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(Introduction) (Previous work on Pristiograptus) (Material) (Biostratigraphy) (Terminology) (Systematic descriptions) (References) and ... (Figures)

Pristiograptus (Graptoloidea) from the perneri - lundgreni biozones (Silurian) of Lithuania

Sigitas Radzevičius

Department of Geology and Mineralogy, Vilnius University, M. K. Čiurlionio 21/27, Vilnius, LT2000 (Lithuania)
sigitas.radzevicius@gf.vu.lt

Manuscript online since November 22, 2003

Abstract

Two new forms of Pristiograptus dubius, here designated as varieties "A" and "B", are described and figured. Both were found in core samples representing the perneri through lundgreni biozones of the Wenlock epoch. The cores are from boreholes Šiupyliai-69, Parovėja-9, Likėnai-396, Paežeriai-222 and Sutkai-87 in central and northern Lithuania. The associated graptolites Cyrtograptus perneri Bouček, C. radians Törnquist, C. lundgreni Tullberg, Monograptus flemingii flemingii (Salter), M. testis testis (Barrande), Monoclimacis flumendosae (Gortani) and Pristiograptus pseudodubius (Bouček) (= P. parvus) are all indicative of the perneri, radians and lundgreni biozones of the Wenlock. Two species of Pristiograptus occur in this time-stratigraphic interval: P. dubius and P. lodenicensis. In P. dubius the degree and manner of extension of the thecal apertural lip (thecal hood) onto the succeeding theca distinguish it from otherwise very similar taxa. The thecal morphology of P. lodenicensis differs from that of a typical Pristiograptus in that the apertural lip has a central depression and is rounded into small lobes laterally.

Key Words

Pristiograptus; Graptoloidea; graptolite; Wenlock; Silurian; Lithuania.

Citation

Radzevičius S. (2003).- Pristiograptus (Graptoloidea) from the perneri-lundgreni biozones (Silurian) of Lithuania.- Carnets de Géologie / Notebooks on Geology, Maintenon, Article 2003/07 (CG2003_A07)

Résumé

Pristiograptus (Graptoloidea) des biozones à perneri, radians et lundgreni (Silurien) de Lithuanie.- Deux formes nouvelles de Pristiograptus dubius, appelées ici variétés "A" et "B", sont décrites et illustrées. Toutes deux ont été découvertes dans des échantillons de carottes dans un intervalle rapporté aux biozones à perneri, radians et lundgreni du Wenlock. Les carottes proviennent des forages Šiupyliai-69, Parovėja-9, Likėnai-396, Paežeriai-222 et Sutkai-87 de Lithuanie centrale et septentrionale. Les graptolites rencontrés en association : Cyrtograptus perneri Bouček, C. radians Törnquist, C. lundgreni Tullberg, Monograptus flemingii flemingii (Salter), M. testis testis (Barrande), Monoclimacis flumendosae (Gortani) et Pristiograptus pseudodubius (Bouček) (= P. parvus), sont tous caractéristiques des biozones à perneri, radians et lundgreni du Wenlock. Deux espèces de Pristiograptus sont connues dans cet intervalle stratigraphique : P. dubius et P. lodenicensis. Chez P. dubius le développement (importance et morphologie) des lèvres aperturales des thèques le long du rhabdosome permet de le distinguer d'autres taxons autrement très voisins. La morphologie des thèques chez P. lodenicensis est distincte de celle d'un Pristiograptus typique dans la mesure où la lèvre aperturale présente une dépression médiane et s'arrondit latéralement en petits lobes.

Mots-Clefs

Pristiograptus ; Graptoloidea ; graptolite ; Wenlock ; Silurien ; Lithuanie.

Introduction

In the Baltic states the graptolite genus Pristiograptus occurs throughout the cyphus to parultimus - ultimus biozones of the Llandovery to Pridoli. Previously, the genus has been studied only cursorily in the interval comprising the upper Sheinwoodian-lower Homerian perneri through lundgreni biozones, probably because the index taxa for those biozones are Cyrtograptus species, quite distinct from those used for zonation of the upper Homerian where Pristiograptus species are the markers (Fig. 1 ). Consequently, this paper is a taxonomic review of the representatives of the genus Pristiograptus found in these middle Wenlockian levels where it has been passed over.

