GFF Agnostid trilobites from the Ordovician of ...

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Agnostid trilobites from the Ordovician of Jämtland, Sweden Per Ahlberg

Online publication date: 06 January 2010

To cite this Article Ahlberg, Per(1988) 'Agnostid trilobites from the Ordovician of Jämtland, Sweden', GFF, 110: 3, 267 —

278

To link to this Article: DOI: 10.1080/11035898809455451 URL: http://dx.doi.org/10.1080/11035898809455451

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Agnostid trilobites from the Ordovician of Jamtland, Sweden

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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: Arfhrorhachiselspefhi Raymond, 1925, Geragnosfirsspp. 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 aniiafusThorslund; 1940, from the Caradocian Dalby Limestone in the centraI Lockne area, Jamtland, is considered to be a subjcctive 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. Afanicscripfreceived 14 January 1988. revised 7 June 1988.

Despite their decline in diversity and abundance Jaanusson et al. 1982, and Karis 1982 for recent after the Cambrian, agnostid trilobites are syntheses). The deposits belong to two distinct characteristic elements of many Ordovician tectonic settings: the thin carbonate-dominated shelly faunas. It has become apparent that sev- autochthonous platform successions in the east, eral Ordovician species are geographically wide- and the generally much thicker and largely silicispread and, as in the Cambrian, their distri- clastic sequences of the allochthonous Caledonbution may have been largely independent ofthe ides in the west (e.g. Thorslund 1960a; Karis in biogeographical differentiation of the benthic Gee& Kumpulainen 1980, fig. 8; Fig. 1 herein). The autochthonous succession crops out in a faunas. Agnostids thus would seem t o have potential for providing additional corroboration narrow belt along the Caledonian thrust front of time-correlation between sequences in differ- (Fig. 2). The Ordovician sequence of this belt rests unconfonnably on Cambrian rocks or ent biogeographic provinces. Agnostid trilobites were first reported from locally on the Precambrian crystalline basement the Ordovician of Jamtland, central Sweden, by (Thorslund 1960a; Jaanusson et al. 1982; Wiman (1 893), who noted their presence in beds Lindstrom et al. 1983). No Tremadoc rocks have corresponding to the Middle Ordovician Seger- been recorded in the autochthonous sequence stad and Dalby Limestones. Subsequently, and the earliest Ordovician rocks belong to the Thorslund (1 940) described Trinodiis annatiis lower Arenig (Tjernvik 1956). The Ordovician (-Artfirorhachis elspethi Raymond, 1925) from sequence consists predominantly of carbonate the Dalby Limestone in the central Lockne area, rocks with some intercalated shale units, and and Tjernvik (1 956, p. 170) recognized Gerag- extends upwards into the lower Ashgill (Karis nostus wirnani Tjernvik, 1956 from the Are- iri Gee & Kumpulainen 1980, fig. 8; Jaanusson nigian Megistaspis planilirnbata Zone at Klox- & Karis 1982, fig. 1). Detailed biostratigraphical Asen (KlocksAsen), about 15 k m southwest of studies on the autochthonous Ordovician have Brunflo, south-central Jamtland. Additional been presented by Thorslund (1940), Larsson material and the material of Thorslund (1 940) (1 9 7 3 , and Lofgren (1 978). The Ordovician successions of the allochthonare redescribed in this paper. ous Caledonides are much affected by folding and thrusting, and belong to a major tcctonic Geological setting unit, referred to as the Lower Allochthon. In the Ordovician sedimentary rocks are widely distri- Ordovician there is a general facies change from buted in Jamtland (see Jaanusson & Karis 1982; limestones in the east via shales to greywackes

268 PerAhlberg

GFF I I0 (1 988)

Graptolite zones

Baltoscandian stages

:

Autochthon

Lockne area

an

Anderson area

I I I I I Kyrk6s Quartzite

Kogsta

I

I- I

Dlcranograptus cllnganl


Lower Allochthoi

Bay of Brunllo

om Limestone

b r a Formation

I

..--.-..-

I

Limeslone

Dldymograptus murchlsonl

Dldymograptus

artus

Kundan

+r Dldymograptus hlrundo

I

.Y I _U , , _l. n _ _n _ U V l:d -.l.l

I

Limestone

Toyen

Billingenian

~

Dldymograptus baltlcus Tetrsgraptur

,anna

----------

Phyllograptus ang. dongatus Phyllogrsptus densus

Helen Limestone

Hunnebergian

Toyen

Shale

-

Latorp Limestone

~

1h yllo grsp t 0 Id.;

I-?

