The Carboniferous period spans from 359.2 to 299 million years ago. The Carboniferous period gets its name from the coal measures found in deposits of this age. In the United States this period is often divided into the Mississippian 359.2-318.1 mya (characterized by limestone deposits in the state of Mississippi) and the Pennsylvanian 318.1-299 mya (characterized by coal deposits in the state of Pennsylvania). Once again life recovers from the crises; new evolutionary variations and first appearances occur.

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Primary Producers & Reefs

The dominant primary producers in the oceans continue to be cyanobacteria, green and red algae (Knoll, Summons, Waldbauer, and Zumberge, 2007, p. 148). Fusulinids, a type of Foraminiferan, make their appearance in the Carboniferous. Fusulinids are single-celled amoeba-like organisms with shells made of calcium carbonate (calcite). Like today’s Foraminiferans, these fusulinids probably had a symbiotic relationship with algae (Stanley, 1987, pp. 92-93). After the Devonian crises reef building became almost non-existent. Calcarious algae formed some small mound-like reefs in warm, shallow seas (Stanley, 1987, p. 92).

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Plants

The first major coal deposits were formed during the Carboniferous. Within the coal measures are found thin marine sediment layers, which may represent interglacial periods (Kenrick and Davis, 2004, p. 81) or the periodic deposition and erosion of delta lobes (Selden and Nudds, 2004, p. 59). The large coal deposits in the eastern U.S. and Western Europe formed between 295 and 320 mya. These coal-forming forests grew in humid, tropical environments. Lycopsids (clubmosses) and shenopsids (horsetails) would reach their greatest diversity in these Carboniferous forests, which have no analogues today (Kenrick and Davis, 2004, p. 35). Lycopods (Lycophyta) represented the dominant tree form. Lepidodendron was a lycopsid that could reach a height of 30 m and a width of 1m near its base. The trunk was tapering and pole-like, studded with diamond-shaped leaf scars, graced with a crown of bifurcating branches atop and a crown of bifurcating roots at its base. Needle-like leaves were clustered around spore-bearing cones at the end of branches (Janssen, 1979, p. 36). Sigillaria was similar to Lepidodendron, but exhibits a different leaf scar pattern on its bark, did not tend to branch, and bore cones at the end of stems erupting from the trunk (Janssen, 1979, p. 54). Both lycopsids were fast growing, had trunks with soft inner tissues surrounded by a protective layer of bark. These trees probably had photosynthetic tissue in the bark, stems, and leaves. Calamites were sphenopsids (Sphenophyta), represented today by horsetails (Equisetum). Calamites grew up to 10 m in height. This tree form spread with rhizomes, grew a ribbed, segmented trunk adorned with needle-like whorled leaves. The whorled leaves are known as Annularia. Spores grew in sacs organized into cones. Psaronius was a tree-fern (Pterophyta), which grew to a height of 10 m. The trunk of the tree was composed of vascular tissue surrounded by a root mantle. Fronds adorned the top and reproduction was accomplished with spores. The fronds of Psaronius are known as Pecopteris. Medullosa was a seed fern (Pteridospermales) that grew as a shrub-like plant reaching heights of 3.5 m. Fern-like foliage bore seeds on the midribs and margins. The stems of these plants were made of many leaf bases. Neuropteris is the frond of a seed fern. Pteridosperms are actually early gymnosperms (Cleal & Thomas, 2009, p. 139). Another early gymnosperm, the cordaites (Cordaitales), possessed the wood, cones, pollen, and seeds of a conifer, but had wide, strap-like leaves. Cordaites were shrub-like plants (Kenrick and Davis, 2004, pp. 84-94). Primitive Walchian conifers also appear in the Carboniferous.

Coal Balls

A special type of fossil, the coal ball, can be found in the coal deposits of the Pennsylvanian and Permian periods. Coal balls are calcareous concretions that can disrupt the mining of bituminous coal bearing strata. Coal balls contain swamp vegetation, which has been permineralized with calcium carbonate, preserving 3-D cellular structure. Although the formation of the coal balls is not totally understood there is evidence for both marine incursions and ground water percolation as sources for the carbonate (Kenrick & Davis, 2004, p. 115). Coal balls are studied in serial section using the cellulose acetate peel method to reveal microscopic structure. Serial sections can be used to reconstruct organs and entire plants. The five major groups of plants found in coal balls include: Lycophytes, sphenopsids, ferns, seed ferns, and cordaiteans (Rothwell, 2002, p. 40). The in situ preservation of plant materials allows paleontologist to study plant associations that tell us something about the palaeoecology of the coal swamps. Coal balls reveal that the arborous fern Psaronius became the dominant canopy tree after the extinction of Lepidondendrales near the Middle Pennsylvanian. Certain species of small ferns and horsetails have been found, which grew in association with the roots of Psaronius (Rothwell, 2002, p 42). One may argue that coal balls represent a kind of lagerstatten, although they are found across multiple time periods.

