Agnostid trilobites from the Ordovician of Jämtland, Sweden

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<ul><li><p>This article was downloaded by: [Purdue University]On: 29 August 2014, At: 02:45Publisher: Taylor &amp; FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office:Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK</p><p>Geologiska Freningen i StockholmFrhandlingarPublication details, including instructions for authors and subscriptioninformation:</p><p>Agnostid trilobites from the Ordovician ofJmtland, SwedenPer AhlbergPublished online: 06 Jan 2010.</p><p>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</p><p>To link to this article:</p><p>PLEASE SCROLL DOWN FOR ARTICLE</p><p>Taylor &amp; Francis makes every effort to ensure the accuracy of all the information (the Content)contained in the publications on our platform. However, Taylor &amp; Francis, our agents, and ourlicensors make no representations or warranties whatsoever as to the accuracy, completeness, orsuitability for any purpose of the Content. Any opinions and views expressed in this publicationare the opinions and views of the authors, and are not the views of or endorsed by Taylor &amp;Francis. The accuracy of the Content should not be relied upon and should be independentlyverified with primary sources of information. Taylor and Francis shall not be liable for anylosses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilitieswhatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.</p><p>This article may be used for research, teaching, and private study purposes. Any substantialor systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, ordistribution in any form to anyone is expressly forbidden. Terms &amp; Conditions of access and usecan be found at</p><p></p></li><li><p>Agnostid trilobites from the Ordovician of Jamtland, Sweden PER AHLBERG </p><p>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. </p><p>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. </p><p>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. </p><p>Geological setting Ordovician sedimentary rocks are widely distri- buted in Jamtland (see Jaanusson &amp; Karis 1982; </p><p>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&amp; Kumpulainen 1980, fig. 8; Fig. 1 herein). </p><p>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 &amp; Kumpulainen 1980, fig. 8; Jaanusson &amp; Karis 1982, fig. 1). Detailed biostratigraphical studies on the autochthonous Ordovician have been presented by Thorslund (1940), Larsson (1 973 , and Lofgren (1 978). </p><p>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 </p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Purd</p><p>ue U</p><p>nive</p><p>rsity</p><p>] at</p><p> 02:</p><p>45 2</p><p>9 A</p><p>ugus</p><p>t 201</p><p>4 </p></li><li><p>268 PerAhlberg GFF I I0 (1 988) </p><p>Graptolite zones </p><p>Autochthon Lower Allochthoi Baltoscandian </p><p>stages Lockne area Bay of Brunllo Anderson area </p><p>Dlcranograptus cllnganl I- </p><p>: an I I I I I Kyrk6s Quartzite Kogsta om Limestone I I b r a Formation </p><p>Dldymograptus murchlsonl </p><p>Dldymograptus artus </p><p>I </p><p>I Limeslone ..--.-..- </p><p>Helen Limestone Kundan </p><p>Dldymograptus hlrundo I . I _ , , _ . _ _ _ :-.. YUlnnUVld l l ,anna Limestone </p><p>---------- +r Phyllograptus Toyen ang. dongatus I-? ('Lower Didymograptus') Toyen Phyllogrsptus </p><p>densus I Billingenian ~ </p><p>Dldymograptus baltlcus </p><p>T e t r s g r a p t u r 1 h yllo grsp t 0 Id.; </p><p>Shale Shale </p><p>~ Latorp Limestone </p><p>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 &amp; Karis (1982, fig. 1). </p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Purd</p><p>ue U</p><p>nive</p><p>rsity</p><p>] at</p><p> 02:</p><p>45 2</p><p>9 A</p><p>ugus</p><p>t 201</p><p>4 </p></li><li><p>CFF llO(1988) Ordovician agnostid trilobites 269 </p><p>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). </p><p>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). </p><p>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). </p><p>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). </p><p>Genus Arthrorhachis Hawle &amp; Corda, 1847 Sysfenrafic posifion. - Class Trilobita Walch, 1771; order Agnostida Salter, 1864 (=Miomera Jaekel, 1909); suborder Agnostina Salter, 1864; family Metagnostidae Jaekel, 1909. </p><p>Type species. - Baftirs tardiis Barrande, 1846, from the Kriltiv Dvdr Formation (Ashgill) of Libomysl, near Zdice, Czechoslovakia. </p><p>Remarks. - I follow Fortey (1980) in restricting the genus Trinodirs to the holotype of the type species, T. agnosfiforniis M'Coy, 1846. </p><p>Artlirorhachis elspethi Raymond, 1925 Fig. 3A-G. </p><p>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. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Purd</p><p>ue U</p><p>nive</p><p>rsity</p><p>] at</p><p> 02:</p><p>45 2</p><p>9 A</p><p>ugus</p><p>t 201</p><p>4 </p></li><li><p>210 PerAhlberg GFF I I0 (1988) </p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Purd</p><p>ue U</p><p>nive</p><p>rsity</p><p>] at</p><p> 02:</p><p>45 2</p><p>9 A</p><p>ugus</p><p>t 201</p><p>4 </p></li><li><p>GFF I I0 ( I 988) Ordosician agnostidtrilobites 27 1 </p><p>Thorslund, p. 127, PI. 9, fig. 9. 