Pristiograptus of a dubius type dominate in the perneri - lundgreni biozones. Pristiograptus of a new lodenicensis type appear first in the lundgreni Biozone (see below). It has proved very difficult in the past to subdivide the dubius group and, thus, indiscriminate identifications like Pristiograptus ex gr. dubius and/or P. dubius (Suess) sensu lato abound in the literature.

Previous work on Pristiograptus

The Pristiograptus group was investigated in detail by Přibyl (1943) who distinguished two 'types' based on P. dubius and P. vulgaris (= Colonograptus ludensis) respectively. He described 23 species of a P. dubius type and 19 of a P. vulgaris (= Colonograptus ludensis) type ranging in age from the Late Llandovery to the Ludlow of the Czech Republic. Urbanek (1953, 1959, 1997) and Teller (1964) investigated the genus in Poland. Teller (1964) described ten species and subspecies of the dubius group from the Ludlow, eight of which were new. Jaworowski (1965) studied Pristiograptus from the lundgreni Biozone. He illustrated five taxa and described a new subspecies: P. lodenicensis peribalticus. Jaeger (1959, 1991) studied the genus in the upper Wenlock of Germany and described two new species. Some species originally described as Pristiograptus by these authors are now assigned to other recently established genera such as Colonograptus (e.g. Koren and Urbanek, 1994; Lenz and Kozłowska-Dawidziuk, 2002) and Pseudomonoclimacis (e.g. Urbanek, 1997). Lenz and Melchin (1991) recorded nine Pristiograptus species as occurring in the Wenlock of Arctic Canada, and described three of them. Lenz (2001) described only P. dubius from the lundgreni zone of Arctic Canada; he did not separate the dubius group into other species or subspecies. Holland et alii (1969) described the new species P. jaegeri from the upper Wenlock and lowermost Ludlow of Great Britain and Poland. Zalasiewicz and Williams (1999) recorded the Pristiograptus group from the Wenlock of central Wales, recognizing only one subspecies, P. dubius pseudolatus (Rickards).

Ulst (in Gailitė et alii, 1967) worked on the graptolites of the Baltic area. She described 12 species and subspecies of Pristiograptus from the Silurian of Latvia, including one new species, P. piltenensis Koren et Ulst (in Gailitė et alii, 1967), from the upper Wenlock. Paškevičius (1974, 1979) studied Pristiograptus in Lithuania and described the new species P. virbalensis from the upper Wenlock ludensis Biozone.

Material

The new Lithuanian material is from the perneri - lundgreni biozones. Much of the material consists of specimens (some three-dimensional) chemically extracted from the rock. The Pristiograptus discussed here are from boreholes Šiupyliai-69 (interval 1080.5-1004.8 m), Parovėja-9 (interval 619.3-545.7 m), Likėnai-396 (interval 638.5-602.9 m), Paežeriai-222 (interval 736.0-690.6 m) and Sutkai-87 (interval 821.3-803.7 m) (Fig. 2 ). In these boreholes the maximum thickness of strata of Silurian age is 850 m. In all, about 150 samples were collected and investigated. Rhabdosomes were isolated with HCl from 50 core samples. About 300 isolated fragments of semi-flattened and 3-dimensionally preserved rhabdosomes were collected, including unusually early astogenetic stages. The associated graptolites Cyrtograptus perneri Bouček, C. radians Törnquist, C. lundgreni Tullberg, Monograptus flemingii flemingii (Salter), M. testis testis (Barrande), Monoclimacis flumendosae (Gortani), Pristiograptus pseudodubius (Bouček) (= P. parvus) and P. lodenicensis Přibyl are indicative of the perneri, radians and lundgreni biozones of the Wenlock.

Photographs were made using the scanning electronic microscope (SEM) in the Laboratory of the Materials Research at Tallinn Technical University (Estonia).

The material is stored at the Department of Geology and Mineralogy of Vilnius University (Lithuania).

Biostratigraphy

The perneri - lundgreni biozones are located within a locally named regional entity called the Jaagarahu Stage (Fig. 1 ), which includes the upper part of Riga Formation and the Birštonas, Dukstyna and Verknė formations (Fig. 2 ) (Lapinskas, 2000). There are no graptolites in the Dukstyna and Verknė formations.