('Lower Didymograptus') Shale

I

Fig. 1. The Arenig-Ashgill stratigraphy in some selected areas of Jamtland, and stratigraphic ranges of agnostid trilobites. For the occurrences of agnostids, an open symbol indicates the approximate stratigraphic position, 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 geological map of the Caledonian Front in southcentral Jamtland, and localities referred to in the text. Based on Stromberg 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 Allochthon shares several features with the Olso Region 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 basal 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 tutdersokning, SGU), Uppsala, and the Palaeontological Institute, University of Lund (LO).

Genus Arthrorhachis Hawle & Corda, 1847 Sysfenraficposifion. - 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. agnosfiforniisM'Coy, 1846.

Artlirorhachis elspethi Raymond, 1925 Fig. 3A-G. Synotiynry. - 0 * 1925 Artlirorhachis elspefhi, sp. nov. - Raymond, pp. 13-14, PI. 1, figs. 2-4. 0 1926 AgIlOSfZISSP. - Butts, p. 102, P1. 19. fig. 12. 0 v. 1940 Trinodrrs arntafiis n. sp. -


210 PerAhlberg

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Thorslund, p. 127, PI. 9, fig. 9. 0.1941 Arthrorltacliis elspethi Raymond - Butts, p. 71, PI. 8 1, figs. 1 1- 14. .I94 1 Arthrorhacliis cf. A. elspethi Raymond - Butts, p. 132, PI. 101, figs. 23-26.0 .I946 Arthrorhachiselspethi Raymond - Cooper & 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 - 4 7 . 0 ?I 982 Triiiodtrs eIspethi (Raymond) - Koroleva, pp. 28-29, PI. 1, figs. 16-17. 0 .I985 Artlirorhachis elspetki (Cooper) - Thomas & Fortey, p. 210, PI. 9.5.5.

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). Material. - One complete but partially exfoliated cephalon and one incomplete cephalon from the centraI Lockne area, collected by Per Thorslund in 1935. Internal moulds of a nearly complete cephalon and a n 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 flattened 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 assigned to the species. Diagnosis. - A species of Arthrorliachis characterized by a subquadrate cephalon with the maximum width at the antero-lateral comers, well-developed cephalic fulcra1 spines, and welldefined 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 a n elongate genal area where the exoskeleton is thinner. The

Ordosician agnostidtrilobites 27 1

pygidium is provided with a tapered rhachis and prominent postero-lateral spines, commencing on a transverse line well behind the rhachis.

Description. - The cephalon is moderately convex, 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 dorsal furrows, gently tapered forward, and moderately 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; sagittally 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, connected 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 5 P and 6P of Fortey (1980, fig. 4). Their margins are deepened on the internal mould. The posterior set (2P) consists of elliptical impressions on the external surface of the exoskeleton with the long axes directed posterolaterally from the mid-line of the glabella. The glabella is not preserved well enough to express 3P and 4P. 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 separated 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

Fig. 3. Arthrorhachis elspelhi Raymond, 1925. I3 A-C. Partially exfoliated cephalon. 1.2 km W of the Tandsbyn raiIway station, the central Lockne area. Upper Dalby Limestone. SGU Type 26. Figured by Thorslund A. Dorsal view. x16. 0 B. Right lateral view. x16. 0 C. Anterior view. X I S . 0 D.Inter(1940, PI. 9, fig. 9). (I 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 . E. 0 Partially exfoliated and slightly distorted cephalon. unsVeda 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.

