This list of the prehistoric life of Illinois contains the various prehistoric life-forms whose fossilized remains have been reported from within the US state of Illinois.

The Paleobiology Database records no known occurrences of Precambrian fossils in Illinois.

• †Achatella
• †Actinoceras
• †Alethopteris
  • †Alethopteris decurrens
  • †Alethopteris lonchitica
  • †Alethopteris owenii
  • †Alethopteris serlii

• 

  †Amphibamus – type locality for genus
  • †Amphibamus grandiceps – type locality for species
• †Amphiscapha
• †Amynilyspes – type locality for genus
• †Annularia
  • †Annularia asteris – or unidentified comparable form
  • †Annularia stellata
• †Anomphalus – type locality for genus
• †Anthracoceras
• †Archaeocidaris – tentative report
• †Archaeopteris
• †Archimedes
  • †Archimedes communis
  • †Archimedes compactus
  • †Archimedes invaginatus
  • †Archimedes lativolvis
  • †Archimedes macfarlani
  • †Archimedes swallovanus
  • †Archimedes terebriformis
• †Archimylacris
• †Arctinurus

• 

  †Arthrolycosa – type locality for genus
• †Artisia
• †Asterotheca
  • †Asterotheca miltoni – or unidentified comparable form
• †Asyncritus – type locality for genus
• †Athyris
• †Atrypa
  • †Atrypa reticularis – report made of unidentified related form or using admittedly obsolete nomenclature
• †Aviculopecten
  • †Aviculopecten fasciculatus
• † Avonia
• †Bandringa – type locality for genus
• †Bassleroceras
• †Bellerophon
  • †Bellerophon spergensis
• †Belinurus
• †Bembexia
• †Blattoidea

• 

  †Brachydectes
  • †Brachydectes newberryi
• †Bumastus
  • †Bumastus armatus – or unidentified comparable form
  • †Bumastus chicagoensis
  • †Bumastus cuniculus – or unidentified comparable form
  • †Bumastus graftonensis
  • †Bumastus insignis
  • †Bumastus springfieldensis
  • †Bumastus transversalis
• †Calamites
  • †Calamites suckowii
• †Callipteridium
• †Calvinia
• †Calymene
  • †Calymene breviceps
  • †Calymene celebra
• †Calyptaulax
• †Camarotoechia

• 

  †Caseodus
• †Cephalerpeton – type locality for genus
  • †Cephalerpeton ventriarmatum – type locality for species
• †Ceratiocaris
• †Ceratocephala
• †Ceraurinella
• †Ceraurinus
• †Ceraurus
• †Cheirurus
• †Chonetes
  • †Chonetes ornatus
• †Cleiothyridina
  • †Cleiothyridina atrypoides
  • †Cleiothyridina sublamellosa
• †Clepsydrops – type locality for genus
• †Cliftonia

• 

  †Coelacanthus
• †Composita
  • †Composita subtilita
  • †Composita trinuclea
• †Conocardium
• †Cooperoceras
• †Cordaicarpus
• †Cordaites
  • †Cordaites principalis
• †Cornulites
• †Crania
• †Craniops

• 

  †Cricotus – type locality for genus
• †Crotalocrinites
• †Ctenacanthus
  • †Ctenacanthus buttersi – type locality for species
• †Curriella – tentative report
• †Cyathocrinites
• †Cyclonema
• †Cyclus
• †Cyphaspis
• †Cyrtoceras
• †Dalmanites
• †Deiphon
• †Dicoelosia
• †Dictyonema
• †Dimerocrinites
  • †Dimerocrinites occidentalis

• 

  †Diplocaulus – type locality for genus
• †Domatoceras
• †Echinaria
• †Edestus
• †Edmondia
• †Elonichthys
• †Encrinurus
• †Eodictyonella
• †Eophacops
  • †Eophacops handwerki
• †Eospirifer
  • †Eospirifer radiatus
• †Erratencrinurus
• †Eucalyptocrinites
  • †Eucalyptocrinites crassus
• †Euomphalus
  • †Euomphalus planodorsatus

• 

  †Euphoberia – type locality for genus
  • †Euphoberia armigera – type locality for species
• †Favosites
• †Fenestella
• †Fletcheria
• †Gerarus – type locality for genus
• †Glikmanius
  • †Glikmanius occidentalis
• †Glyptambon
• †Glyptopleura
• †Gondolella
• †Graeophonus
  • †Graeophonus scudderi – type locality for species
• †Greenops

• 

  †Greererpeton
• †Hallopora – tentative report
• †Halysites
• †Harpidium
• †Helenodora – type locality for genus
  • †Helenodora inopinata – type locality for species
• †Heliomeroides
• †Holia
• †Hyolithes
• †Ilyodes
• †Isodectes

• 

  †Isotelus
  • †Isotelus gigas
  • †Isotelus maximus
• †Kionoceras
• †Latzelia – type locality for genus
  • †Latzelia primordialis – type locality for species
• †Lepidodendron
  • †Lepidodendron aculeatum
• †Lepidophyllum
• †Lepidostrobus
• †Lichas
• †Lingula
• †Liroceras
• Lithophaga – tentative report