The perneri Biozone is composed of dark gray argillite with rare clay limestone interbeds in the Šiupyliai-69, Likėnai-369 and Parovėja-9 boreholes. In the Sutkai-87 and Paežeriai-222 holes, strata representing the perneri Biozone are gray marls with interbeds of micro-grained limestone but it is impossible to define its boundaries accurately because graptolites are rare. Where graptolites are present in quantity this association defines the perneri Biozone: Monograptus flemingii flemingii (Salter), Monoclimacis flumendosae (Gortani), Cyrtograptus perneri Bouček, C. ellesae Gortani. This biozone ranges in thickness from 29.5m (Šiupyliai-69) to 33.5m (Parovėja-9).

The lithology of the radians Biozone is like that of the perneri Biozone. The association of graptolites: Monograptus flemingii flemingii (Salter), M. flumendosae (Gortani), Cyrtograptus radians Törnquist, C. carruthersi Lapworth, is indicative of the radians Biozone. Its thickness ranges between 7.5m (Šiupyliai-69) and 10.7m (Parovėja-9).

The lithology of the lundgreni Biozone is identical to that of the other two. The association of graptolites that defines it is made up of two groups. One consists of species previously recognized: Monograptus flemingii flemingii (Salter), M. flumendosae (Gortani), Pristiograptus pseudodubius (Bouček) (= P. parvus) and the other is made up of more recently recognized grouping that together with the older group define the biozone: Eisenackograptus eisenacki (Obut et Sobolevskaya), Monograptus flemingii compactus Elles et Wood, M. testis testis (Barrande), M. t. inornatus Elles, Cyrtograptus lundgreni Tullberg, C. hamatus (Baily). Its thickness ranges from 31.5m (Parovėja-9) to 50.2m (Šiupyliai-69).

Terminology

This paper employs terminology (Fig. 3 ) modified from Teller (1964), Urbanek (1997) and Radzevičius and Paškevičius (2000):

ß - the angle between the thecal apertural lip and the succeeding metathecal wall. Variations in this angle allows Pristiograptus to be split into groups (see below).
The thecal apertural lip (Fig. 4 ) is a thickening around the aperture. Its form is an important taxonomic feature in subdividing the genus (e.g. P. deubeli (Jaeger) (= Colonograptus deubeli), P. lodenicensis, etc.; see Fig. 4 ).
The end of the thecal apertural lip is at the disto-lateral margin of the thecal aperture. Its location is a very important taxonomic feature in distinguishing forms of the dubius group.

The author is of the opinion that the thecal apertural lip (thecal hood) may be linked to the development of the ectocortex. It is not the result of deformation because some of material is three-dimensional. The internal structure of Pristiograptus was not studied, but will be investigated in the future. These morphological elements should be excellent features for discriminating between the several forms assigned to Pristiograptus ex gr. dubius, making their recognition easier.

The following abbreviations are used in the descriptions below: L - length of rhabdosome including sicula; W - rhabdosome dorso-ventral width; th₁, th₂, etc. - theca 1, theca 2, etc.

Systematic descriptions

Using these newly recognized morphological elements, the author describes two forms of Pristiograptus ex gr. dubius. They may well be found to be subspecies as material from other areas becomes available to demonstrate their geographic extent and persistence.

Family Monograptidae Lapworth, 1873

Genus Pristiograptus Jaekel, 1889

Type species. Pristiograptus frequens Jaekel, 1889

Pristiograptus dubius group, Přibyl, 1943

Type species. Pristiograptus dubius dubius (Suess), Suess, p. 115, pl. 9, figs. 5.a-b, riccartonensis Biozone, lower Wenlock, Silurian of Vyskočilka, Male Chuchle (Czech Republic).

Diagnosis. The rhabdosome is robust. The proximal end generally has a slight ventral curvature up to th₄-th₆; sometimes it is straight. Thecae are uniform, cylindrical, and inclined to the rhabdosomal axis at 25-35º. There is a distinct thickening of the thecal apertural margin, the thecal apertural lip, which extends onto the lateroventral wall of the succeeding theca. The angle (ß) between the apertural margin of the theca and the succeeding thecal wall is always obtuse, especially in the more proximal thecae. The sicula is conical with a short and thin, slightly curved virgella.

Age and geographic distribution. Pristiograptus of the dubius type is widespread having been recorded from: Lithuania, Latvia, Poland, Russia, the Czech Republic, Sweden, Germany, Bulgaria, Arctic Canada, Australia, British Isles, France, Italy, Tunisia, Algeria, and Morocco, etc. The group ranges from the Upper Llandovery to the Prídolí.

Pristiograptus dubius var. "A"

Fig. 5 A-I , 6 A-E , 7 A-C

Type. No S.P9-220, Fig. 5 A , Parovėja-9 borehole, depth 605.25 m, Cyrtograptus perneri Biozone.

