The Cerin Lagerstätte is a fossil deposit of the Jura Mountains located in Cerin, a hamlet belonging to the commune of Marchamp in the department of Ain. The site is internationally known for its surprising diversity.

The deposits are located in what was a tropical lagoon at the end of the Kimmeridgian age (Late Jurassic). It is dated from the Aulacostephanus pseudomutabilis biozone, whose equivalent in the Mediterranean area is the Aulacostephanus eudoxus biozone, which dates to 153 Ma.Schweigert, G. 2007b. Ammonite biostratigraphy as a toolfor dating Upper Jurassic lithographic limestones from South Germany – first results and open questions. NeuesJahrbuch für Geologie und Paläontologie Abhandlungen,245(1), 117–125

Situation

The site belonging to the Bugey historical region was erected at 560 meters above the sea level and is located 20 km off Belley, 75 km off Lyon, 80 km off Grenoble and 90 km off Geneva.IGN maps on Géoportail.

Lithographic limestone

Cerin was reputed by the end of the 19th century for the quality of its lithographic limestone. The area was, during the Late Jurassic, a tropical lagoon. Lithographic limestone is formed by sedimentation of a very thin carbonated mud deposited at the bottom of a lagoon 153 million of years ago. Those deposits are disposed in strata.

The quarry exploitation, which began in 1835 during the lithography golden age, allowed to periodically uncover prints of prehistoric animals and plants fossilized in stone. Paleontology was at its beginning and those discoveries were still little-known.

Site discovery

In 1838, thanks to the engineer Aimé Drian, a passionate amateur geologist, and Lyonese geologist, and Victor Thiollière, those fossils were discovered and the existence of the paleontological site of Cerin was revealed to the scientific world. The site has an international reputation and rivals with the Solnhofen Limestone, in Bavaria. Until his death, Victor Thiollière never ceased to collect and study a maximum of fossils from the site, and it was, in large part, thanks to his study of the Cerin fossils that he was recognized in the palaeontology world.

His works showed the similarities of the lithographic limestones of Cerin and Solnhofen. He described several new species of fish. He published in 1854 the first part of his "Description des poissons fossiles provenant des gisements coralliens du Jura dans le Bugey" ("Description of the fossil fish from the Jura corallian deposits in the Bugey"), but died shortly before the publication of the second part, the descriptions and lithographic plates already finished..

Research on site

Research on the Late Kimmeridgian site (Late Jurassic, around −153 Ma),. occurred from 1975 to 1995, it was a unique operation and involved advanced technology. Directed by geologists from Claude Bernard University in Lyon, it necessitated heavy equipment of civil engineering.

This operation allowed the discovery of algae, ferns, conifers, molluscs, sea urchins, starfish, crustaceans such as the holotypes of Cyclerion bourseaui and Soleryon amicalis, reptiles, fish, as well as tracks of turtles and other reptiles. A thorough study permitted to determine the age and nature of the site (a tropical lagoon 153 million years old) and to understand the reasons of this exceptional fossilization.

A one-of-a-kind Late Jurassic fossilized track of prehistoric marine turtle was discovered in Cerin.

Fossilization process in Cerin

To understand the fossilization process in Cerin, scientists went on an expedition to Aldabra in the Indian Ocean, to observe a fossilization process similar to the one that happened on the Cerin site.

153 million years ago, the climate was tropical. The lagoon had only few contacts with the open sea, and evaporation was intense. Terrestrial animals venturing in the lagoon shores left their tracks in a quickly drying mud.

During storms, a large quantity of saltwater, carrying mud, vegetal remains, and dead or alive animals, entered the lagoon; clearwater, brought by rains and water runoff, brought as well large quantities of particulate matters. When the calm was settling, the fine particles deposited in a regular layer which carpeted the depths and covered remains and tracks.

This layer gave then birth to a strata of lithographic limestone.

When evaporation made the water level drop again, it became under oxygenated and over-concentrated on salt, which led to the death of many living beings, while protecting their bodies from scavengers. The microbial mats, who thrived in this environment, covered the corpses and vegetal remains, helping their conservation as fossils.

