The Norwegian Upper Ordovician Illaenid Trilobites
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The Norwegian Upper Ordovician illaenid trilobites DA VID L. BRUTON & ALAN W. OWEN Bruton, D. L. & Owen, A. W.: The Norwegian Upper Ordovician illaenid trilobites. Norsk Geologisk Tidsskrift, Vol. 68, pp. 241-258. Oslo 1988. ISSN 0029-196X. New camera lucida techniques used in the measurement of illaenid trilobites are described and allow quantification of convexity of cephala and pygidia. Application of these techniques has demonstrated that previously identified dimorphs of Stenopareia glaber are only end members of continuous variation and that the species ranges from the upper Caradoc well into the Ashgill. A quantitative and qualitative distinction between S. glaber and S. linnarssoni is presented toget,her with descriptions of Il/aenus ( Paril/aenus) roemeri and specimens termed /. (P.) cf. da/ecarlicus. The latter throws light on the possible relationship between Octillaenus and other illaenids. Comparative material from Sweden is discussed and figured as is new material of Panderia from Norway. D. L. Bruton, PaleontologiskMuseum, Sars gate I, N-{)562 Oslo 5, Norway; A. W. Owen, Department of Geology, The University, Dundee DDJ 4HN, Scotland. In a previous paper (Owen & Bruton 1980) we shape are made becomes critical. To overcome described Caradoc and earliest Ashgill illaenid this, Bruton (1968, pp. 2-3) distinguished trilobites from Norway and Sweden. In doing so, between the dorsal view in which the plane pass we noted the occurrence of other illaenids in ing through the posterior margin of the cranidium the overlying Ordovician formations of the Oslo is vertical and the palpebral view in which the Region. These were described in an unpublished palpebral lobes are in the horizontal plane. A thesis (Owen 1977) and subsequently listed by longitudinally convex cranidium/cephalon would Owen (1981, table l) who summarized the Ashgill appear proportionally shorter in dorsal than in lithostratigraphy of the region (1981, fig. l; Fig. palpebral view. The sagittal length of convex cran l herein). The main purpose of the present paper idia or pygidia is especially difficult to assess is to describe these Ashgill species whose distri (Bruton 1971), but a technique to overcome some bution is summarized in Fig. 2. In doing so we of these problems is described here. have developed quantitative techniques which may have wider application in the Illaenidae and other groups. Measurement As with other smooth trilobites, the virtually featureless exoskeleton of illaenids makes taxo Cephala, cranidia and pygidia were orientated in nomic differentiation very difficult. Terrace line lateral view under a binocular microscope and the and muscle scar patterns, eye position, thoracic sagittal profile drawn using a camera lucida (Fig. segment number and shape of the rostral plate, 3A). The chord connecting the ends of the profile rostral flange, hypostoma and pygidial doublure was drawn and measured (12) as was the per are usefulin some illaenids but many species have pendicular from the chord to the highest part of been distinguished largely in terms of pro the profile(h). The distance along the chord from portional differences. The work of Jaanusson the perpendicular to the posterior end of the (1957) on Scandinavian Lower Ordovician illa cranidium or anterior end of the pygidium (11) enids exemplifies the usefulness of a quantitative was also measured. The percentage ratio h: 12 analysis of such differences, but most diagnoses gives a measure of convexity and 11:12 a measure have been essentially qualitative. The problem is of the shape of the profile.In more weakly convex compounded by the convexity of many cephala material the precise position of the perpendicular and pygidia. Not only is this prone to diagenetic and hence the value of 11 is difficult to define and and tectonic distortion, but also the orientation thus 11:12 becomes no more than a general guide in which measurements of assessments of overall to shape. The use of ratios in this analysis over- 242 D. L. Bruton & A. W. Owen NORSK GEOLOGISK TIDSSKRIFT 68 (1988) series Oslo-Asker Ringerike Hadeland & stages E Vv W E NW � SE ou t hern �,..._......, \ Norway i -- ·.? HADELAND ,...., \ .i NGERIKE .' Kjorrven .l � � OSG,LO-A SKER-- ; -·_:Fm. � "'V i 'l>� ø� -'-r' l .,. -i''� �Sk ogerholmen _Grina � \=_Sha l�� �'l> -.--.- Mbr. r _ 200k m ��Gagnu m Lfi Limestone dominant litho log ies Fm. Sorbakk en � _L u nner � Skjaerholmen p;:Fm. Fm.= sandstone Fm. -� limestone ): & siltstone �� msoya Fm. Lunner Kirke -Gagnu m shale/mudstone = Mbr . � Venstop Fm . Sha le '\ Mbr . 