Gastropods from the Late Jurassic-Early Cretaceous seep deposits in Spitsbergen, Svalbard.

Eleven gastropod species from seven latest Jurassic-earliest Cretaceous hydrocarbon seep deposits from central Spitsbergen, Svalbard are described and illustrated. Six new species and one new genus are introduced. Sassenfjordia gen. nov. is tentatively classified as a naticoid and is characterized by a naticiform gross teleoconch morphology and a large protoconch ornamented with spiral ribs, similar to those present in the subfamily Sininae. The patellogastropod ?Pectinodonta borealis sp. nov. represents the oldest occurrence of pectinodontid limpets in chemosynthesis-based associations. The colloniid Hikidea svalbardensis sp. nov. is the oldest confirmed occurrence of this genus so far. The eucyclid Eucycloidea bitneri sp. nov. is the second report of an occurrence of this genus at hydrocarbon seeps (both of which come from the Paleo-Barents Sea). Abyssomelania sp. is the third known, and the oldest occurrence of this genus. Hudlestoniella hammeri sp. nov. represents another occurrence of this genus at seeps in the Paleo-Barents Sea; its current higher-rank assignment is reviewed herein. It may belong either to the rissoids, as previously suggested, or to the abyssochrysoids (or zygopleurids). A fossil identified as Cretadmete sp. is here classified in the Purpurinidae, a family of stem neogastropods or tonnoideans. The heterobranch Hyalogyrina knorringfjelletensis sp. nov. is the oldest occurrence of hyalogyrinids in chemosynthesis-based associations. Several oldest occurrences of seep taxa in moderately shallow-water seeps in Spitsbergen suggest that the previously questioned onshore-offshore hypothesis of seep and vent colonization should be carefully re-investigated.

Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard.

The bivalve fauna from the latest Jurassic-earliest Cretaceous hydrocarbon seep deposits from central Spitsbergen, Svalbard comprises at least 17 species, four of which belong to chemosymbiotic taxa often found at seeps. These are the solemyid Solemya (Petrasma) cf. woodwardiana; Nucinella svalbardensis sp. nov., which belongs to a group of large Nucinella species known from seeps and deep water environments; the lucinid bivalve, Tehamatea rasmusseni sp. nov., included in a genus widely distributed in other Jurassic-Cretaceous seeps; and Cretaxinus hurumi gen. et sp. nov., which is the oldest known thyasirid and is discussed in relation to other large seep-restricted genera in this family. The remaining species in the fauna belong to 'background' genera known from coeval normal marine sediments, mostly from the Boreal area. These include the nuculid Dacromya chetaensis, two new malletiids (Mesosaccella rogovi sp. nov. and M. toddi sp. nov.), the oxytomiid Oxytoma octavia, at least three Buchia species, at least two pectinids, including Camptonectes (Costicamptonectes) aff. milnelandensis and Camptonectes (Camptochlamys) clatrathus, the limid Pseudolimea arctica, the arcticid Pseudotrapezium aff. groenlandicum, and the pholadomyid Goniomya literata. The large number of 'background' species in the bivalve fauna is probably a reflection of the shallow-water setting of the Svalbard seeps. This might also explain the lack of the seep-restricted modiomorphid bivalve Caspiconcha from the fauna. With solemyids, Nucinella, lucinids and thyasirids, the latest Jurassic-earliest Cretaceous bivalve seep fauna of Svalbard contains typical representatives of the Mesozoic bivalve seep faunas, both long established and young evolutionary colonists.

Brachiopods from Late Jurassic-Early Cretaceous hydrocarbon seep deposits, central Spitsbergen, Svalbard.

Late Jurassic-Early Cretaceous (Late Volgian-latest Ryazanian) rhynchonellate brachiopods are described from eight out of 15 hydrocarbon seep deposits in the Slottsmøya Member of the Agardhfjellet Formation in the Janusfjellet to Knorringfjellet area, central Spitsbergen, Svalbard. The fauna comprises rhynchonellides, terebratulides (terebratuloids and loboidothyridoids) and a terebratellidine. The rhynchonellides include: Pseudomonticlarella varia Smirnova; Ptilorhynchia mclachlani sp. nov.; and Ptilorhynchia obscuricostata Dagys. The terebratulides belong to the terebratuloids: Cyrtothyris? sp.; Cyrtothyris aff. cyrta (Walker); Praelongithyris? aff. borealis Owen; and the loboidothyridoids: Rouillieria cf. michalkowii (Fahrenkohl); Rouillieria aff. ovoides (Sowerby); Rouillieria aff. rasile Smirnova; Uralella? cf. janimaniensis Makridin; Uralella? sp.; Pinaxiothyris campestris? Dagys; Placothyris kegeli? Harper et al.; and Seductorithyris septemtrionalis gen. et sp. nov. The terebratellidine Zittelina? sp. is also present. Age determinations for all but one of the brachiopod-bearing seeps are based on associated ammonites. Five of the seep carbonates have yielded Lingularia similis?, and it is the only brachiopod species recorded from two of the seeps. Other benthic invertebrate taxa occurring in the seeps include bivalves, gastropods, echinoderms, sponges, and serpulid and non-serpulid worm tubes. The brachiopod fauna has a strong Boreal palaeobiogeographic signature. Collectively, the Spitsbergen seep rhynchonellate brachiopods exhibit high species richness and low abundance (<100 specimens from 8 seeps). This contrasts markedly with other Palaeozoic---Mesozoic brachiopod-dominated seep limestones where brachiopods are of low diversity (typically monospecific) with a super-abundance of individuals. The shallow water environmental setting for the Spitsbergen seeps supported a diverse shelf fauna, compared to enigmatic Palaeozoic-Mesozoic brachiopod-dominated seeps.