0.1941 Arth- rorltacliis elspethi Raymond - Butts, p. 71, PI. 8 1, figs. 1 1 - 14. .I94 1 Arthrorhacliis cf. A. els- pethi Raymond - Butts, p. 132, PI. 101, figs. 23-26.0 .I946 Arthrorhachis elspethi Raymond - Cooper &amp; Cooper, PI. 3, figs. 3-6. 0 .I953 Trinodtrs elspethi (Raymond) - Cooper, pp. 7-8, PI. 1 , figs. 1-12. 0 ?I965 Trinodirsdoirlargensis sp. nov. - Tripp, pp. 578-579, PI. 80, figs. 1-4. 0 .I967 Triiiodirs elspethi (Raymond, 1925) - Hunt, pp. 203-208, PI. 22, figs. 1-47 .0 ?I 982 Triiiodtrs eIspethi (Raymond) - Koroleva, pp. 28-29, PI. 1 , figs. 16-17. 0 .I985 Artlirorhachis elspetki (Cooper) - Thomas &amp; Fortey, p. 210, PI. 9.5.5. </p><p>Type data. - The synrypes (Harvard Museum of Comparative Zoology 1626 and 1627) from the Effna Limestone on the Thomas Farm, about 5 km east of Blacksburg, Virginia, were figured by Raymond (1925, P1. 1, figs. 2-3). </p><p>Material. - One complete but partially ex- foliated cephalon and one incomplete cephalon from the centraI Lockne area, collected by Per Thorslund in 1935. Internal moulds of a nearly complete cephalon and an incomplete pygidium from Norderon, collected by Gustaf C. von Schmalensee in 1884. One partially exfoliated cephalon from Onsveda, collected by Per Thorslund in 1936. Internal moulds of a flat- tened cephalon and a nearly complete pygidium (Fig. 4A-B), collected by G. C. von Schmalensee in 1884 from a loose boulder of Chasmops Limestone at Pilgrimstad, are tentatively as- signed to the species. </p><p>Diagnosis. - A species of Arthrorliachis charac- terized by a subquadrate cephalon with the maximum width at the antero-lateral comers, well-developed cephalic fulcra1 spines, and well- defined dorsal and border furrows. Four pairs of distinct muscle insertion areas are present on the glabella, evident both on the parietal and the dorsal exoskeletal surface. At the postero-lateral portion of the genae there is an elongate genal area where the exoskeleton is thinner. The </p><p>pygidium is provided with a tapered rhachis and prominent postero-lateral spines, commencing on a transverse line well behind the rhachis. </p><p>Description. - The cephalon is moderately con- vex, subquadrate in outline, and slightly wider than long (maximum width at the antero-lateral corners). The highest point of the cephalon is at the posterior part of the glabella. The glabella, occupying about 62-64 Oh of the sagittal cephalic length, is well defined by deeply impressed dor- sal furrows, gently tapered forward, and modera- tely rounded in front. It is faintly constricted at about mid-length. A median node is situated slightly anterior to the midpoint of the glabella. Transversely, the glabella is highly convex; sagit- tally it is moderately convex. In dorsal view the glabellar rear is obtusely angulate. In front of the occipital band there is a narrow median ridge, extending up the slope from the occipital furrow. The basal lobes are entire, wider than long, con- nected medially, and provided with transverse muscle impressions medially. Three pairs of muscle insertion areas are visible on the glabella in a complete cephalon (SGU Type 26; Fig. 3A). The arrangement andbutline of the muscle impression areas are of typical Arthrorhachis/ Geragnostirs type (see Fortey 1980, fig. 4). The two anterior sets are expressed on the parietal surface, flank the median glabellar node, and correspond to 5P and 6P of Fortey (1980, fig. 4). Their margins are deepened on the internal mould. The posterior set (2P) consists of ellipti- cal impressions on the external surface of the exoskeleton with the long axes directed postero- laterally from the mid-line of the glabella. The glabella is not preserved well enough to express 3P and 4P. </p><p>The acrolobe is evenly rounded. The genae are equal in width anteriorly and laterally, and slope steeply downwards; most steeply laterally. The border is moderately wide, convex, and sep- arated from the genae by a well defined border furrow. It is widest antero-laterally, expanding in width from the posterior border, and slightly narrowing adaxially again towards the mid-line. Postero-laterally, the border is partly tucked </p><p>Fig. 3. Arthrorhachis elspelhi Raymond, 1925. I3 A-C. Partially exfoliated cephalon. 1.2 km W of the Tands- byn raiIway station, the central Lockne area. Upper Dalby Limestone. SGU Type 26. Figured by Thorslund (1940, PI. 9, fig. 9). (I A. Dorsal view. x16. 0 B. Right lateral view. x16. 0 C. Anterior view. X I S . 0 D.Inter- nal mould of an incomplete cephalon. 1.2 krn W of the Tandsbyn railway station, the central Lockne area. Upper Dalby Limestone. SGU Type 6099. ~ 1 4 . 0 E. Partially exfoliated and slightly distorted cephalon. uns- Veda about 3 km S of Sunne church. Anderso Shale. SGU Type 6100. x14. 0 F. Internal mould of a nearly complete cephalon. Norderon. Basal part of Kogsta Siltstone or uppermost Dalby beds. SGU Type 6101. x17. 0 G. Internal mould of an incomplete pygidium. Norderon. Basal part of Kogsta Siltstone or uppermost Dalby beds. SGU Type 6102. x19. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Purd</p><p>ue U</p><p>nive</p><p>rsity</p><p>] at</p><p> 02:</p><p>45 2</p><p>9 A</p><p>ugus</p><p>t 201</p><p>4 </p></li><li><p>272 Per Ahlberg GFF llO(1988) </p><p>beneath the genae. Sagittally, the border and border furrow occupies about 12 O/o of the total cephalic length. The posterior border is sepa- rated from the genae by a deeply incised...</p></li></ul>