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Hansen, 2009

Trilobites of the Middle Ordovician Elnes Formation of the Oslo Region, Norway

Hansen, T.
AjakiriFossils and Strata


The Eines Formation, which succeeds the Baltoscandian ‘Orthoceratite limestone, can be traced all over the Oslo Region. The largely mudstone-dominated formation, which in places has been heavily faulted and folded during the Caledonian Orogeny, reaches thicknesses of nearly 100 m in the central Oslo Region and possibly significantly more in the northern part of the region, where it extends higher in the stratigraphical record. Sediments of the Eines Formation were laid down in what may possibly have been a foreland basin on the north-western margin of the isolated continent Baitica and where conditions were generally quiet and periods of slightly dysoxic bottom conditions pre- vailed around or somewhat below normal storm wave base. The fairly rich trilobite fauna mainly occurs in the lower and upper part of the formation, deposited in slightly more oxic waters at or above storm wave base. The lower part of the Eines Formation in the northern Mjøsa district is considered equivalent to the Heggen Member further south, while the middle part is regarded as a possible distal variety of the Engervik Member from the central Oslo Region. Based on the biostratigraphic results from a number of sections of the Eines Formation throughout the Oslo Region, the boundaries between various members have been defined more accurately, resulting in some adjustment to previous work. Of the close to 100 trilobite taxa known from the Eines Formation only four seem to have immigrated from elsewhere, two of which have a clear Laurentian origin. All four species are restricted to the northern Mjøsa district. Inferences on the autecology of the most common trilobite groups are summarised. Systematic descrip- tions are presented of Ampyx, Asaphus, Atractopyge, Botrioides, Bronteopsis, Cnemi- dopyge, Cybellela, Cybelurus, Cyrtometopus, Geragnostus, Gravicalymene, Icelorobergia n. gen. (type species Robergiella brevilingua Fortey 1980), Illaenus, Lonchodomas, Megistaspis, Metopolichas, Nileus, Niobe, Ogmasaphus, Ogygiocaris, Pliomera, Porterfiel- dia, Primaspis, Proetus, Pseudasaphus, Pseudobasilicus, Pseudomegalaspis, Pterygometo- pus, Robergia, Scotoharpes, Sculptaspis, Sculptella, Sphaerocoryphe, Sthenarocalymene, Telephina, Raymondaspis (Cyrtocybe) and Volchovites. Nine new taxa are described: Asaphus narinosus n. sp., A. raaenensis n. sp., Lonchodomas cuspicaudus n. sp., Megistaspis (Μ.) gigantens runcinatus n. ssp., Ogmasaphus furnensis n. sp., Ogygiocaris isodilatata n. sp., Ogygiocaris henningsmoeni n. sp., Pseudasaphus limatus longistriatus n. subsp. and P. (P.) truncatus n. sp. The asaphid subgenus Megistaspis (Heraspis) Wandas is considered a junior subjective synonym of Μ. (Megistaspidella) Jaanusson. The genus Ogygiocaris is totally revised and all former subspecies of Ogygiocaris sarsi Angelin are assigned species rank. Ogygiocaris regina Henningsmoen is regarded a junior subjective synonym of O. lata Hadding, reducing the number of taxa to seven species and two subspecies. The trinucleid Botrioides efflorescent Hadding occurs before B. foveolatus Angelin, and is considered its ancestor. Atractopyge dentata is redescribed and its range reduced, and Upper Ordovician specimens from Norway and the United Kingdom are reassigned to Cybellela grewingki (Schmidt). Lonchodomas striolatus Månsson and Ampyx clavifrons Hadding are considered subjective synonyms of L. rostratus Sars and A. mammilatus Sars, respectively. The two genera Valdaites Balashova, 1976 and Mischynogorites Balashova, 1976 are both regarded as junior subjective synonyms of Pseudasaphus Schmidt, 1904.

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