Mazon Creek

Pecopteris, a leaf type which is not found after the early Permian.

Plants of the Carboniferous swamps.

Lepidodendron was a common plant at Mazon Creek. This is just a branch; it grew to be a large tree.

Mazon Creek is a lagerstätte in northeast Illinois. It is a site of exceptional fossil preservation from the Pennsylvanian sub-period of the Carboniferous, about 309 million years ago.

The area was declared a National Historic Landmark in 1997.

The site's greatest find is the oldest-known beetle.

Palaeoecology

The sediments were laid in a gigantic tropical estuary, a river delta with shifting channels and mud and sand bars. Much of the continent was then covered by a great internal epeiric sea, which sometimes swept over the area, and then withdrew.

Coal measures are formed like this. A low-lying tropical swamp or rain forest forms a peat swamp which is short of oxygen. Dead plant material builds up, and does not decompose entirely. The plant material gets buried as a peat bog. From time to time rivers bring down a load of sediment from nearby mountains, covering the swamp. Eventually layers of coal alternate with layers of sandy deposit. Pressure turns it all into hard rock. Later on, the rocks are uncovered by erosion, and fossils can be found by humans.

The huge growth of vegetation at this time produced coal measures, and the fossils were found in concretions (hard balls) in the spoil heaps of coal mines. The concretions are caused by iron carbonate, FeCO₃, which makes the parcel of mud and organic material into hard balls.

Flora and fauna

Fossil specimens have been found which belong to these groups of plants and animals:

• 140 species of insects and other arthropods.
• Freshwater aquatic faunas include fish, amphibia, bivalves, crustacea & horseshoe crabs.
• Fauna from brackish water include ostracodes, lungfish scales, xenacanth sharks, polychaetes, crustacea, cephalopods with soft tissues, hydrozoa, siphonophores, chitons, sea cucumbers, eurypterids, echinoderms.
• One unique species cannot be placed in a taxon: the 'Tully Monster', Tullimonstrum gregarium, a soft-bodied marine invertebrate. It is the Illinois state fossil.

• 400 species of plants: Lepidodendron, horsetails, ferns, pteridosperms and primitive conifers and ginkgoes.

Mazon Creek Media

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  Tullimonstrum gregarium, an enigmatic soft bodied bilaterian animal known only from the Mazon Creek deposits.

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  Bandringa rayi fossil shark, Mazon Creek Lagerstatte. This is the holotype specimen of the species. Its size and lack of developed cartilaginous skeletal structures indicate that this is a juvenile.