272

Per Ahlberg

GFF llO(1988)


beneath the genae. Sagittally, the border and border furrow occupies about 12 O/o of the total cephalic length. The posterior border is separated from the genae by a deeply incised furrow, and produced into a distinct postcro-lateral spine. The external surface of the cephalic exoskeleton is indistinctly covered with a reticulate pattern of fine raised lines. The two pygidia at hand are specifically indistinguishable from the pygidia described and illustrated by Cooper (1953) from Virginia. The pygidial rhachis is highly convex, and provided with a prominent median ridge which is highest posteriorly.

Dinlensions (mm)

SGU Type 3.30 0.40 2.90 2.10

0.35 2.55 1.80

1.10

1.00 3.15 1.15

26

Length (sag.) ofcephalon Length (sag.) ofcephalic border(inc1. border furrow) Length(sag.)ofcephalicacrolobe Length (sag.) ofglabella Distancc(sag.) from rcarofglabella to high spot ofmedian node Maximum width(tr.)ofcephalon Maximumwidth(tr.)ofglabella

SGU Type 6101 2.90

3.55 1.35

Remarks. - Raymond’s (1 925) description of Artfirorhachis elspethi is short and his figures are vague. It is thus difficult t o recognize diagnostic features. The species was, however, redescribed by Cooper (1 953) on the basis of excellent material from the type area and localities elsewhere in Virginia and Tennessee. The ontogeny and intraspecific variability of A. elspethi is well known through the study of Hunt (1967). The specimens from Jamtland correspond well with the detailed description given by Cooper (1953) and Hunt’s (1 967) excellent illustrations of A . elspethi. The elongate genal area, figured by Cooper (1953, P1. 1, fig. 6) and Hunt (1 967, P1. 22, fig. 47) is a distinctive character in A. elspethi. Among the Jamtland material, it is present in cephala preserved with the exoskeleton (SGU Type 26; Fig. 3B). Thorslund (1 940, p. 127, PI. 9, fig. 9) named a new species, Trirtoditsarnratirs, from the Dalby Limestone in the central Lockne area, southcentral Jamtland. I have examined the holotype (SGU Type 26; Fig. 3A-C) and a topotype cephalon (SGU Type 6099; Fig. 3D) of this taxon, and becauss all characters of the type specimens agree with the diagnosis above, T. artnatits is considered t o be conspecific with Arthrorhachis elspethi. The cephalon from Pilgrimstad (Fig. 4A) is

Fig. 4. Arihrorhachis cf. elspeihi Raymond, 1925 from a loose boulder o f ‘Chasmops Limestone’ at Pilgrimstad. OA. Internal mould o f a flattened cephalon. SGU Type 6103. x 1 8 . 0 B. Internal mould o f a nearly complete pygidium. SGU Type 6104. x18.

closely comparable to Arthrorhachis elspethi. It differs mainly in having a narrower and nearly parallel-sided glabella. In addition, the lateral cephalic border is slightly wider posteriorly and not tucked beneath the gena. In these respects, it is close to A. tarda (Barrande, 1846).

Occirrretice. - Llandeilo-lower Caradoc. Southern Appalachians; south-central Jamtland, Sweden; Kazakhstan(?). Arthrorhachis elspethi is widely distributed in the Middle Ordovician of the Southern Appalachians, and it has been recorded from several localities in Virginia, Tennessee, and Alabama (Cooper 1953, p. 8, table 1). In Virginia and Tennessee it occurs in the Effna Limestone, Edinburg, Liberty Hall, and Athens Shale Formations, and also in correlative units. Conodonts recovered from these strata are indicative o f a stratigraphic interval ranging from the Pygo-


GFF 110 (1988)

