Trilobites from the Middle and Upper Ordovician Andersön Shale Formation in Jämtland and the equivalent Killeröd Formation in Skåne, Sweden

Sammanfattning: Trilobites from the Middle and Upper Ordovician Andersön Shale Formation in Jämtland, central Sweden, and the Middle Ordovician Killeröd Formation in Skåne (Scania), southern Sweden, are revised and discussed. These formations have several genera and a few species in common, e.g. Botrioides efflorescens (Hadding, 1913a) and Pseudomegalaspis patagiata (Törnquist, 1884). The regional and vertical distribution of the trilobite faunas are also discussed. The Killeröd Formation is very thin, about 70 cm thick (at the type locality), and is situated within the Scanian Lithofacies Belt. The formation is found only in SE Skåne and consists of alternating limestones and shaly mudstone beds. It is richly fossiliferous and has yielded a fairly extensive fauna consisting mainly of trilobites, but also of brachiopods, conodonts, and ostracodes. The trilobite fauna is dominated by the trinucleid trilobites Botrioides bronnii (Boeck, 1838) and B. efflorescens. The Killeröd Formation belongs to the upper part of the Hustedograptus teretiusculus Zone, and it is considered to be an equivalent of the lower part of the Andersön Shale. The Andersön Shale of Jämtland is situated within the Caledonian Front and belongs to the Lower Allochthon. It can usually be divided into a lower and an upper part. At the type locality, the lower part consists of 7 m of faulted and folded grey shales, which are nodular in the upper 5 m. On top of this is 1.5 m of dark grey bedded Biseriata Limestone, characterized by the trilobite Telephina biseriata (Asklund 1936a). The upper part of the Andersön Shale is more than 10 m thick and consists of dark grey shales with sandstone intercalations and lenses of limestone. The lower part of the formation is assigned to the Hustedograptus teretiusculus Zone, whereas the upper part of the Andersön Shale extends from the basal Nemagraptus gracilis Zone well up to Diplograptus multidens Zone. The Andersön Shale is generally richly fossiliferous, with both benthic and pelagic elements represented. The macrofauna found on Andersön includes trilobites, bivalves, gastropods, nautiloids, brachiopods and graptolites. The total number of species is almost three times as many in the lower part of the Andersön Shale as in the upper part of the formation. Five olenid, one dionidid and six raphiophorid trilobites were described in detail. Four of these were new: Triarthrus latissimus Månsson, 1998a, Dionide jemtlandica Månsson, 2000, Ampyxoides minor Månsson, 2000, and Lonchodomas striolatus Månsson, 2000. Almost all stages of growth of T. latissimus, from the anaprotaspid stage and onwards, are represented and the ontogeny was described. Meraspid and early holaspid material of D. jemtlandica was also described. The Andersön Shale Formation is a variable unit both with respect to the faunal content and the lithology. Trilobite associations from three localities in Jämtland (Andersön, Marielund and the NW shore of Frösön) are discussed. Comparisons are made with trilobite associations of different ages elsewhere and provide some insights into the palaeoenvironments represented in the Andersön Shale. Two clearly distinguishable trilobite associations are found: a raphiophorid association and an olenid association. A palaeogeographical model is proposed for the disposition of the associations. The composition of the trilobite faunas from the Andersön Shale in Jämtland suggests deposition in fairly deep water environments, probably on the outer shelf.