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Carboniferous Introduction

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.

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).

Marine Invertebrates

Corals that flourish are not the reef-building varieties. Stony bryozoans decrease, while lacey forms evolve and diversify. Brachiopods once again become abundant. Nautiloids with coiled shells become more abundant than their straight-shelled relatives. Other mollusks, such as ammonoids, bivalves, and gastropods also recover and diversify. Trilobites become increasingly rare. Echinoderms become very successful during the early Carboniferous, especially the crinoids, which form vast “ocean meadows” on the sea floor (Kazley, 2002, Carboniferous page). Delocrinus missouriensis is a crinoid from the Pennsylvanian period, which was chosen as Missouri's state fossil. The tabulate coral Lithostrotionella from the Mississippian is West Virginia's state gem. Lithostotionella specimens from West Virginia are permineralized with chalcedony. Impurities in the chalcedony give these gem quality fossils blue, grey, pink, and red hues.


Jawless fish become less numerous, with some groups becoming extinct. While the class of spiny sharks (Acanthodii) would survive, the armored Placoderms (Placodermi) would meet with extinction. Many sarcopterygians (Lobe-finned fish and lungfishes) would go extinct, making the subclass Sarcopterygii less diverse and less common. Primitive ray-finned fish made a recovery and diversified. Cartilaginous fish flourish during the Carboniferous undergoing an adaptive radiation that included the appearance of an entire new group, the holocephalians or chimaeras (subclass holocephali) (Dixon, 1988, pp. 28-29).


Traditionally, the term amphibian has been used to refer to all tetrapods that are not amniotes (reptiles, birds, and mammals). However, it is now clear that this is a paraphyletic term. The class Amphibia now refers to present-day amphibians and their extinct sister groups. Amphibians underwent an adaptive radiation during the early Carboniferous. The Carboniferous and Permian are often referred to as the age of the amphibians. Two extinct groups of amphibians make their first appearance during the Carobniferous.

Temnospondyls (Order Temnospondyli) are primitive amphibians that dominated the terrestrial and freshwater habitats of the Carboniferous. Temnospondyli, Ichthyostegalia, and Anthracosauria use to be grouped together as Labyrinthodonts. Labyrinthodtia is now an obsolete term. In general, temnospondyls had long bodies, large flat skulls, and short legs. Dendrerpeton from the Upper Carboniferous of Novia Scotia, Canada is found associated with the hollows of lycopod trees.

Lepospondyls (Superorder Lepospondyli) are early amphibians that range from the Carboniferous to the Permian. Microsaurs and nectrideans are the best-known lepospondyls. Microsaurs (Order Microsauria) were the largest group of lepospondyls and had a body form reminiscent of salamanders or lizards. Most microsaurs were terrestrial feeding on arthropods although, some became secondarily adapted to aquatic environments. The nectrideans (Order Nectridia) were aquatic organisms that had newt-like bodies with long tails. Their heads were equipped with horn-like structures that grew as the animal aged. Diplocaulus is a well-known nectridean with a “boomerang” shaped skull. Biomechanical studies on models of a Diplocaulus head provide evidence that it acted as a hydrofoil, providing lift. Diplocaulus swam in streams and lakes feeding on fish (Benson, 2005, pp. 89-90).

Modern amphibians are never free of the aquatic stage. Amphibians lay their eggs in water. As larvae, just like fish, they possess gills, a lateral line, tail, and a single loop circulatory system with a two-chambered heart. Limbs, lungs (for most), a tympanic membrane, and a double loop circulatory system with a three-chambered heart are developed through the process of metamorphosis. Many adult amphibians supplement breathing with their skin and mouth. Fossil amphibians are found with traces of gill structures and canals for lateral lines. Fossils amphibians representing different stages of metamorphosis have also been found (Dixon, 1988, p. 46).