dirs anserinis Zone to the Priotiiodirs gerdae Subzone (S.M. Bergstrom 1971a, and written communication, June 1987). In terms of the British-Baltoscandicgraptolite zonal succession, this interval corresponds to the upper part of the Glyptograpttts teretiitsctrlirs Zone, the Nemagrapttrs gracilis Zone, and the lower part of the Diplograptiis tnitltidens Zone (Bergstrom 19716). Thus, in terms of the British sequence the Southern Appalachian specimens of Artkrorlrachis elspethi seem to be of late Llandeilian and early Caradocian age. The species has, howvever, been reported from the Little Oak Limestone at Pratt Ferry, Alabama (Butts 1940, p. 157; cf. Cooper 1953, table 1). Conodont evidence suggest that it was recovered from a level high in the Pygodiis serra Zone or very low in the P. ariseritiirs Zone ( S . M. Bergstrom 1971a, p. 117, fig. 10, and written communication, June 1987). Hence, the base of its range in the Southern Appalachians appears to fall within strata corresponding to the lower Llandeilo, provided that thc identification of Butts ( I 940, p. 157) is correct. In Jam t la n d , A rth rorhachis elspethi has been recovered from (1) thc upper Dalby Limestone (formerly Lower Chasmops Limestone) 1.2 km W of thc Tandsbyn railway station, the central Lockne area, (2) the Anderso Shale of the Lower Allochthon at Onsveda south of Sunne, and (3) the uppermost Dalby Limestone or basal Kogsta Siltstone of the Lower Allochthon on the northern shore of Norderon, a n island in Lake Storsjon. A closely comparable form (Fig. 4A-B) is known from a loose boulder of ‘Chasmops Limestone’, found in a gravel-pit near the railway at Pilgrimstad. The specimens from the central Lockne area (see Thorslund 1940, 19606, and Lindstrom et al. 1983 for geological descriptions of the area) were probably recovered from the lower part of thc upper Dalby Limestone. In the conodont zonal scheme, this unit evidently falls within the lower Caradocian Priotiiodirs gerdae Subzone of the Aniorphogtiatliiu tvaeretisis Zone ( S . M. Bergstrom, written communication, June 1987). Thc cephalon from Onsveda was recovered from a dark grey shale (Anderso Shale; formerly Ogygiocaris Shalc) about 3 km S of Sunne church. Thc stratigraphic position within the Anderso Shale is ?ot known, and the unsveda specimen may be of Llandeilian or early Caradocian age (cf. Karis 1982, pp. 57-58). The Norderon specimens were collected from a dark grey, argillaceous limestone adjacent to “Nils Hans Husa” (caves) on thc northern shore

Ordovician agnostid trilobites 213

of the island (G.C. von Schmalensee, unpubl. ficld-notes from 1884, SGU Central Archives; stop 15 of Thorslund 19606, p. 49). The lithology indicates that they were recovered from the basal Kogsta Siltstone or the uppermost Dalby beds, and it is likely that the Norderon specimens come from erratic boulders (L. Karis, pers. comm., October 1987). Koroleva (1982, pp. 28-29, PI. 1, figs. 16- 17) dcscnbed two cephala from strata corresponding t o the middle Caradoc (Majlisor horizon) in northern Kazakhstan, and assigned them to Artlirorhachis elspethi. I cannot, however, determine from the illustrations or description whether these specimens represent A. elsperhi or a closely comparable species.

Genus Geragnostus Howell, 1935 Systematic position. - Family as for Arthroriiachis. Typespecies. - Agtiostits sidenbladiii Linnarsson, 1869 (pp. 82-83, PI. 2, figs. 60-61), from the upper Tremadocian Ceratopyge Limestone (Apatokephalus serratirs Biozone) at Mossebo, Hunneberg, Vastergotland, south-central Sweden; by original designation.

Geragttosttisspp. aff. fabizis (Billings, 1865) Fig. SA-E.

Material. - Two nearly complete cephala and one incomplete pygidium. The specimens are preserved with the exoskeleton and they were collected by Lars Karis in 1973. Description. - Thc cephalon is moderately to highly convcx, subquadrate in outline, and subequal in length and width. The glabella, occupying 61 -62 Yo of the total cephalic length, is moderately convex (sag. and tr.), rounded in front, and slightly constricted at about mid-length. In the large cephalon (SGU Type 6 105; Fig. 5A-B), the glabella is subparallel-sidc and moderately wide. In the small ccphalon (SGUType 6106; Fig. 5C-D) it is proportionately wider and tapered fonvard. A low median node is situated slightly anterior to the midpoint of the glabella. The glabellar rcar is obtusely angulate. The dorsal furrows are almost effaced and the glabella is set off from the genae mainly by change in cxoskcletal slope. The basal lobes arc entire,


