| Mazon Creek fossil beds | |
|---|---|
| Stratigraphic range: ~309–307 Ma[1] |
Mazon Creek Fossil Beds | |
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| Nearest city | Benson Road,Morris, Illinois |
|---|---|
| NRHP reference No. | 97001272 |
| Significant dates | |
| Added to NRHP | September 25, 1997[2] |
| Designated NHL | September 25, 1997[3] |
TheMazon Creek fossil beds are a conservationlagerstätte found nearMorris, inGrundy County, Illinois. The fossils are preserved inironstoneconcretions, formed approximately309 million years ago in the mid-Pennsylvanian epoch of theCarboniferous period. These concretions frequently preserve both hard and softtissues of animal and plant materials, as well as many soft-bodied organisms that do not normally fossilize. The quality, quantity and diversity of fossils in the area, known since the mid-nineteenth century, make the Mazon Creeklagerstätte important topaleontologists attempting to reconstruct thepaleoecology of the sites.[4] The locality was declared aNational Historic Landmark in 1997.[3][5]
The Mazon Creek fossils are found in the Upper CarboniferousFrancis Creek Shale.[6] Thetype locality is theMazon River (or Mazon Creek), a tributary of theIllinois River nearMorris,Grundy County, Illinois. The 25 to 30 meters ofshale were formed approximately309 million years ago, during thePennsylvanian period. The fossiliferousconcretions are usually found within the thickest deposits of the Francis Creek Shale. The concretions occur in localized deposits within the silty to sandymudstones, in the lower four metres of the formation. The paleoecosystem was once believed to be a largeriver delta system, deposited by at least one majorriver system flowing from the northeast, but it has been reinterpreted as a bay into which some rivers brought some freshwater.[7] The sediments, which show tidal cycles,[8] are believed to derive from theAppalachian orogeny events. The bay had atropical climate, a result of the area being within 10° northlatitude of theequator during thePennsylvanian.
The remains of plants and animals were rapidly buried by thesediment deposited in the deltaic system. Bacterialdecomposition of the remains producedcarbon dioxide that combined with dissolved iron from thegroundwater. This process formedsiderite in the sediments surrounding the remains, forming detailed casts of their structure.Lithification of the sediments formed protective nodules of ironstone around the now fossilized remains. This mode of preservation is known asauthigenic mineralisation.
The fossiliferous concretions are found in the Mazon River area ofGrundy,Will,Kankakee, andLivingston counties. Additional fossils are found inLaSalle County, Illinois; between theVermilion River andMarseilles, Illinois. The ironstone concretions are recovered from exposures along streams, roadcuts and in active or abandonedcoal mine areas.
Most concretions are found among the shale and sandstone that was piled up in either aspoil tip of an older underground mine, or ridges of the latersurface mining. ThePeabody Energy company operated several large strip mines in Grundy County that are now abandoned. They were named Pit 1 through Pit 11. The northern pits 1-8 are known for theBraidwood Biota with mostly terrestrial fauna and flora. Pit 11, which was located southwest of the town of Braidwood, Illinois, is known for itsEssex Biota with a greater abundance of marine species.[9] Pit 11 is nowBraidwood State Fish and Wildlife Area, anIllinois state park. Fossil collecting is allowed at the park with a permit.[10]
The site's importance was realized in the mid-nineteenth century: "the nodules of Mazon Creek, where fragments of plants, even of the softest texture, have been preserved in their integrity".[11]
The Mazon Creek flora comprises over 400 species from at least 130 genera. However, the true number of species is difficult to determine.Paleobotanists name separate plant structures with different names by convention, inflating the number of fossil plant taxa. Paleobotanists are currently determining which taxa are valid.
Mazon Creek flora includes:lycopsids, related to modern club moss, with arborescent forms namedLepidophloios,Sigillaria andLepidodendron, and herbaceous forms calledLycopodites andCormophyton;sphenopsids likeCalamites a tree-likehorsetail relative, with common foliage names ofAnnularia andAsterophyllites, and a vine-like form calledSphenophyllum; Pteridophyta as marattitalean tree ferns and Filicales and Zygopteridales understory ferns, with common foliage names ofPecopteris,Acitheca andLobatopteris;pteridosperms, also known as seed ferns, an extinct group of plants that grew both as trees and smaller shrubs, with features like pinnated leaves similar to true ferns, but reproduced by seeds instead of spores; they had common foliage namesMariopteris,Alethopteris,Odontopteris,Neuropteris,Laveineopteris andMacroneuropteris; extinctgymnospermCordaites, believed to be closely related to and sharing many features with modernconifers.
