agnostid trilobites from the ordovician of jämtland, sweden
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Agnostid trilobites from the Ordovician ofJmtland, SwedenPer AhlbergPublished online: 06 Jan 2010.
To cite this article: Per Ahlberg (1988) Agnostid trilobites from the Ordovician of Jmtland, Sweden,Geologiska Freningen i Stockholm Frhandlingar, 110:3, 267-278, DOI: 10.1080/11035898809455451
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Agnostid trilobites from the Ordovician of Jamtland, Sweden PER AHLBERG
Ahlberg, P., 198809 15: Agnostid trilobites from the Ordovician of Jamtland, Sweden. Geologiska Foreningens i Sfockholrn Forhandlingar, Vol. 110, Pt. 3, pp. 261-278. Stockholm. ISSN 0016-786X. Agnostid trilobites from the Lower and hliddle Ordovician in Jamtland, central Sweden, are described. The following taxa are recognized: Arfhrorhachis elspefhi Raymond, 1925, Geragnosfirs spp. aff./abiurs (Billings, 1865), G. cf. clusus Whittington, 1963. and G. spp. The stratigraphical range of Arlhrorhachis elspefhi in the Southern Appalachians and in Jamtland is discussed, and it is concluded that all its known occurrences are in Llandeilian-lower Caradocian strata. Trinodirs aniiafus Thorslund; 1940, from the Cara- docian Dalby Limestone in the centraI Lockne area, Jamtland, is considered to be a sub- jcctive junior synonym of Arrhrorhachir elspefhi. 0 Trilobifa. Agnosfida, Afetagnostidae. Arthrorhachis, Geragnostus, laxonoriiy, biosfrafigraphy, Ordovician, Caledonian Fronf. Jariifland, Sweden. Per Ahlberg, Avd. f i r historisk geologi och paleontologi, Solvegatan 13, S-223 62 Lund. Sweden; 11 Decernber 1987. Afanicscripf received 14 January 1988. revised 7 June 1988.
Despite their decline in diversity and abundance after the Cambrian, agnostid trilobites are characteristic elements of many Ordovician shelly faunas. It has become apparent that sev- eral Ordovician species are geographically wide- spread and, as in the Cambrian, their distri- bution may have been largely independent ofthe biogeographical differentiation of the benthic faunas. Agnostids thus would seem to have po- tential for providing additional corroboration of time-correlation between sequences in differ- ent biogeographic provinces.
Agnostid trilobites were first reported from the Ordovician of Jamtland, central Sweden, by Wiman (1 893), who noted their presence in beds corresponding to the Middle Ordovician Seger- stad and Dalby Limestones. Subsequently, Thorslund (1 940) described Trinodiis annatiis (-Artfirorhachis elspethi Raymond, 1925) from the Dalby Limestone in the central Lockne area, and Tjernvik (1 956, p. 170) recognized Gerag- nostus wirnani Tjernvik, 1956 from the Are- nigian Megistaspis planilirnbata Zone at Klox- Asen (KlocksAsen), about 15 km southwest of Brunflo, south-central Jamtland. Additional material and the material of Thorslund (1 940) are redescribed in this paper.
Geological setting Ordovician sedimentary rocks are widely distri- buted in Jamtland (see Jaanusson & Karis 1982;
Jaanusson et al. 1982, and Karis 1982 for recent syntheses). The deposits belong to two distinct tectonic settings: the thin carbonate-dominated autochthonous platform successions in the east, and the generally much thicker and largely silici- clastic sequences of the allochthonous Caledon- ides in the west (e.g. Thorslund 1960a; Karis in Gee& Kumpulainen 1980, fig. 8; Fig. 1 herein).
The autochthonous succession crops out in a narrow belt along the Caledonian thrust front (Fig. 2). The Ordovician sequence of this belt rests unconfonnably on Cambrian rocks or locally on the Precambrian crystalline basement (Thorslund 1960a; Jaanusson et al. 1982; Lindstrom et al. 1983). No Tremadoc rocks have been recorded in the autochthonous sequence and the earliest Ordovician rocks belong to the lower Arenig (Tjernvik 1956). The Ordovician sequence consists predominantly of carbonate rocks with some intercalated shale units, and extends upwards into the lower Ashgill (Karis iri Gee & Kumpulainen 1980, fig. 8; Jaanusson & Karis 1982, fig. 1). Detailed biostratigraphical studies on the autochthonous Ordovician have been presented by Thorslund (1940), Larsson (1 973 , and Lofgren (1 978).
