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Fossil specimen of Tullimonstrum gregarium, perhaps the most infamous, and well known organism from the Mazon Creek Fossil Beds.

This is a list of the fauna of the Mazon Creek Fossil Beds, a Carboniferous lagerstätte located in Grundy County, Illinois. The site is famous for its exceptional preservation of soft bodied fossils inside of ironstone concretions, and was one of the first fossil sites discovered to preserve soft bodied fossils.

The Mazon Creek Fossil Beds are around 309-307 million years old, being approximately Moscovian in age. During the Carboniferous, this area was a lush, tropical delta system, about 10° north latitude of the equator. This ecosystem was formed by at least one river system that flowed in from the northeastern united states, and met the ocean to form an Estuarine environment. This large amount of habitat led to the creation of a very diverse ecosystem, which has been divided into two main sections, the marine Essex fauna, and the terrestrial/freshwater Braidwood fauna. Since the site's discovery in the early 20th century, more than 700 species of flora and fauna have been discovered, some of which have only been found at this site. Because of the diverse habitat types, many different groups, such as arthropods, mollusks, hemichordates, cnidarians, chondrichthyes, actinopterygii, and various other fish groups are represented.

Fauna

Cnidaria

A number of Cnidarians are known from Mazon Creek, and show the diversity of taxa in the ecosystem.

Cnidaria
Genus	Class	Higher taxon	Species	Notes	Images
Essexella	Cnidaria	Actiniaria	E. asherae	One of the most abundant taxa from Mazon Creek, Originally described as a scyphozoan in 1979, but was later re-described as an actiniarian in 2023
Reticulomedusa	Cnidaria	Incertae sedis	R. greenei	Another cnidarian once described as a scyphozoan, but most likely represents a synonym of Essexella
Anthracomedusa	Cnidaria	Cubozoa	A. turnbulli	A jellyfish, though not a member of the scyphozoa (true jellyfish) but instead a cubozoan (box jellyfish)
Octomedusa	Cnidaria	Coronatae?	O. pieckorum	A bizarre scyphozoan, originally considered to be a basal coronatan (crown jellyfish), however this has been questioned by later studies
Drevotella	Cnidaria	Hydroidolina	D. proteana	A cnidarian, and a member of the hydroidolina within the hydrozoa
Mazohydra	Cnidaria	Hydroidolina	M. megabertha	Another hydrozoan, and one of the first fossil taxa from the class to be discovered

Annelida

A number of annelid worms are known from Mazon Creek.

Annelida
Genus	Class	Higher taxon	Species	Notes	Images
Coprinoscolex	Annelida	Echiura	C. ellogimus	Originally considered to be the oldest definitive echiuran (spoon worms), before the discovery of taxa from Castle Bank
Rhaphidiophorus	Annelida	Amphinomida	R. hystrix	A member of the amphinomida, and one of the earliest known members of the group, nicknamed the 'Oliver Hardy Worm'
Esconites	Annelida	Eunicida	E. zelus	A member of the eunicida, showing the distinctive mouthparts, of the group, and reaches around 14 cm.
Mazopherusa	Annelida	Terebellida	M. prinosi	A member of the flabelligeridae family (fan worms), and one of the few known fossil taxa from the family
Didontogaster	Annelida	Phyllodocida	D. cordylina	A phyllodocid often nicknamed the 'Tummy Tooth Worm', named after the swollen section of its body containing the worms mouthparts
Levisettius	Annelida	Phyllodocida	L. campylonectus	A phyllodocid often nicknamed the 'Riccardo Worm'
Astreptoscolex	Annelida	Phyllodocida	A. anasillosus	A phyllodocid often nicknamed the 'Plain Worm'
Rutellifrons	Annelida	Phyllodocida	R. wolfforum	A phyllodocid often nicknamed the 'Shovel Nose Worm'
Pieckonia	Annelida	Phyllodocida	P. helenae	A phyllodocid often nicknamed the 'Fish Worm'
Fossundecima	Annelida	Phyllodocida	F. konecniorum	A phyllodocid often nicknamed the 'Simple Jaw Worm', one of the most common worms of the Essex Fauna
Dryptoscolex	Annelida	Phyllodocida	D. matthiesae	A phyllodocid often nicknamed the 'Rat Worm'
Hystriciola	Annelida	Phyllodocida	H. delicatula	A phyllodocid often nicknamed the 'Baby Tooth Worm'
Fastuoscolex	Annelida	Phyllodocida	F. gemmatus	A phyllodocid often nicknamed the 'Papillae Worm'

Cephalopoda

A number of cephalopods are known from Mazon Creek, including shelled taxa, and basal coleoids.

Cephalopoda
Genus	Class	Higher taxon	Species	Notes	Images
Jeletzkya	Cephalopod	Belemnoidea	J. douglassae	A genus of basal coleoid within the belemnoidea, however it is too poorly understood for a more specific placement, known from arms and radulas

Non-Arthropod Panarthropoda

A small number of non-arthropod panarthropods (notably onychophorans and similar taxa) are known from Mazon Creek.