Reptile-Like Amphibians

The superorder Reptiliomorpha includes reptile-like amphibians that range from the Early Carboniferous to the Early Triassic as well as the amniotes that evolved from them. The classification of tetrapods that have both reptile and amphibian-like characteristics continues to be debated. The line between amphibians and amniotes is blurred among advanced reptiliomorphs. However, many paleontologists regard some reptile-like amphibian groups as tetrapod evolutionary lines that branched long after the amniotes split off (Prothero, 2004, p. 378). Representatives of the reptiliomorph order Anthracosauria make their first appearance in the Early Carboniferous. These reptile-like amphibians were fish eaters adapted to both terrestrial and aquatic habitats. Proterogyrinus from the Lower Carboniferous of West Virginia, USA and Scotland was a 1 meter long anthracosaur. Proterogyrinus had legs well adapted for land and a flattened tail good for swimming. Some anthracosaurs became secondarily adapted for life in the water (Benton, 2005, p. 95). Sometime in the Late Devonian or Early Carboniferous reptile-like amphibians gave rise to amniotes that evolved a fully terrestrial life cycle.

Reptiles were the first amniotes to conquer land. The evolution of the egg would allow reptiles to free themselves from the aquatic larval stage of the amphibians. A watertight egg along with protective scales and a rib cage that enhances breathing would help reptiles better exploit dry terrestrial environments. This would give them an advantage during the Permian period.

Reptiles & The Amniotic Egg

Reptiles and the amniotic egg make their first appearance in the late Carboniferous. The amniotic egg is a water tight, independent life support unit that affords the developing embryo protection, nutrients, and waste disposal. A protective leathery shell and four membranes (amnion, yolk sac, allantois, and chorion) help to define this evolutionary innovation. The embryo is suspended in amniotic fluid contained within the amnion. A yolk sac attached to the embryo’s gut acts as a source of nutrients. Waste products are excreted through the allantois and stored in the allantoic cavity. The chorion lines the inside of the shell and allows oxygen to enter the egg. The amniotic egg can be laid on land and frees amniotes from the aquatic larval, tadpole stage, a great adaptation for dry conditions.

The interrelationships between amniotes and their descendents can be studied by examining four basic skull structures. The anapsid type (subclass Anapsida) includes the earliest reptiles and modern turtles; the diapsid type (subclass Diapsida), which includes lizards, snakes, crocodiles, dinosaurs, sphenodonts and the extinct marine reptiles (formerly grouped as the subclass Euryapsid); and the synapsid type (class Synapsida and class Mammalia), which encompasses protomammals (formerly referred to as mammal-like reptiles) and mammals (Johnson and Stucky, 1995, pp 55-56 and Dixon, 1988, p. 61). The subclass Anapsida and Diapsida are in the reptilian class Sauropsida. Synapsids include all mammals (class Mammalia) and protomammals (Class Synapsida). Sauropsids (reptiles) and Synapsids (protomammals and mammals) make up the two major groups of amniotes.

The first reptiles (class Sauropsida) were small lizard-sized tetrapods living in damp forests of the mid-Carboniferous feeding on insects and worms. In Nova Scotia there are sedimentary deposits that contain upright lycopod tree stumps. Since 1852 thirty Sigillaria tree stumps have produced abundant tetrapod remains. These tetrapods were living in the rotted out Sigillaria trunks. Hylonomus and Paleothyris are two tetrapods that were preserved in the hollow tree stumps and represent the oldest known amniotes (Benton, 2005, pp 110-111). Hylonomus had an anapsid type skull, which is solid and box-like with no temporal openings. These small insectivores could not open their mouth very wide or close it with much force. Temporal openings allow the jaw muscles to be longer and larger.

Reptiles with diapsid skulls also appear in the late Carboniferous. These reptiles have skulls with two pairs of openings on either side of the skull behind the eyes. Initially, these holes reduced the weight of the skull. Later muscles and ligaments would stretch across these holes, resulting in a mouth that could open wider and close with greater force (Dixon, 1988, p. 84). The ealiest diapsid was the lizard-like Petrolacosaurus. The jaws and sharp, pointed teeth of Petrolacosaurus are similar to Hylonomus.