274 PerAhlberg

CFF I 1 0 (1988)

Fig. 5. Geragnosftrs spp. aff. jabitu (Billings, 1865). Lisbetodlingen in the Brunflo area. 0 A-B. Cephalon. Top of Lanna Limestone, Alegisfarpis litnbafa Zone. SGU Type 6 1 0 5 . 0 A. Dorsal view. x l 2 . 0 B. Left lateral view. x 1 2 . 0 C-D. Cephalon. Basal part of Holen Limestone, Asaphus e.rpanstrs Zone. SGU Type 6 1 0 6 . 0 C. Dorsal view. x18. 0 D. Left lateral view. x20. 0 E. Incomplete pygidium. Top of Lanna Limestone, Afegisfaspislitnbafa Zone. SGU Type 6 107. x 13.

subtriangular, and wider than long. The basal furrows are weak. The genae are moderately convex, smooth, and equal in width anteriorly and laterally. They slope steeply downwards, most steeply laterally. In the small cephalon (SGU Type 6106) there is an ovate, smooth, and gently inflated genal area at the postero-lateral portion of the genae (Fig. SD).The border is gently declined and it is widest antero-laterally. Sagittally, the border occupies 9- 12 Yo ofthe total cephalic length. The posterior border is narrow (exsag.) and defined by a distinct border furrow. It is steeply declined beyond the fulcrum and then curved fonvard to become the narrow lateral border. The pygidium is not preserved well enough to permit a detailed description. It is obvious, however, that e!t posterior border is comparatively wide and provided with a pair of spines commencing on a transverse line well behind the rhachis. The pygidial rhachis is defined by change in the slope of the exoskeleton, and it is divided into three well defined lobes. It is of

maximum width across the anterior lobe and constricted at the posterior rhachial furrow. The posterior lobe is about as long (sag.) as the anterior and middle lobes combined. It is rounded behind with a minute median node posteriorly. The anterior and second rhachial lobes are crossed by a median ridge which is highest posteriorly. Dirtletisions (mm) Length (sag.)ofcephalon Length (sag.) ofcephalic border (incl. border furrow) Length(sag.)ofcephalicacrolobe Length (sag.) ofglabella Distance (sag.) from rear ofglabella to high spot ofmedian node Maximum width (tr.)ofcephalon Maximum width (tr.)ofglabella

SGU

SGU

Type 6105 4.25

Type 6106 2.10

0.40 3.85 2.60

0.25 1.85 1.30

1.50 4.20 1.50

0.70 2.15

0.90

Reniarlis. - In the partial effacement of the furrows and lobes, the present specimens resemble Geragtiostw fabirrs (Billings, 1865) from the

Ordovician agnosiid irilobites 275


GFF 110 (1988)

Fig. 6. Geragnostus cf. clusus Whittington, 1963 from the top o f the Segerstad Limestone (Vikarby Limestone). 0 A-B. Cephalon. Gammalbodberget. LO 58591. 0 A. Dorsal view. x13. 0 B. Left lateral view. x13. OC. hleraspidid pygidium. Lunne quarry. LO 58601. x21.

Llanvirnian Table Cove Formation (Middle Table Head Formation) of western Newfoundland. G.fabitis was redescribed by Whittington (1965, pp. 302-304, PI. 2, figs. 1-23, 25, 26) who also illustrated type material of this species. The Jamtland material differs from G. fabiiis primarily by the presence of pygidial border spines, in having a wider border, and lacking a transverse glabellar furrow on the external surface of the exoskeleton. Furthermore, the specimens at hand are less effaced. There are considerable morphological differences between the small (SGU Type 6106) and the large cephalon (SGU Type 6105), and it is likely that they may actually belong to two separate species. A distinctive feature in the small cephalon is the presence of gently inflated lateral genal areas (Fig. 5D).In this respect, the small cephalon is similar to Galbagriosttu galba (Billings, 1865) from the Table Cove Formation of western Newfoundland (see Whittington 1965, p. 307, fig. 2, PI. 2, fig. 24, PI. 3, figs. 2, 9- 10, 13, 15): The lateral genal areas are, however, not as conspicuous as in G. galba.