Non-Arthropod Panarthropoda
Genus	Class	Higher taxon	Species	Notes	Images
Carbotubulus	Panarthropoda	Hallucigeniidae?	C. waloszeki	The youngest known non-onychophoran lobopodian in the fossil record, originally assigned to Hallucigeniidae, however this has been questioned by later studies.
Helenodora	Panarthropoda	Onychophora (stem-group)	H. inopinata	One of the oldest known onychophorans (velvet worms), originally considered a synonym of the contemporary Ilyodes.
Ilyodes	Panarthropoda	Incertae sedis	I. divisa I. elongata	Originally named as a myriapod in 1890, however, it may represent a nomen dubium, due to neither species being properly diagnosed.
Palaeocampa	Panarthropoda	Aysheaiidae	P. anthrax	First known freshwater and toxic lobopodian, bears spines unlike any other known animal

Chelicerata

A high number of arthropods belonging to the chelicerata subphylum are known from Mazon Creek.

Chelicerata
Genus	Class	Higher taxon	Species	Notes	Images
Adelophthalmus	Eurypterida	Adelophthalmidae	A. mazonensis	The only eurypterid from Mazon Creek, and the youngest member of the eurypterina
Euproops	Xiphosura	Belinuridae	E. danae	A genus of horseshoe crab, interestingly, juveniles may have been semi-aquatic, due to their bristle covered undersides
Paleolimulus	Xiphosura	Paleolimulidae	P. sp.	A wide ranging, and well known genus of xiphosuran, known from number of other deposits

Arachnida

A large number of arachnids are known from Mazon Creek, with some belonging to extinct groups, while others represent some of the earliest appearances of crown-group lineages.

Arachnida
Genus	Class	Higher taxon	Species	Notes	Images
Douglassarachne	Arachnida	Pantetrapulmonata	D. acanthopoda	A unique species distinguished by the long spines on its limbs, it is unclear where this species places specifically within arachnida

Cyclostomi

Members of Cyclostomi (hagfish and lampreys) are known from Mazon Creek.

Cyclostomi
Genus	Class	Higher taxon	Species	Notes	Images
Mayomyzon	Petromyzontida	Mayomyzontidae	B. pieckoensis	One of the first discovered fossil lampreys, it differs from modern taxa by having fewer gill openings, more teeth, and larger eyes
Pipiscius	Petromyzontida	Petromyzontiformes	P. zangerli	Another lamprey, often nicknamed “Push me-Pull you” in reference to each end being very similar in shape, may represent a synonym of Mayomyzon
Gilpichthys	Myxini	Myxiniformes	G. greenei	A basal hagfish, was considered enigmatic until a placement within the myxini was accepted
Myxinikela	Myxini	Myxiniformes	M. siroka	One of the oldest known hagfish, which shares several features with modern taxa (including large velar cavity and a cardinal heart)

Chondrichthyes

A number of chondrichthyes (cartilaginous fish) are known from Mazon Creek, including a number of elasmobranchs and holocephalans.

Chondrichthyes
Genus	Class	Higher taxon	Species	Notes	Images
Bandringa	Chondrichthyes	Elasmobranchii (placement uncertain)	B. rayi	A bizarre elasmobranch that possessed a hyper-elongated upper jaw. Juvenile specimens are known from Mazon Creek, and adults are known from parts of Pennsylvania and Ohio, suggesting the area of Mazon Creek was a mating ground for these fish.
Dabasacanthus	Chondrichthyes	Elasmobranchii	D. inskasi	A close relative of modern sharks, and a member of the Lonchidiidae family within the hybodontiformes order.
Holmacanthus	Chondrichthyes	Elasmobranchii	H. keithi	Tiny hybodontiform.
Orthacanthus	Chondrichthyes	Xenacanthida	O. sp.	A large xenacanthid fish, the species from Mazon Creek is unnamed.
Jimpohlia	Chondrichthyes	Holocephali	J. erinacea	A bizarre, tadpole-like holocephalian fish that somewhat resembled the similar genus Delphyodontos.
Polysentor	Chondrichthyes	Holocephali	P. gorbairdi	A chimaera-like holocephalian fish, and is one of the various holocephalians known from Mazon Creek.
Similhariotta	Chondrichthyes	Holocephali	S. dabasinskai	A small sized chimaeriform fish that possessed an elongated upper jaw, however a paper published by Grogan & Lund, 2008 suggests it may represent a specimen of the already known Bandringa.

Acanthodii

Some members of the acanthodii are known from Mazon Creek.

Acanthodians
Genus	Class	Higher taxon	Species	Notes	Images
Acanthodes	Acanthodii	Acanthodiformes	A. beecheri A. marshi A. sp.	While genus Acanthodes is widespread, from Mazon Creek juvenile specimens that were named as "Acanthodes beecheri" are known, although this species is dubious.
Trichorhipis	Acanthodii	Incertae sedis	T. praecursor	An acanthodian only known from a pectoral fin impression, once classified as a gyracanthid, now considered an enigmatic taxon.

