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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 biota 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. The flora is also diverse, with lycopsids, sphenopsids, pteridophyta, and pteridosperms being represented.

Fauna

Chondrichthyes

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

Chondrichthyes
Genus	Class	Higher taxon	Abundance	Notes	Images
Bandringa	Chondrichthyes	Elasmobranchii (placement uncertain)	Various specimens	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		A close relative of modern sharks, and a member of the Lonchidiidae family within the hybodontiformes order.
Holmacanthus	Chondrichthyes	Elasmobranchii		Tiny hybodontiform.
Jimpohlia	Chondrichthyes	Holocephali		A bizarre, tadpole-like holocephalid fish that somewhat resembled the similar genus Delphyodontos.
Polysentor	Chondrichthyes	Holocephali		A chimaera-like holocephalid fish, and is one of the various holocephalans known from Mazon Creek.
Similhariotta	Chondrichthyes	Holocephali		A small sized chimaeriform fish that possessed an elongated upper jaw.

Acanthodians

Chondrichthyes
Genus	Class	Higher taxon	Abundance	Notes	Images
Acanthodes	Acanthodii	Acanthodiformes	Various juvenile specimens	While genus Acanthodes is widespread, from Mazon Creek juvenile specimens that were named as "Acanthodes beecheri" are known, although this species is dubious.