Occurrence. - Abandoned quarry at Lisbetodlingen in the Brunflo area, south-central Jamtland (grid reference X6999 15N1453 19). The pygidium and the large cephalon are from the top of the Volkhovian Lanna Limestone (Megistaspis limbata Zone), and the small cephalon is from the base of the Kundan Holen Limestone (basal Asaphtts expatisfis Zone). 19-GFF 111988

Gerag,iostiis cf. eltisits Whittington, 1963 Fig. 6A-C.

Malerial. - A nearly complete cephalon from Gammalbodberget in the Asarna area, and a meraspidid pygidium from the Lunne quarry in the Brunflo area. The material is preserved with the exoskeleton, and it was collected by Kent Larsson in 1969. Dirnensions(mm) LO 58591 Length (sag.) ofcephalon 4.00 Length (sag.) of cephalic border (incl. border furrow) 0.50 Length (sag.) ofcephalicacrolobe 3.50 2.50 Length (sag.) ofglabella Distance (sag.) from rear of glabella to 1.45 high spot of median node hlaximum width (tr.)ofcephalon 4.20 hlaximum width (tr.) ofglabella 1.55

Remarks. - The cephalon from Gammalbodberget compares most closely with Geragrtostiis elitstis Whittington (1963, pp. 28-32, PI. 1, figs. 1 - 17) from the Llanvirn of Lower Head, western Newfoundland. It differs, however, in having a shorter glabella (occupying about 63 Yo of thc total cephalic length) and a slightly more posteriorly situated glabellar node. In addition, the cephalon at hand seems to be less convex than cephala from Newfoundland, but this may be due to taphonomic processes. The external exoskeletal surface on the Gammalbodberget


276

GFF IlO(1988)

PerAhlberg

Fig. 7. Gerugnostirs spp. 0 A. Incomplete cephalon. Lisbetodlingen in the Brunflo area. Basal part of HoIen Limestone, Asuphis e.rpurtsus Zone. Coll. L. Karis 1973. SGU Type 6108. x20. 0 B. Incomplete cephalon. Road-section in the north-wcstern part of Brunflo. Basal part of Holen Limestone, Asuphiis e.rpu,uus Zone. Coll. P. Thorslund axid T. Tjernvik 1954. SGU Type 6109. x20.0 C. Incomplete pygidium. Lunne quarry. Top of Holen Limestone, Afegisruspis gigas Zone. Coll. K. Larsson 1969. LO 58611. x14.

specimen bears an indistinct reticulate pattern of raised lines. The glabellar muscle insertion areas are only faintly indicated and hardly discernible. The basal lobes, however, are provided with distinct muscle impressions posteromedially. The small pygidium from the Lunne quarry resembles Hunt’s (1 967) “Meraspid Instar 2” in the ontogeny of Arthrorhachis elspethi Raymond, 1925. It is about 1.1 mm long, widest posteriorly, and provided with postero-lateral spines. The rhachis is well defined and highly convex. Geragnosiiis clirsirs, C. sytntnetrictrs Zhou (in Zhou et al. 1982) from the Llanvim of northwest China, and G. hadros Wandas, 1984 from the Llanvirn of Norway, are morphologically similar to C. longicollis (Raymond, 1925), and they may prove to be synonymous with that species.

Occurrence. - In Jamtland, Geragtiostiis cf. chiis presently known from the top of the Segerstad Limestone (Vikarby Limestone; upper Llanvim) at Gammalboberget, about 6.5 km SSW ofAsarna church, and in the Lunne quarry, about 3 km ESE of Brunflo church, The former locality was briefly described by Thorslund (1940, p. 68), and in detail by Larsson (1973, pp. 34-44) and Lofgren (1978, pp. 17-18). The succession encountered in the Lunne quarry has been described principally by Wiman (1 893), Larsson (1973. pp. 11-21, in Gee & Kumpulainen 1980, pp. 33-34), Lofgren (1978, pp. I I- 12), and Karis& Larsson( 1982, pp. 69-71).