• 

  †Lysorophus – type locality for genus
• †Marsupiocrinus
• †Meristina
• †Metacoceras
• †Milosaurus – type locality for genus
• †Monograptus
  • †Monograptus dubius
• †Murchisonia
• †Naticopsis
  • †Naticopsis carleyana
  • †Naticopsis planifrons
  • †Naticopsis waterlooensis – type locality for species
• †Nemastomoides – type locality for genus
• †Neospirifer
  • †Neospirifer dunbari
  • †Neospirifer triplicatus

• 

  †Neuropteris
  • †Neuropteris ovata
  • †Neuropteris rarinervis
  • †Neuropteris scheuchzeri
  • †Neuropteris tenuifolia
• †Nodonema
• Nucula
• †Nuculoidea – tentative report
• †Onychopterella – tentative report
• †Palenarthrus – type locality for genus
  • †Palenarthrus impressus – type locality for species
• †Paraisobuthus
• †Pecopteris
  • †Pecopteris plumosa
• †Pentremites
  • †Pentremites tulipaformis
• †Periechocrinus
• †Phacops

• 

  †Phlegethontia
  • †Phlegethontia longissima – type locality for species
• †Phragmolites
• †Platyceras
• †Platysomus
• †Platystrophia
• †Plicochonetes
• †Polysentor – type locality for genus
• †Priapulites – type locality for genus
  • †Priapulites konecniorum – type locality for species
• †Prodentalium
• †Proetus

• 

  †Protophasma
• †Protopilio
• †Pseudophlegethontia – type locality for genus
  • †Pseudophlegethontia turnbullorum – type locality for species
• †Pterochiton
• †Pterotheca
• †Rasstriga – type locality for genus
  • †Rasstriga americana – type locality for species
• †Remopleurides
• †Rhodea
• †Rhynchonella
• †Saccoglossus
• †Sagenodus
• †Samaropsis
• †Savagella
• †Scepasma – type locality for genus

• 

  †Sigillaria
• †Silphion – type locality for genus
• † Simplicius
• †Skenidioides
• Solemya
• †Solenochilus
• †Sphaerexochus
• †Sphaerocoryphe
• †Sphenophyllum
  • †Sphenophyllum cuneifolium
  • †Sphenophyllum emarginatum
  • †Sphenophyllum longifolium – or unidentified comparable form
• †Sphenopteris
• †Stearoceras
• †Stethacanthus
  • †Stethacanthus altonensis
• †Stigmaria
• †Strophomena
• †Symmorium – type locality for genus
• †Syringopora
• †Tainoceras
• †Tentaculites
• †Tranodis

• 

  †Tullimonstrum – type locality for genus
• †Wilkingia
• †Worthenia
• †Xyloiulus
• Yoldia

The Paleobiology Database records no known occurrences of Mesozoic fossils in Illinois.

• Agabus
  • †Agabus praelugens – type locality for species
  • †Agabus savagei – type locality for species
• †Anomodon – type locality for genus
  • †Anomodon snyderi – type locality for species
• †Araloselachus
  • †Araloselachus cuspidata
• Arvicola
• †Bananogmius
  • †Bananogmius crieleyi – or unidentified comparable form
• Bison

  • 

    †Bison antiquus
  • †Bison bison
• Blarina
• †Bootherium
  • †Bootherium bombifrons
• Carabus
  • †Carabus maeander
• Castor
  • †Castor canadensis
• †Castoroides
  • †Castoroides ohioensis
• †Cervalces
  • †Cervalces latifrons – tentative report

  • 

    †Cervalces scotti
• Chelonia
• Chlaenius
  • †Chlaenius plicatipennis – type locality for species
• Conopeum
  • †Conopeum damicornis
• Donacia
  • †Donacia stirioides – type locality for species
• Equus
• Esox
• †Euceratherium
  • †Euceratherium collinum
• Geomys
  • †Geomys bursarius
• †Ischyodus
  • †Ischyodus williamsae
• Lasiopodomys
  • †Lasiopodomys deceitensis
• †Mammut
  • †Mammut americanum
• †Mammuthus

  • 

    †Mammuthus primigenius
• Marmota
  • †Marmota monax
• †Megalonyx
  • †Megalonyx jeffersonii
• Mesalia
  • †Mesalia alabamiensis
• Microtus
  • †Microtus paroperarius
• Natica
  • †Natica reversa
• Olophrum
  • †Olophrum henryi – type locality for species
• Ostrea
  • †Ostrea pulaskensis
• †Pachyrhizodus
  • †Pachyrhizodus caninus
• Patrobus
  • †Patrobus henshawi – type locality for species
• Peromyscus
• Pitar – tentative report
  • †Pitar ripleyanus
• Pituophis

• 

  †Platygonus – type locality for genus
  • †Platygonus compressus – type locality for species
  • †Platygonus cumberlandensis – tentative report
• Platynus
  • †Platynus calvini – type locality for species
  • †Platynus pleistocenicus – type locality for species
  • †Platynus subgelidus – type locality for species
• Procyon
  • †Procyon lotor – type locality for species
• †Propenser
  • †Propenser hewletti
• †Ptychodus
• Rangifer

  • 

    †Rangifer tarandus
• †Sangamona
  • †Sangamona fugitiva
• Sorex
• †Strepsidura
  • †Strepsidura contorea
• Sylvilagus
  • †Sylvilagus floridanus
• Ursus
  • †Ursus americanus
• Venericardia
  • †Venericardia smithii

• Various Contributors to the Paleobiology Database. "Fossilworks: Gateway to the Paleobiology Database". Retrieved 7 August 2017.