The Mazon Creek fauna has over 320 species of animals that have been identified. The fauna has traditionally been divided into two components: themarineEssex fauna and the land and purportedlyfreshwaterBraidwood fauna, that were washed into the deltaic sediments. However, as for many otherPaleozoic fossiliferous localities, the degree of marine influence seems to have been under-estimated,[12][13] and the latest studies questioned the presence of a trulyfreshwater fauna at Mazon Creek and interpreted it instead as deposits from a bay into which some rivers brought some fresh water; no unequivocally freshwater taxon is known there.[7] Thus, the Essex and Braidwood biotas simply represent fossil assemblages preserved far (Essex) or close (Braidwood) to the coast and deltas. Many of the fossils of different plants and animals from the carboniferous that have been found there are on display at theIllinois State Museum inSpringfield in their changes dynamic Illinois environments exhibit, and at theField Museum of Natural History inChicago in their evolving planet exhibit.
The Essex fauna includes the invertebrates such as a species ofjellyfish calledAnthracomedusa turnbulli,[14] seaworms (Nemertea,Priapulida,Chaetognatha,Annelida),snails, saltwaterscallops, such asAviculopecten mazonensis,crustaceans, such asKallidecthes richardsoni,Belotelson,Tyrannophontes, andCyclidas such asAmericlus americanus, andApionicon,sea scorpions, such asAdelophthalmus mazonensis,chaetognaths,Etacystis communis,Escumasia roryi,cephalopods such asJeletzkya douglassae andPohlsepia mazonensis, two of the three species ofHorseshoe crab found the area known asLiomesaspis laevi, and the more rarePaleolimulus mazonensis,Reticulomedusa grennei, the possiblehemichordateEtacystis,[6] and the most common species found there, theEssexella asherae sea anemone. It consists of 42% of all fossils finds in the Essex biota.[9] The Essex invertebrate fauna also includes the most famous faunal member of the Illinois state fossilTullimonstrum, known popularly as the "Tully Monster". Essex fish that have been found includeactinopterygians such asPlatysomus circularis, some finnedsarcopterygians,Agnathas such asGlipichthys greenei,Myxinikela siroka,Mayomyzon pieckoensis, andpipiscius zangerli, andchondricthyans such asdabasacanthus inskasi, and a species of shark calledBandringa rayi which sort of resembles a modernPaddlefish, aSawfish, or agoblin shark that when the area was still a shallowbrackish waterEstuary, used it as a breeding ground to have their babies.
The Braidwood fauna includes land invertebrates such asinsects such as the extinctroachoidPlatymylacris paucinervis,Gerarus which has several species such asgeratus vetus, andgerarus mazonus that are both found in the mazon area, the possible beetleAdiphlebia, a winged insect calledProtodictyon pulchripenne, and the biggest insect found at the creek theMazothairos enormis. Other invertebrate fauna types that have been found includemyriapods such asMazoscolopendra richarsoni, and the giantArthropleura cristata.Arachnids that have been found include thephalangiotarbus laceoi,Arthrolycosa antiqua,Geralinura carbonaria, and the newly discoveredDouglassarachne acanthopoda. A rare freshwater lobopodian (formerly mistaken for afireworm) calledPalaeocampa anthrax is also found in the Braidwood fauna.[15] Vertebrate fossils include TheEmbolomeri calledSpondylerpeton spinatum which sort of resembled aCrocodile and was probably the top predator of the area at the time,temnospondyls such asPlatyrhinops lyelli,[16]lepospondyls such asPseudophlegethontia turnbullorum,Infernovenator steenae, andNagini mazonense, and aMicrosauria calledDiabloroter bolti.[17] Fresh water/brackish water creatures include actinopterygians (Illiniichthys[18]),shrimps,ostracods, and the third species ofhorseshoe crab known from the area calledEuproops danae, which was common and probably lived in both saltwater and brackish water. A dubious species ofAcanthodes namedA. beecheri, an unknown species ofOrthacanthus, preserved tetrapod larva fossils,Mazonova, andHelenodora inopinata have also been described from the Braidwood fauna.[19][20]
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41°19′16″N88°20′46″W / 41.321°N 88.346°W /41.321; -88.346
