The paleoecology of a coal-ball deposit from the middle Pennsylvanian of Iowa dominated by cordaitalean gymnosperms
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Paleoecological investigations of a cordaitalean-dominated coal-ball deposit from Urbandale, Iowa, of Middle Pennsylvanian age, suggests that cordaitaleans were the first peat-forming community preserved within coal balls from this deposit. Associations dominated by Psaronius tree-ferns and medullosan seed-ferns colonized the Urbandale swamp after the cordaitaleans. Lycopods, preserved only as stigmarian rootlets, were the last association to colonize the swamp. In Urbandale coal balls, stigmarian rootlets penetrate the debris of all other groups, and these rootlets are frequently surrounded by a calcite matrix of lighter color than occurs throughout the rest of the coal ball. These factors, along with the absence of lycopod aerial debris, suggest that lycopods may have colonized the swamp surface after peat accumulation ceased. The ratio of shoot debris to root debris within Urbandale coal-ball peats suggests that most of this deposit formed in a freshwater swamp. However, coal-ball peats with extremely low shoot-root ratios (no shoots to 0.1) also occur in the Urbandale deposit. These are dominated by cordaitalean roots and may have formed in saltwater swamps. Additional evidence for saltwater cordaitaleans comes from the distribution of pyrite around cordaitalean rootlets. Within the Urbandale deposit, only cordaitalean rootlets have pyrite rims without internal pyrite, a distribution which suggests that some cordaitaleans remained alive in salt water. This distribution of pyrite does not appear to occur around rootlets in coal balls from the freshwater Herrin Coal of Illinois. The shoot-root ratio of all cordaitalean debris in the Urbandale deposit is 0.64, a ratio indicative of a freshwater group. However this value conflates the individual shoot-root ratios of four types of cordaitaleans, each of which produced a different type of ovule (Mitrospermum leanum, M. florinii, Nucellangium glabrum, or Cardiocarpus spinatus). The shoot-root ratio of cordaitalean debris in coal balls containing the foliage assigned to Cordaites-Mitrospermum (Cordaites crassus) as the only cordaitalean foliage, is 1.8, and suggests that these plants inhabited freshwater swamps. The shoot-root ratio of cordaitalean debris in coal balls containing the foliage assigned to Cordaites-Cardiocarpus spinatus (Cordaites principalis) as the only cordaitalean foliage is 0.15, and suggests that these plants inhabited saltwater environments within the Urbandale swamp. Cordaites-Cardiocarpus spinatus probably had a broad ecological amplitude similar to some recent tropical mangroves that can flourish in freshwater and saltwater swamps. 1988.