Actinopterygii

A large number of early actinopterygians are known from Mazon Creek, notably the Palaeonisciformes.

Actinopterygii
Genus	Class	Higher taxon	Species	Notes	Images
”Elonichthys”	Actinopterygii	Elonichthyiformes	E. peltigerus E. hypsilepus E. disjunctus E. wolffi E. remotus	”E. peltigerus" and various other species known, none actually belong to the genus Elonichthys, but they haven’t been reassigned yet.
Illiniichthys	Actinopterygii	Palaeonisciformes	I. cozarti	Mainly differentiated by scale morphology
Nozamichthys	Actinopterygii	Palaeonisciformes	N. contorta	Relatively similar to Illiniichthys
Haplolepis	Actinopterygii	Palaeonisciformes
Microhaplolepis	Actinopterygii	Palaeonisciformes	M. sp.
Parahaplolepis	Actinopterygii	Palaeonisciformes	P. sp.
Palaeoniscoid. sp.	Actinopterygii	Palaeonisciformes		Various undescribed palaeoniscoids
Amphicentrum	Actinopterygii	Eurynotiformes	A. orbiculare	a member of the basal Eurynotiformes order of ray finned fish
Pyritocephalus	Actinopterygii	Haplolepiformes	P. comptus P. gracilis	One of the youngest known members of the Haplolepiformes order
Platysomus	Actinopterygii	Platysomiformes	P. circularis	A long lasting genus, some species most likley belong to other genera

Non-Tetrapod Sarcopterygii

Several groups of non-tetrapod sarcopterygians, including basal coelacanths and lungfish are known from the site.

Sarcopterygii
Genus	Class	Higher taxon	Species	Notes	Images
Megalichthys	Sarcopterygii	Megalichthyiformes	M. cf. hibberti	Also known from the Catskill Formation
Rhizodopsis	Sarcopterygii	Megalichthyiformes	R. cf. robustus ?R. mazonius
Rhabdoderma	Sarcopterygii	Coelacanthiformes	R. exiguum R. robustus R. elegans	A widespread and long lasting early coelacanth
Conchopoma	Sarcopterygii	Dipnoi	C. edesi C. arctatum	Also known from Permian aged deposits
Ctenodus	Sarcopterygii	Dipnoi	C. cristatus	a widespread Carboniferous lungfish
Megapleuron	Sarcopterygii	Dipnoi	M. zangerli	Also known from Permian aged deposits from France
Palaeophichthys	Sarcopterygii	Dipnoi	P. parvulus	Another basal lungfish

Miscellaneous Taxa

This section documents faunal taxa whose taxonomic affinities are not fully understood, or do not fit into any of the above groups.

Miscellaneous Taxa
Genus	Class	Higher taxon	Species	Notes	Images
Tullimonstrum	Incertae sedis	Incertae sedis	T. gregarium	Perhaps the most well known, and infamous species from the site, recent research suggests a placement as either a non-vertebrate chordate, or a protostome of uncertain placement.
Pohlsepia	Incertae sedis	Incertae sedis	P. mazonensis	Originally considered to be the oldest known octopus, specifically within the cirroctopoda, however recent research has questioned this placement, and may possibly represent a cnidarian of uncertain placement.
Lascoa	Cnidaria	Semaeostomeae	L. mesostaurata	Originally considered as a true jellyfish within the semaeostomeae, possessing frilly arms, but affinity is questioned.
Escumasia	Incertae sedis	Incertae sedis	E. roryi	Often nickanmed the 'Y-animal', this problematic organism has been compared to cnidarians, but no concrete placement has been given
Etacystis	Hemichordata?	Pterobranchia?	E. communis	Also known as the 'H-animal', this colonial invertebrate is normally compared to hemichordates, but also hydrozoans
Esconichthys	Chordata	Incertae sedis	E. apopyris	An enigmatic jawed vertebrate, often nicknamed 'blade', most often identified by its prominent eyes
Archisymplectes	Nemertea	Palaeonemertea	A. rhothon	A featureless worm belonging to the nemertea (ribbon worms).
Priapulites	Priapulida	Priapulimorphida	P. konecniorum	One of the earliest known crown-group priapulid worms, and a close relative of the priapulidae
Paucijaculum	Chaetognatha	Incertae sedis	P. samamithion	A member of the chaetognatha (arrow worms), and shares much in common with modern taxa
Microconchida. sp	Lophophorata	Tentaculita	Various species	Originally considered a member of the sabellida, but was later discovered all pre-Bathonian sabellids were instead microconchids

Further reading

• Wittry, Jack 2012, "The Mazon Creek Fossil Fauna", Chicago, ESCONI Associates, 202 pp. ISBN 9780578111483