Mammal-like reptiles or protomammals (class Synapsida) with the synapsid type skull also make their first appearance in the late Carboniferous. The synapsid skull has a single large opening behind each eye socket. This opening allows the jaw muscles to be larger and longer, resulting in a wider, more powerful bite. The pelycosaurs (order Pelycosauria) were the first protomammals to evolve. Archaeothyris was a small lizard-like pelycosuar. Archaeothyris was an insectivore with different sizes of sharp, pointed teeth. Synapsids were the first to evolve heterodont dentition (specialized tooth shapes).

Vertebrate herbivory gets its start in the late Carboniferous. Edaphosaurus was a primitive herbivorous pelycosaur, which had a sail similar to Dimetrodon. Edaphosaurus had teeth specialized for chopping up plant material. Herbivory represents an important evolutionary innovation in digestion, as it requires hosting a community of bacteria within the gut that can help chemically process the cellulose. Edaphosaurus would evolve into different species during the Permian. Prior to organisms like Edaphosaurus all vertebrates were carnivores and detritivores.

Invertebrates on Land

Spiders and scorpions undergo an adaptive radiation during the Carboniferous (Rich, 1996, p. 228). Millipedes become abundant and also diversify. Insects with wings (Pterygota) make their first appearance during the Carboniferous (Carpenter & Burnham, 1985, p 298). The fact that 99% of living insects belong to the subclass Pterygota is a testament to the success of insect flight. There is evidence to suggest that insects with fixed wings (Paleoptera), like mayflies, came before those with folded wings (Neoptera), like cockroaches. Folded wings allow insects to hide and hunt in small spaces. Over geologic time the percentage of insects with folded wings would increase. Today insects with fixed-wings (Paleoptera) make up less than 1% of insect species (Carpenter & Burnham, 1985, p. 299).

Insects with wings (Neoptera) can be subdivided into two informal groups. Exopterygota undergo incomplete metamorphosis (egg, nymph, and adult), while endopterygota undergo complete metamorphosis (egg, larva, pupa, adult). Insects with complete metamorphosis do not make their first appearance until the Permian, although paleontologists speculate they evolved sometime during the Carboniferous period. Many orders of winged insects (Pterygota) make their first appearance during the Carboniferous. Over half of these insect orders would go extinct. We will mention just a few. Among fixed-winged insects the mayflies (Ephemeroptera) and dragonfly-like insects (Protodonata) appear. Among insects with folded wings and incomplete metamorphosis (exopterygota Neoptera) cockroaches (Blattodea) make their first appearance (Carpenter & Burnham, 1985, p.302).

Evidence for herbivory in insects appears in the Carboniferous. Like vertebrates, the first insects were carnivores and detritivores. Herbivory requires hosting cellulose-digesting bacteria through a symbiotic relationship within the gut. The oldest examples of marginal and surface feeding is on Carboniferous seed fern leaves of Neuropteris and Glosspteris (Grimaldi & Engel, 2005, p. 52). It is estimated that only 4% of the leaves in Carboniferous deposits exhibit damage from feeding. Herbivores do not make a significant impact on plant life until the Permian (Kenrick & Davis, 2004, pp. 166-167).

Galls are excessive growths on stems, leaves, cones, and flowers caused by insect feeding or egg laying. The earliest fossil galls are found on the petioles of Psaronius tree ferns of the Late Carboniferous. Insect gall fossil diversity and abundance takes off with the advent of flowering plant evolution in the Cretaceous. Insects produce tunnels in wood known as borings or galleries. Some insects eat the cambial layer while others eat fungus that grows within the galleries, still others eat the wood itself. The oldest borings and galleries in wood, attributed to mites, are known from the Carboniferous (Grimaldi & Engel, 2005, pp. 53 & 54).

We think of insects as small in size, but during the Carboniferous insects and some other arthropods attained spectacular sizes. Coal deposits in France have produced dragonfly-like insects with wingspans of 2 feet. A 14-inch fossil spider was discovered in Argentina. A 6 ft long, 1 ft wide millipede was found in Nova Scotia (Johnson and Stucky, 1995, p. 58). Giant insects probably indicate higher oxygen levels in the atmosphere and fewer predators.


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.

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.

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.

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).

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.


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