Name: Mazon Creek.
Location: Illinois.
Age: Pennsylvanian, Carboniferous.
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Fauna & Flora

The Mazon Creek concretions preserve a wide variety of animal and plant fossils, reflecting the complexity of terrestrial and near-shore marine habitats [4]. The Braidwood biota comprises a diverse selection of non-marine flora and faunas that inhabited terrestrial and freshwater habitats within the delta complex [2, 3]. The Essex fauna is equally diverse and typical of estuarine marine organisms, suggesting that the Mazon Creek delta prograded out into a large marine area, although conditions were not suitable within the river-influenced estuary for most marine shelf taxa [2, 3].


Faunas

Braidwood Biota:

The Braidwood fauna of the Francis Creek Shale includes over 140 species of insect (Figure 2), millipedes (Figure 3), centipedes, scorpions and spiders (Figure 4) [1]. Freshwater aquatic faunas include fishes, amphibians, bivalves such as mussels and clams, shrimp-like crustaceans (Figure 5), xiphosurans (horseshoe crabs; Figure 6), and arthropods (Figure 7) [1, 7, 8]. Nearshore Braidwood aquatic habitats may also have been slightly brackish as indicated by ostracodes, lungfish scales and xenacanth sharks [3].

Figure 2A. Heterologus langfordorum
Coin = 19 mm
Figure 2B. Thesoneura americana 
Coin = 19 mm
An insect wing from an extinct group of large insects, Protorthoptera. The group is most closely related to modern cockroaches, mantids, grasshoppers and walking sticks. (Both part and counterpart are shown; courtesy Illinois State Museum.)
An insect wing from an extinct group of large insects, Palaeodictyoptera. The group is most closely related to modern mayflies and dragonflies. (Courtesy Illinois State Museum.)

Figure 3. Euphoberia armigera
Coin = 19 mm
A Pennsylvanian myriapod, which would have occupied terrestrial habitats and is closely related to modern millipedes. (Both part and counterpart are shown; courtesy of Illinois State Museum).

Figure 4A. Orthotarbus robustus
Coin = 19 mm
Figure 4B. Pleophrynus
Coin = 19 mm
A member of an extinct group of terrestrial arachnids called architarbids. (Courtesy Illinois State Museum.)
A member of an extinct group of terrestrial arachnids called trigonotarbids. (Courtesy Illinois State Museum.)

Figure 5. Cyzicus sp.
Coin = 19 mm
Figure 6. Euproops danae
Coin = 19 mm
Conchostracans or clam shrimps, small freshwater crustaceans related to water fleas. Modern clam shrimps live in small temporary alkaline pools such as those on floodplains. (Courtesy Illinois State Museum.)
A small xiphosuran, holotype of Euproops thompsoni, believed to have lived in both terrestrial and freshwater habitats and closely related to modern horseshoe crabs. (Courtesy Illinois State Museum.)

Figure 7A. Acanthotelson stimpsoni
Coin = 19 mm
Figure 7B. Palaeocaris typus
Coin = 19 mm
Figure 7C. Anthracaris gracilis
Coin = 19 mm
Small shrimp-like aquatic arthropods. Acanthotelson stimpsoni and Palaeocaris typus belonged to a group called the syncarids and Anthracaris gracilis belonged to a group called the eocarids. Their modern relatives include isopods and amphipods. (Courtesy Illinois State Museum.)

Essex Fauna:

Marginal marine and estuarine organisms of the Essex fauna include various polychaetes and crustaceans, cephalopds with soft tissues, medusae (Figure 8), hydrozoans, siphonophores, chitons, holothurians, xiphosurans (horseshoe crabs), eurypterids ('sea scorpions'; Figure 9), echinoderms, numerous fish species including sharks, and problematical taxa such as the Tully monster (Tullimonstrum gregarium; Figure 10) [1].

Figure 8. Essexella asherae
Coin = 19 mm
Figure 9. Lepidoderma mazonense
Coin = 19 mm
A scyphomedusan jellyfish manubrium (bell). Similar to modern jellyfish, this organism would have captured and eaten smaller animals using stinging cells (nematocysts). (Courtesy Illinois State Museum.)
A eurypterid. (Courtesy of Illinois State Museum.)

The Tully Monster:

The Tully Monster, Tullimonstrum gregarium, is a soft-bodied marine invertebrate, often found preserved as a compressed outline (Figure 10) [8]. It had an elongated segmented body, round or oval in cross section and tapered at both ends [8, 11]. The animal had eyes on stalks, which protruded sideways from the body, a long proboscis with a jaw at the end containing eight minute, sharp teeth, and a tail with two horizontal fins and a dorsal fin, all of which were triangular [8, 11]. It is believed that the proboscis did not contain the throat and may have been a muscular organ used to pass food to the mouth [8]. With its streamlined and flexible body, the Tully monster may have been an active swimming carnivore that may have preyed upon other marine animals such as jellyfish and shrimps [8, 11].

