# Agardhfjellet Formation

The Agardhfjellet Formation is a geologic formation in Svalbard, Norway. It preserves fossils dating back to the Oxfordian to Berriasian stages, spanning the Late Jurassic-Early Cretaceous boundary. The formation contains the Slottsmøya Member, a highly fossiliferous unit (Lagerstätte) where many ichthyosaur and plesiosaur fossils have been found, as well as abundant and well preserved fossils of invertebrates.

## Description

The formation overlies the Knorringfjellet Formation and is overlain by the Rurikfjellet Formation. The formation comprises the lower Oppdalsåta and upper Slottsmøya Members. The Slottsmøya Member, which averages 55 to 60 metres (180 to 197 ft) in thickness in the study area, consists of dark-grey to black silty mudstone, often weathering to paper shale, and discontinuous silty beds with local occurrences of red to yellowish sideritic concretions as well as siderite and dolomite interbeds. It consists of a mix of shales and siltstones which were deposited in a shallow marine environment, near a patch of deeper marine sediment. The seafloor, which was located about 150 metres (490 ft) below the surface, seems to have been relatively dysoxic, or oxygen-poor, although the diversity of the benthic fauna suggest that these environments were likely not truly anoxic and that oxygen-depletion may have been a seasonal phenomenon. Although direct evidence from Slottsmøya is currently lacking, the high latitude of this site and relatively cool global climate of the Tithonian suggest that some sea ice may have been present at least in the winter.
These sites represent shallow-water methane seeps which were spread over a relatively large geographic area, and like modern day seeps, they supported high biodiversity. Near the top of the member, various assemblages of invertebrates have been discovered; these include ammonites, bivalves, lingulate brachiopods, rhynchonellate brachiopods, tubeworms, belemnoids, tusk shells, sponges, crinoids, sea urchins, brittle stars, starfish, crustaceans, and gastropods, numbering 54 taxa in total. The most common and abundant of these taxa were bivalves and brachiopods, each of which make up 27.8% of the known seep fauna. Several chemosymbiotic species are known, such as the bivalve Nucinella. Outside of the cold seeps, several non-seep-restricted invertebrates were also present in abundance. In addition to the invertebrates, the Slottsmøya Member has also revealed a diverse assemblage of marine reptiles, including several taxa of ichthyosaurs and plesiosaurs. Many of these specimens are relatively complete and in articulation, which is rare among Jurassic marine fossil sites. It thus provides a unique and detailed glimpse into the boreal seas of the Late Jurassic. As it spans the Jurassic-Cretaceous boundary, it is also important to understanding how marine ecosystems changed going into the Cretaceous.

## Paleobiota

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
The formation, especially the Slottsmøya Member, has provided an abundance of marine fossils dating to the Late Jurassic-Early Cretaceous.
The timeline below follows stratigraphic data provided in Delsett et al. 2018 and Roberts et al. 2020.

### Vertebrates
#### Bony fish

Bony fish from the Agardhfjellet Formation :
| Genus | Species | Location | Member | Material | Notes | Images |
| Leptolepis | L. nathorsti | | Oppdalssåta & Lardyfjellet Members | | | |

#### Plesiosaurs

Plesiosaurs from the Agardhfjellet Formation :
| Genus | Species | Member | Material | Notes | Images |
| Tricleidus | T. svalbardensis | Slottsmøya Member | | Reassigned to the genus Colymbosaurus after further analysis |
| Colymbosaurus | C. svalbardensis | Slottsmøya Member | | A colymbosaurine cryptoclidid, also known from the Kimmeridge Clay | Specimen from an unknown location |
| Djupedalia | D. engeri | Slottsmøya Member | | A cryptoclidid plesiosaur | |
| Ophthalmothule | O. cryostea | Slottsmøya Member | Extensive skeletal material including a complete cranium | A cryptoclidid plesisosaur which has unusually large orbits. | |
| Spitrasaurus | S. wensaasi | Slottsmøya Member | | A cryptoclidid plesiosaur related to Ophthalmothule | |
| Spitrasaurus | S. larseni | Slottsmøya Member | | A cryptoclidid plesiosaur related to Ophthalmothule | |
| Pliosaurus | P. funkei | Slottsmøya Member | Known from two partial skeletons | A large thalassophonean pliosaurid which was informally known as "Predator X" prior to being named. This species is notable for having unusually large paddles in comparison to its body size. | |