Biota

Algae Genus Species Classification Note Picture Campbelliella C. striata Dasycladales Clypeina C. jurassica Dasycladales Clusters of this species attain more than 10 cm. Goniolina G. janeti Dasycladales

Plants Genus Species Classification Note Picture Apoldia A. latifolia Cycadales Brachyphyllum B. desnoyersii Pinales B. elegans Pinales B. speciosa Pinales Crossozamia Cycadopsida Formerly described as Cycalacis. Cupressinoxylon C. sp Coniferales Cycadopteris C. jurensis Pteridospermatophyta Pachypteris Pteridospermatophyta Pagiophyllum P. cirinicum Araucariaceae Zamites Z. feneonis Bennettitales Very abundant in the site. Z. pumilio Bennettitales

Cnidarians

Genus Species Classification Note Picture Bipedalia B. cerinensis Cubozoa Paracarybdea P. lithographica Cubozoa Paraurelia P. cerinensis Scyphozoa P. sp Scyphozoa

Lophotrochozoans Genus Species Classification Note Picture Aspidoceras A. cf. catalaunicum Aspidoceratidae Hibolites Belemnoidea LaevaptychusL. latusA kind of Aptychus. Formerly referred as Aptychus latus. Lithacoceras L. cf. ulmense Ataxioceratidae L. sp. gr. siliceus Ataxioceratidae L. sp Ataxioceratidae Potentially a new species. NanogyraN. virgulaBivalviaNerineaN. suprajurensisGastropoda Rhynchonellida Rhynchonellida Subplanites S. cf. rueppellianus Perisphinctidae Terebratulidae Terebratulidae

Arthropods Genus Species Classification Note PictureAntrimposA. speciosusPenaeidae Cycleryon C. bourseaui Eryonidae C. propinquusEryonidaeDusaD. monoceraPenaeidaeIts presence at Cerin might be a mistake caused by confusion due to misattribution of specimens hailing from the Solnhofen Limestone.ErymaE. modestiformisErymidaeEryonE. arctiformisEryonidaeIts presence at Cerin might be a mistake caused by confusion due to misattribution of specimens hailing from the Solnhofen Limestone. Gigacerina G. saemanni Glypheoidea One of the largest known glypheids. Formerly referred to as Glyphea saemanni. GlypheaG. pseudoscyllarusGlypheoideaMacrourites cerensis is a synonym.MecochirusM. brevimanusGlypheoideaIts presence at Cerin might be a mistake caused by confusion due to misattribution of specimens hailing from the Solnhofen Limestone. This species is also considered a synonym of M. longimanatus, likely to represent a female morphotype. M. sp Glypheoidea Mesolimulus M. walchi Limulidae Soleryon S. amicalis Eryonidae

Echinoderms Genus Species Classification Note PictureAcrocidarisA. nobilisEuechinoidea ?Ceramaster ? C. chantrei Goniasteridae CidarisC. cariniferaCidaroida Comptoniaster C. meyeri Goniasteridae Ophiopetra O. lithographica Aplocomidae Pentasteria P. lithographica Astropectinidae Pseudodiadema P. cf. pseudodiadema Pseudodiadematidae Pygurus Clypeidae Solanocrinites S. thiollieri Crinoidea

Chondrichthyes Genus Species Classification Note Picture Belemnobatis B. sismondae Asterodermidae Corysodon C. cirinensis Carcharhiniformes The genus Corysodon has been synonymized with Palaeoscyllium by Cappetta, 1987. However the validity of Corysodon was maintained by Thies & Candoni, 1998, this was related by later studies. Phorcynis P. catulina Squatiniformes Spathobatis S. bugesiacus Asterodermidae

Sarcopterygians Genus Species Classification Note Picture Undina U. cirinensis Latimeriidae This species was classified as a member of the genus Holophagus by Wenz et al., 1994. However, Ferrante & Cavin, 2025 considered H. gulo as the only Holophagus species.