Tullimonstrum gregarium is the official state fossil of Illinois. It does not appear to be closely related to any known living or extinct animal and has been placed in its own phylum, although it may be related to snails and other molluscs [8]. Read more about the Tully Monster at the Illinois State Geological Survey's web site.

Figure 10. Tullimonstrum gregarium
A soft-bodied active swimming marine invertebrate. (Courtesy Illinois State Museum.)


Floras

The Mazon Creek flora is extremely diverse, 400 species from at least 30 different genera have been described, and all are characterised by excellent preservation [8]. Many of these plant specimens are component organs, for example bark, leaves, stems and seeds belonging to common groups such as the extinct and extant varieties of pteridophytes and the extinct spenophytes [7]. Spore-bearing pteridopsidan ferns, the fern-like seed bearing ferns, the pteridosperms and the primitive ancestors of conifers and ginkgoes are also present [7, 8].

Pteridophytes:

Lycopsids such as the extinct Lepidodendron were particulary profuse during the Carboniferous (Figure 2). They were arborescent (tree-like) plants (Figure 11), estimated to have grown to a height of 54 metres and an important component of worldwide Carboniferous coal swamps [7, 8].

Figure 11A. Lepidodendron sp.
Coin = 19 mm
Figure 11B. Lepidophyllum sp. (Lepidophylloides sp.)
Coin = 19 mm
Figure 11C. Lepidostrobus sp.
Coin = 19 mm
Figure 11D. Lepidostrobophyllum sp.
Coin = 19 mm
The bark of the "Lepidodendron tree". The diamond-shaped pattern represents the individual basal leaf scars from leaves that were shed.
The leaf of the "Lepidodendron tree". Although this is a small leaf specimen, some Lepidophyllum reached over a metre in length.
The cone of the "Lepidodendron tree". The cones produced spores and megaspores and may have exceeded 30 cm in length. The megaspores are Lepidocarpon.
The cone scale (bract) from a Lepidostrobus. One Lepidostrobus is composed of numerous individual Lepidostrobophyllum.

Sphenophytes:

Sphenopsids or horsetails were also widely distributed in the swampy wet habitats of the Carboniferous. These were arborescent or tree-like and may have grown to a height of 10 metres [7, 8]. Horsetail plant components include Calamites sp. (Figure 12), Asterophyllites equisetiformis (Figure 12), Spenophyllum sp. (Figure 13), and Equisetites sp. (Figure 14). Scouring rushes such as Equisetum sp. are modern representatives of this group [8].

Figure. 12A. Calamites sp.

Coin = 19 mm
Figure 12B. Asterophyllites equisetiformis

Coin = 19 mm
The pith cast of the stem of an extinct horsetail. (Courtesy Illinois State Museum.)
The leaf whorls of an extinct horsetail that would have grown on plants with stems similar to Calamites sp. (Courtesy Illinois State Museum.)

Figure 13. Spenophyllum sp.
Coin = 19 mm
Figure 14. Equisetites sp.
Coin = 19 mm
The leaf whorls of an extinct horsetail. (Courtesy Illinois State Museum.)
The partially fused leaves of a horsetail. (Courtesy Illinois State Museum.)

Ferns:

Ferns were also very common during the Carboniferous. Pteridopsids or tree ferns bore spores on their leaves as they do today [8]. Psaronius (Figure 15) was a very common tree fern during the Pennsylvanian, and its increasing abundance in the Late Palaeozoic may have been caused by changing climates and an increase in drier habitats. It may have resembled a modern palm tree and reached a height of 10 metres [8].

Figure 15A. Pecopteris sp.
Coin = 19 mm
Figure. 15B. Asterotheca sp.
Coin = 19 mm
The foliage of a tree fern, Pecopteris, grew on the tree fern Psaronius. (Courtesy Illinois State Museum.)
The fertile fronds of a fern. (Courtesy Illinois State Museum.)

The seed ferns or Pteridospermales (Figure 16) became extinct after the Carboniferous [7, 8]. Although the pteridospermale foliage was very similar in appearance to modern-day ferns, these ferns reproduced by means of seeds, instead of by spores, as in modern ferns [8].

Figure 16A. Alethopteris sp.
Coin = 19 mm
Figure 16B. Diplothmema sp.
Coin = 19 mm
The foliage of a seed fern that may have been similar to a small tree and have grown to a height of 4 m. The seeds were up to 4 cm in length. (Courtesy Illinois State Museum.)
The foliage of a seed fern. (Courtesy Illinois State Museum.)


Palaeoecology

The location of Illinois during the Pennsylvanian and the abundance of lush tropical vegetation surrounding a deltaic margin suggest that Mazon Creek was a swampy habitat that prograded out into an estuarine environment. Cyclic repetitions within the mud and siltstone laminae associated with the Essex fauna are suggestive of sequential flood- and ebb-tide events [2, 3]. There is an abrupt boundary between the Essex faunas and the Braidwood biotas, indicating a change in ecological habitat and salinity [1]. Southwestward (seaward) current flow may have transported non-marine Braidwood biotas from upstream localities into shallow bay areas of the Essex estuary [1, 2].