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.Delsett et al., 2019, p.741Agardhfjellet Formation at Fossilworks.org
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 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 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
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 Location Member Material Notes ImagesTricleidusT. svalbardensis Slottsmøya Member Reassigned to the genus Colymbosaurus after further analysisE. M. Knutsen, P. S. Druckenmiller, and J. Hurum. 2012. Redescription and taxonomic clarification of ‘Tricleidus’ svalbardensis based on new material from the Agardhfjellet Formation (Middle Volgian). Norwegian Journal of Geology 92:175-186 Colymbosaurus C. svalbardensis Slottsmøya Member A colymbosaurine cryptoclidid, also known from the Kimmeridge Clay 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, indicating possible deep-diving and/or nocturnal behaviors. Spitrasaurus S. wensaasi Slottsmøya Member A cryptoclidid plesiosaur related to Ophthalmothule S. larseni Pliosaurus P. funkei Slottsmøya Member Known from two partial skeletons A very large thalassophonean pliosaurid which was informally known as "Predator X" prior to being named. P. funkei is among the largest of all pliosaurs and is notable for having unusually large flippers in comparison to its body size. This genus is also known from Kimmeridge Clay.
Ichthyosaurs Ichthyosaurs from the Agardhfjellet Formation Genus Species Location 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. lundiSlottsmøya 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. A. hoybergeti A. chrisorum 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. 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 Location Member Material Notes Images Craspedites C. okensis Slottsmøya Member C. originalis Borealites B. rossicus Slottsmøya Member B. sp. Hectoroceras H. kochi Slottsmøya Member H. sp. Kachpurites K. sp. Slottsmøya Member Surites S. spasskensis Slottsmøya Member S. spasskensoides S. tzikwinianus S. mesezhnikovi Tollia T. tolli Slottsmøya Member
Echinoderms Echinoderms from the Agardhfjellet Formation Genus Species Location Member Material Notes Images Chariocrinus C. sp. Slottsmøya Member A form of isocrinid crinoid Hemipedina H. sp. Slottsmøya Member A species of pedinoid sea urchins related to the extant Caenopedina Polarasterias P. janusensis Slottsmøya Member Nearly complete specimen A forcipulatid sea star Savignaster S. septemtrionalis Slottsmøya Member A pterasterid sea star 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 Location Member Material Notes Images Pseudotrapezium P. aff. groelandicum Slottsmøya Member An arcticid clam which constitutes nearly half of the fauna recovered from the seep assemblages. Mesosaccella M. rogovi Slottsmøya Member M. toddi 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 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 Goniomya G. literata Slottsmøya Member Rare
Serpulids Serpulids from the Agardhfjellet Formation Genus Species Location Member Material Notes Images Propomatoceros P. sp. Slottsmøya Member A serpulid tube worm 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
Gastropods Gastropods from the Agardhfjellet Formation Genus Species Location Member Material 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
See also List of fossiliferous stratigraphic units in Norway Tithonian formations Berriasian formations
References
Bibliography