#### Ichthyosaurs

Ichthyosaurs from the Agardhfjellet Formation :
| Genus | Species | Member | Material | Notes | Images |
| Cryopterygius | C. kristiansenae | Slottsmøya Member | | Junior synonym of Undorosaurus gorodischensis | |
| Undorosaurus | U. gorodischensis | Slottsmøya Member | A single, largely complete specimen | A large platypterygiine ichthyosaur, also known from the Russian Volga. | |
| Janusaurus | J. lundi | Slottsmøya Member | | Junior synonym of Arthropterygius lundi | |
| Palvennia | P. hoybergeti | Slottsmøya Member | | Junior synonym of A. hoybergeti | |
| Keilhauia | K. nui | Slottsmøya Member | | A nomen dubium referred to A. sp. cf. chrisorum | |
| Arthropterygius | A. lundi | Slottsmøya Member | Several specimens of varying ages, previously referred to several other genera | A widespread genus of ophthalmosaurid ichthyosaur, also known from Canada, Russia, and Argentina. Is present both before and directly after the Jurassic-Cretaceous Boundary. | |
| Arthropterygius | A. hoybergeti | Slottsmøya Member | Several specimens of varying ages, previously referred to several other genera | A widespread genus of ophthalmosaurid ichthyosaur, also known from Canada, Russia, and Argentina. Is present both before and directly after the Jurassic-Cretaceous Boundary. | |
| Arthropterygius | A. chrisorum | Slottsmøya Member | Several specimens of varying ages, previously referred to several other genera | A widespread genus of ophthalmosaurid ichthyosaur, also known from Canada, Russia, and Argentina. Is present both before and directly after the Jurassic-Cretaceous Boundary. | |
| Nannopterygius | N. borealis | Slottsmøya Member | | An ophthalmosaurid ichthyosaur present in early Cretaceous strata. This genus is also known from the Kimmeridge Clay and from fossil sites in the Volga. | Specimen from Kimmeridge Bay, England |
| Brachypterygius | B. sp. | Slottsmøya Member | Partial skull | Referred to Brachypterygius by Angst et al. (2010) | |

### Invertebrates

An unnamed galatheid squat lobster is known from a single, poorly preserved fragment.

#### Cephalopods

Large belemnoid arm hooks are known.
Cephalopods from the Agardhfjellet Formation :
| Genus | Species | Member | Images |
| Craspedites | C. okensis | Slottsmøya Member | Specimen from an unknown location |
| Craspedites | C. originalis | Slottsmøya Member | Specimen from an unknown location |
| Borealites | B. rossicus | Slottsmøya Member | |
| Borealites | B. sp. | Slottsmøya Member | |
| Hectoroceras | H. kochi | Slottsmøya Member | |
| Hectoroceras | H. sp. | Slottsmøya Member | |
| Kachpurites | K. sp. | Slottsmøya Member | |
| Surites | S. spasskensis | Slottsmøya Member | |
| Surites | S. spasskensoides | Slottsmøya Member | |
| Surites | S. tzikwinianus | Slottsmøya Member | |
| Surites | S. mesezhnikovi | Slottsmøya Member | |
| Tollia | T. tolli | Slottsmøya Member | |