Actinopterygians

Genus Species Classification Note PictureAiniaA. armataHalecomorphiAllothrissopsA. regleyiIchthyodectiformesFormerly placed in the genus Thrissops.AmblysemiusA. bellicianusCaturidaeAnaethalionA. affinis ElopiformesA. cirinensisElopiformesA. knorriElopiformesA. cf. subovatusElopiformesAnkylophorusA. similisAnkylophoridaeFormerly placed in the genus Pholidophorus.ApomesodonA. comosus Pycnodontiformes Synonym of A. gibbosus. A. gibbosus Pycnodontiformes Formerly referred to as Eomesodon gibbosus. A. surgens Pycnodontiformes Synonym of A. gibbosus.AspidorhynchusA. acutirostrisAspidorhynchidaeBelonostomusB. tenuirostrisAspidorhynchidaeSome authors are more prudent about the identification of this species as B. tenuirostris.B. sp.AspidorhynchidaeAt least one more species is present in Cerin, it is similar to B. kochii.CaturusC. furcatusCaturidaeCerinichthysC. koelblaeHalecomorphiEoprotelopsE. viretiElopiformesGyrodus"G". hexagonusPycnodontiformesThe specimens from Cerin probably belong to a more derived pycnodontiform, it might not be the same species as Gyrodus hexagonus.G. sp.PycnodontiformesSpecimens from Cerin were formerly referred to as Mesturus verrucosus.IonoscopusI. desoriAmiiformesLehmanophorusL. segusianusAnkylophoridaeFormerly referred to as Pholidophorus segusianus.LeptolepidesL. sp.OrthogonikleithridaeThere might be more than one species present in Cerin.MacromesodonM. macropterusPycnodontiformesFormerly referred to as Coelodus jourdani, which is now considered a junior synonym. Macrosemius M. fourneti Macrosemiidae NotagogusN. helenaeMacrosemiidaeN. inimontisMacrosemiidaeOligopleurusO. esocinusHalecomorphiOphiopsiellaO. guigardiHalecomorphiFormerly referred to as Ophiopsis guigardi.O. macrodusHalecomorphiFormerly referred to as Ophiopsis macrodus.O. proceraHalecomorphiSpecimens from Cerin were formerly referred to as Ophiopsis attenuata. Ophiopsis O. muensteri Ionoscopiformes The occurrence from Cerin was formerly described as Furo praelongus. However, this species has been synomymized with Furo muensteri. This species was moved to the genus Ophiopsis after a taxonomic reassessment.OrthocormusO. teyleriPachycormiformesOrthogonikleithrusO. francogalliensisOrthogonikleithridaeSpecimens previously assigned to Leptolepides sprattiformis belong to this species.O. sp.OrthogonikleithridaeThere is at least one more species of Orthogonikleithrus. Specimens previously referred to Ascalabos voithi actually belong to O. sp.PalaeomacrosemiusP. thiollieriMacrosemiidaePholidophorichthysP. inermisPholidophoridaePholidophoristionP. cf. ovatusPholidophoridae"Pholidophorus""P". veliferCaturidaeFormerly known as Caturus velifer, it surely does not belong to Caturus. Its taxonomic statue is still unclear and is being studied.PleuropholisP. thiolleriPleuropholidaePropterusP. microstomusSemionotiformesMaterial from Cerin was previously described as Histionotus falsani. This species was later regarded as a synonym of Histionotus oberndorferi, itself synonymized with Propterus microstomus. Proscinetes P. bernardi Pycnodontiformes P. distantidensPycnodontiformes P. elegansPycnodontiformes P. intermediusPycnodontiformes P. itieri Pycnodontiformes P. sauvanausiPycnodontiformesP. thiolleriPycnodontiformes Scheenstia S. laevis Lepisosteiformes Formerly referred to as Lepidotes laevis.SolnhofenamiaS. elongataAmiidaeFormerly placed in the genus Urocles as U. elongatus.TharsisT. dubiusAscalaboidiformes Thiollierepycnodus T. wagneri Pycnodontiformes Formerly referred to as Gyrodus wagneri. ThrissopsT. cirinensisIchthyodectiformesT. formosusIchthyodectiformes Turboscinetes T. egertoni Pycnodontiformes Formerly referred to as Proscinetes egertoni. Reptiles