1111111111 hiatus lllllillill Onnian � limestone Solvang Fm. Actonian Fig. l. The la test Caradoc and Ashgill stratigraphy of the Oslo--Asker, Ringerike and Hadeland districts of the Oslo Region (based on Owen (1981), fig. 1-see Williams & Bruton (1983) for the Caradoc-Ashgill boundary hiatus in Oslo--Asker). U> Ashgill ei ti a O> "'U> _.:"- ·c: _.: ei spee ies E (J u; ,.,dl o "' >- "- Pusgillian Cautleyan Rawtheyan Hirnantian (J o s - � 2 .s l [ 1 11/aenus (Pari llaenus) roe meri A,R,H -- - 1 1 27 5 1 1 26 l. (P.) fa/lax - R,H all. 2 1 1 1 1 l. (P.) dalecarlicus cf. 1 1 1 -- H 1/laenus ---- o sp. 1 1 Stenopareia glaber .. R,H 3 1 47 11 1 1 2 31 S. linnar ssoni R,O-A-- -· 42 4 22 Panderia hadelandica 1 4 13 1 1 34 ---H P. megalopthalm a aff. 2 2 -H P. insulana --· R 1 Fig. 2. The ranges of Ashgill illaenids in Oslo--Asker (O-A), Ringerike (R) and Hadeland (H), and the number of sclerites of each species examined in the present study. Note that the specimens of Il/aenus (Parillaenus) aff. fal/ax itemized here are from Hadeland; specimens from the lowest Ashgill Høgberg Mbr. of the Solvang Fm. in Ringerike were described by Owen & Bruton (1980). Both I. (P.) aff. fallax and Stenopareia glaber are also known from the latest Caradoc of the Oslo Region. NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Upper Ordovician illaenid trilobites 243 A Sagittal Profile B Palpebral View poster ior CRANIDIUM/CEPHALON anter ior anter ior PYGIDIUM posterior Fig. 3. Length (!), height (h) and width (w) measurements made h herein on: A, the sagittal profile of the illaenid cephalon/cranidium ! and pygidium and B, palpebral ----11---· l.__ ___.l orientation of the cranidium. ------12------- w 1 comes the need to calculate the absolute value of on the posterior cranidial width are very scarce. the camera lucida enlargement. However, various Nevertheless, other transverse measurements are measurements of the cranidium were made using easily obtainable; namely the posterior glabellar vemier calipers and four of these proved useful width (w1), the width of the glabella at the inner in some of our analyses - 12 and three width edges of the lunettes (w2), the maximum anterior variates (Fig. 3B). width of the cranidium (w3, Fig. 3B) and the As a prerequisite for our work on the Ashgill maximum pygidial width. species of Stenopareia and as a test of the use Length:width and width:width plots of pairs fulness of the measurement techniques, a quanti of data (Fig. 4A, B, F, G) show a scatter around tative analysis of S. glaber from the la test Caradoc single growth lines which are isometric (Fig. 4A) was undertaken. This enabled the earlier hypoth or slightly anisometric (Fig. 4B, F, G). Percentage esis that this species is dimorphic to be tested and ratios from the camera lucida profiles(Fig. 4C-E) established a firm basis for the discrimination of also show a wide scatter but without any clustering Ashgill species. into two distinct groups. A Principal Component Analysis of 12 and the cranidial width measure ments also failed to show separation into clusters Stenopareia glaber (Fig. 5). The 'typical' long and short specimens 'Dimorphs' in illustrated by Owen & Bruton (1980, pl. 3, figs. (Kjerulf) 9-13) represent no more than conditions near the Owen & Bruton (1980, p. 16) suggested that extremes in length: width (Fig. 4A) and longi dimorphism is present in S. glaber with 'long' and tudinal profile shape and convexity (Fig. 4C, E) 'short' morphs defined on the ratio of the sagittal with a full range of intermediates present in the length to posterior width of the cranidium or sample. In width:width plots (Fig. 4F, G) these maximum pygidial width. The 'short' form was 'long' and 'short' cranidia plot almost at the same also considered to be more convex longitudinally. point. It is clear therefore that dimorphism is Similar dimorphism was suggested by Warburg not present. No clustering of points into distinct (1925, p. 126) for Swedish material which Owen instars is present on any of the plots, probably & Bruton included in S. glaber. In order to test reflecting a large amount of overlap in variation whether dimorphism is really present, all avail between successive moult stages, as suggested by able sufficiently complete material (60 cranidia, Hughes (1969, p. 49) for other trilobites. 13 pygidia) from the type unit, the latest Caradoc Solvang Formation in the Oslo-Asker district of the Oslo Region, was examined. In addition to the data from camera lucida profiles other Systematic palaeontology measurements were made using vemier calipers, Illaenids are known from several formations in including an accurate value of 12• Very few speci the Ashgill successions in the Oslo Region (Figs. mens have the narrow (exsag.) posterior parts of l, 2) and whilst those from the Hadeland district the fixedcheeks perfectly preserved and thus data are distorted to some extent, specimens from 244 D. L. Bruton & A. W. Owen NORSK GEOLOGISK TIDSSKRIFT 68 (1988) A 15 23 8 <11 ·"' E Q) -;;·� .o ::> <11 'typical short' "O 20 Cl 10 .\ ,·.... -": Ol n-51 · .s= . ,;. r >- ' . c.