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Bear Gulch

In central Montana the upper Mississippian Bear Gulch beds represent a Lagerstatten that preserves a Carboniferous bay ecosystem. Platy limestone lenses contain a diverse fossil assemblage representing paleocommunities in a shallow marine basin. Periodic turbidite sedimentation smothered and buried communities. Soft-tissues, phosphatic fossils, cartilaginous fossils, and molds can be found. Preservation of circulatory tissue, gut contents, skin and eye pigments, are just a few examples of the important soft-tissue finds in Bear Gulch.

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Algae, bacterial mats, and plankton represent the primary producers of Bear Gulch. The most abundant invertebrates are straight and coiled nautiloids, ammonoids, shrimp, and polychaete worms. Horseshoe crabs, gastropods, trilobites, asteroids, bryozoans, brachiopods, and branching sponges are also found. Bear Gulch contains one of the most diverse assemblages of fossil fish in the world. In 30 years, 108 fish species have been documented. Sharks, skates, platysonids, paleoniscids, dorypterids, tarrassiids, and coelacanths have been identified. Coelacanths are the most common, but chondrichthyes are the most diverse. Terrestrial plants such as lycopsid logs, leaves and other plant material that drifted into the bay can also be found.

The diversity and exceptional preservation found at Bear Gulch has allowed paleontologist to reconstruct life habits, feeding strategies, sexual dimorphism, trophic structures, and evolutionary history of some taxa (Hagadorn, 2002, p. 167)

Mazon Creek

Equisite examples of leaves, stems, cones, and seeds of Carboniferous plants along with animal life can be found in the Lagerstatten known as Mazon Creek, which is just 150 km southwest of Chicago, Illinois. Mazon Creek provides the best window into late Carboniferous shallow marine, freshwater, and terrestrial life (Selden and Nudds, 2004, p.60). The soft and hard parts of plants and animals are found in siderite (iron carbonate) concretions and can reveal minute structural details. Subtle pH changes created by the body of a buried organism caused available iron carbonate to precipitate. Thus, the organism became its own nucleation site for the formation of a siderite nodule. When these nodules are split open, the fossil appears as a 3-D external cast and mold. The concretions are small, never larger than 30 cm, thus for larger organisms only small parts are preserved. The siderite nodules are found in the lower layers of the Francis Creek Shale Member, which lies over the Colchester No 2 Coal Member; both included in the Carbondale Formation. Access to the fossils came from the tailings of coal pit mining.

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The different habitats represented by Mazon Creek flora and fauna were associated with a deltaic environment (Selden and Nudds, 2004, p. 66). The Colchester No 2 Coal Member represents a swamp forest composed of lycopsid and sphenopsid trees with an understory of seed ferns. Nodules in the Francis Creek Shale Member represent two biotas. Braidwood nodules represent freshwater and terrestrial environments. Essex nodules represent a shallow marine environment with material drifted in from a terrestrial environment (Selden and Nudds, 2004, p. 63). The bark, leaves, and reproductive structures of Lycopsids (clubmosses) and Sphenopsids (horsetails) are found. Foliage and seeds of seed ferns, cordaites, and gymnosperms are present. A list of some of the animal fauna includes: Cnidaria (jellyfish), Mollusca (chitons, bivalves, gastropods, and cephalopods), Crustacea (shrimp, banacles, and ostracods), Chelicerata (horseshoe crabs, eurypterids, scorpions, spiders, mites), Insecta (cockroaches, dragonfly-like, and grasshopper-like winged insects), Diplopoda (millipedes), Chilopoda (centipedes), Brachiopoda (Lingula), Echinodermata (sea cucumbers), Fish (jawless, cartilaginous, lobe-finned, and lungfish), Amphibia, and Reptilia.

The Illinois state fossil, Tullimonstrum gregarium, is also found in this location. Tully’s Monster has a segmented, sausage-shaped body with a proboscis ending in a claw and teeth. This organism may represent a type of shell-less gastropod predator (Sheldon and Nudds, 2004, p. 67).

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A Transition to Drying Conditions

Ecosystems of the Carboniferous did not experience extinction on a massive scale. As the Permian period unfolded dryer climatic conditions would become the norm. The drying trend would have an impact on which groups of organisms would increase or decrease in diversity.

 

It is interesting to note that the many forests that grew in the U.S. and Western Europe during this time would eventually transform energy from the Carboniferous sunshine into coal. The stored sunshine in this coal would allow humans to power the industrial revolution. Even today, over a third of our electricity is powered by this fossil fuel.