The Ordovician successions of the allochthon- ous Caledonides are much affected by folding and thrusting, and belong to a major tcctonic unit, referred to as the Lower Allochthon. In the Ordovician there is a general facies change from limestones in the east via shales to greywackes
268 PerAhlberg GFF I I0 (1 988)
Autochthon Lower Allochthoi Baltoscandian
stages Lockne area Bay of Brunllo Anderson area
Dlcranograptus cllnganl I-
: an I I I I I Kyrk6s Quartzite Kogsta om Limestone I I b r a Formation
I Limeslone ..--.-..-
Helen Limestone Kundan
Dldymograptus hlrundo I . I _ , , _ . _ _ _ :-.. YUlnnUVld l l ,anna Limestone
---------- +r Phyllograptus Toyen ang. dongatus I-? ('Lower Didymograptus') Toyen Phyllogrsptus
densus I Billingenian ~
T e t r s g r a p t u r 1 h yllo grsp t 0 Id.;
~ Latorp Limestone
Hunnebergian - I Fig. 1. The Arenig-Ashgill stratigraphy in some selected areas of Jamtland, and stratigraphic ranges of agnos- tid trilobites. For the occurrences of agnostids, an open symbol indicates the approximate stratigraphic po- sition, while a filled,symbol represents detailed sampling. Diagonal shading indicates breaks in the scquence. Stratigraphy after Jaanusson & Karis (1982, fig. 1).
CFF llO(1988) Ordovician agnostid trilobites 269
Fig. 2. General geologi- cal map of the Cale- donian Front in south- central Jamtland, and localities referred to in the text. Based on Strom- berg et al. ( 1984).
in the west (e.g. Thorslund 1960a; Jaanusson 1982, p. 7, fig. 3; Karis 1982). With respect to biofacies, the Ordovician of the Lower Alloch- thon shares several features with the Olso Re- gion in Nonvay (Jaanusson 1982, p. 9).
Systematic paiaeontology Teriitiitology. - The descriptive terms used herein are essentially those advocated by dpik (1967, pp. 52-62, fig. 1 3 , Shergold (1 975, pp. 39-44, fig. 14), and Robison (1982, pp. 134- 135, text-fig. 2). The terms rhachis and dorsal furrow are preferred to axis and axial furrow. The glabella is taken to exclude the ba- sal lobes and the occipital band. The symbols amplifying the information in the synonymy list areexplainedbyMatthews(1973,pp. 717-718).
Repositories. - The illustrated specimens are housed in the type collections of the Geological Survey of Sweden (Sserigesgeologiska tutdersok- ning, SGU), Uppsala, and the Palaeontological Institute, University of Lund (LO).
Genus Arthrorhachis Hawle & Corda, 1847 Sysfenrafic posifion. - Class Trilobita Walch, 1771; order Agnostida Salter, 1864 (=Miomera Jaekel, 1909); suborder Agnostina Salter, 1864; family Metagnostidae Jaekel, 1909.
Type species. - Baftirs tardiis Barrande, 1846, from the Kriltiv Dvdr Formation (Ashgill) of Libomysl, near Zdice, Czechoslovakia.
Remarks. - I follow Fortey (1980) in restricting the genus Trinodirs to the holotype of the type species, T. agnosfiforniis M'Coy, 1846.
Artlirorhachis elspethi Raymond, 1925 Fig. 3A-G.
Synotiynry. - 0 * 1925 Artlirorhachis elspef hi, sp. nov. - Raymond, pp. 13-14, PI. 1, figs. 2-4. 12. 0 v. 1940 Trinodrrs arntafiis n. sp. - 0 1926 AgIlOSfZIS SP. - Butts, p. 102, P1. 19. fig.