of peat under each community corresponds to the percentage of debris uncovered in the coal balls. (a) Based on root penetration analysis, cordaitalean trees colonized first, followed by the freshwater tree ferns. (b) Reconstruction of the swamp following the lycopod colonization. Lycopod roots penetrate both cordaitalean and freshwater peat.

 

Iowa Mangroves: A Figment of the Pennsylvanian

After analyzing the pyrite patterns and shoot-to-root ratios of Urbandale coal balls, Raymond asserted that Pennsylvanian cordaitaleans lived mangrove lifestyles. Further, she completed fossil gradient and root penetration analyses to indicate that they preceded the colonization of freshwater tree ferns and lycopods. In light of major paleogeographical events, Iowa’s present lack of marine forests comes as no surprise. The Urbandale coal balls immortalize tropical flora that existed when North American, Europe, and Asia composed a single continent.

References

Raymond, A. 1983. Peat taphonomy of recent mangrove peats and Upper Carboniferous coal-ball peats. Ph.D. Dissertation. University of Chicago, pp 293.

Raymond, A. 1988. The paleoecology of a coal-ball deposit from the Middle Pennsylvanian of Iowa dominated by cordaitalean gymnosperms. Review of Paleobotany and Palynology. 53: 233-250.

Warne, K. 2007. Mangroves: forests of the tide. National Geographic Magazine. ncm.com.

Werner, A. and R. Stelzer. 1990. Physiological responses of the mangrove Rhizophora mangle grown in the absence and presence of NaCl. Plant, Cell, and Environment. 13: 243-255.