#### Echinoderms

Echinoderms from the Agardhfjellet Formation :
| Genus | Species | Member | Material | Notes | Images |
| Chariocrinus | C. sp. | Slottsmøya Member | | A form of isocrinid crinoid | Modern isocrinid crinoid, Neocrinus decorus, from Saint Vincent |
| Hemipedina | H. sp. | Slottsmøya Member | | A species of pedinoid sea urchins related to the extant Caenopedina | Modern Caenopedina hawaiiensis from Hawaii |
| Polarasterias | P. janusensis | Slottsmøya Member | Nearly complete specimen | A forcipulatid sea star | |
| Savignaster | S. septemtrionalis | Slottsmøya Member | | A pterasterid sea star | Modern pterasterid sea star from Washington state |
| Ophiogaleus | O. sp. | Slottsmøya Member | | A ophiacanthid brittle star | |
| Ophioculina | O. hoybergia | Slottsmøya Member | | An ophiurine brittle star | |

#### Bivalves

Remains of possible anomiid clams have been found which are as yet undescribed.
Bivalves from the Agardhfjellet Formation :
| Genus | Species | Member | Notes | Images |
| Pseudotrapezium | P. aff. groelandicum | Slottsmøya Member | An arcticid clam which constitutes nearly half of the fauna recovered from the seep assemblages. | Another arcticid species, Arctica islandica, from an unknown location |
| Mesosaccella | M. rogovi | Slottsmøya Member | | |
| Mesosaccella | M. toddi | Slottsmøya Member | | |
| Nucinella | N. svalbardensis | Slottsmøya Member | The most abundant chemosymbiotic species in the Spitsbergen seeps. | |
| Buchia | B. sp | Slottsmøya Member | | |
| Solemya | S. cf. woodwardiana | Slottsmøya Member | A burrowing chemosymbiotic bivalve which is very abundant in the seep sites | |
| Tehamatea | T. rasmusseni | Slottsmøya Member | A lucinid clam | Specimen of another lucinid species, Divaricella huttoniana, from New Zealand |
| Cretaxinus | C. hurumi | Slottsmøya Member | A thyacirid clam | |
| Camptonectes | C. spp | Slottsmøya Member | A pectinid clam represented by at least two species | |
| Oxytoma | O. octavia | Slottsmøya Member | | |
| Pseudolimea | P. arctica | Slottsmøya Member | | |
| Dacromya | D. chetaensis | Slottsmøya Member | Rare | Dacryomya lacryma, of Middle Jurassic age, from Bavaria, Germany |
| Goniomya | G. literata | Slottsmøya Member | Rare | |

#### Serpulids

Serpulids from the Agardhfjellet Formation :
| Genus | Species | Member | Notes | Images |
| Propomatoceros | P. sp. | Slottsmøya Member | A serpulid tube worm | Modern serpulid, Spirobranchus giganteus, from East Timor |
| aff. Pyrgopolon | aff. P. nodulosum | Slottsmøya Member | A serpulid tube worm which forms mass accumulations in the Echinoderm Bed | |
| Nogrobs | N. sp. | Slottsmøya Member | | |

#### Brachiopods

Brachiopods from the Agardhfjellet Formation :
| Genus | Species | Location | Member | Material | Notes | Images |
| Lingularia | L. similis | | Slottsmøya Member | | The most abundant species of brachiopod that is present in Slottsmøya | |
| Ptilorhynchia | P. mclachlani | | Slottsmøya Member | | A rhynchonellide brachiopod | Specimens of another rhynchonellid species, Rhynchotrema dentatum, from Indiana |

#### Gastropods

Gastropods from the Agardhfjellet Formation :
| Genus | Species | Member | Notes | Images |
| Ambercyclus | A. sp. | Slottsmøya Member | The most abundant species of gastropod that is known from the Slottsmøya seep deposits | |
| Hudlestoniella | H. sp. | Slottsmøya Member | A rissoid gastropod | |
| Cantrainea | C. sp. | Slottsmøya Member | A turbinid gastropod; it is rare and only known from one of the seep sites | Specimen of late Miocene age from Trinidad and Tobago |