Derivation of name. Temporary designation pending additional confirmation of the ubiquity and persistence of the characteristics defining the variety.

Material. About 25 rhabdosomes and additional fragments from the Parovėja-9 borehole, depth 605.25-590.5 m.

Description. The rhabdosome is robust and straight, although some specimens show ventral curvature proximally. L is >11.5mm. The collection includes only one complete specimen (Fig. 5 B ). The rhabdosome widens gradually from 0.8mm at th₁, to 0.9mm at th₂, 1mm at th₃, and 1.2mm at th₄. W_max is 1.7mm. The minimum W (= inter-apertural width) is 0.55mm at th₁, 0.65mm at th₂, 0.85mm at th₃, 0.95mm at th₄, and 1.05mm at th₅. There are 5-5.5 thecae in 5mm. Thecae are cylindrical. Th₁ is 1mm long. The free part of each theca is half of its whole length. The thecal apertural lip is distinct (Fig. 5 A-B & G ) and connects with that of the succeeding theca (Fig. 6 A ) except apparently in th₁ and th₂ (Fig. 6 B ) although it may have been obscured in these thecae by overgrowth of cortical tissue. The apertural lip connection is particularly obvious in the medial and distal portions of the rhabdosome (Fig. 7 ). The thecae are inclined at 30º to the virgula. The sicula is short and narrow. Its L is 1.8mm and apertural W is 0.4mm. The apex of the sicula reaches the base of th₃. Th₁ rises 0.1mm above the aperture. The aperture is concave in profile view. The virgella is short, up to 0.6mm long. In one specimen there is a thickened protrusion (Fig. 5 A & G , 6 D ).

Comparison. Pristiograptus dubius var. "A" differs from P. pseudodubius (= P. parvus), P. dubius dubius and P. dubius var. "B" in the connection of the thecal apertural lip to the succeeding theca. In P. dubius dubius and P. dubius var. "B" the thecal apertural lip does not reach the middle of the free ventral wall of the succeeding theca and in P. pseudodubius it reaches only the middle of the free ventral wall of the succeeding theca. Also, P. dubius var. "B" differs from P. dubius var. "A" in its longer virgella - 1.35 mm, as compared with 0.6 mm. The width of P. dubius var. "B" does not exceed 1.1 mm at th_4-5.

Association. P. dubius var. "A" is found with Cyrtograptus perneri (Bouček), Monograptus flemingii flemingii (Salter), Monoclimacis flumendosae (Gortani), P. pseudodubius (Bouček) (= P. parvus) and P. sp.

Age and geographical distribution. P. dubius var. "A" is widespread throughout northern Lithuania in the upper part of the Riga Formation, in the perneri Biozone of the Sheinwoodian Stage itself comprising the lower portion of a locally denominated entity, the Jaagarahu regional Stage.

Pristiograptus dubius var. "B"

Fig. 8 A-G , 9 A-D

Type. No. S.P222-229, fig. 8 A , Paežeriai-222 borehole, depth 730m, Cyrtograptus lundgreni Biozone.

Derivation of name. Temporary designation pending additional confirmation of the ubiquity and persistence of the characteristics defining the variety.

Material. About 20 rhabdosomes and additional fragments from Paežeriai-222 borehole, depth 734.5-730m, Likėnai-396 borehole, depth 577.0-605.9m, Šiupyliai-69 borehole, depth 1007.0m.

Description. The rhabdosome is straight and narrow. L is >12.5mm. The collection includes only one complete specimen (Fig. 9 A ). W is 0.7mm at th₁, 0.75mm at th₂, 0.85mm at th₃, 0.95mm at th₄, and 1mm at th₅. W_max is 1.1mm. The minimum, inter-apertural width is 0.5mm at th₁, 0.6mm at th₂, 0.75mm at th₃, 0.8mm at th₄, and 0.9mm at th₅. There are 5-5.5 thecae in 5mm. The free part of each theca is half of its entire length. The thecal apertural lip is distinct (Fig. 8 A-B ), extending onto the ventral wall of the succeeding theca, but not reaching its apertural lip (Fig. 9 A ), but ending midway up the ventral wall (Fig. 9 C ). Thecae are inclined at 30º to the virgula. The sicula is 2mm long and its apertural width is 0.35mm. The apex of sicula is at approximately the level of the apertural lip of th₂. Th₁ arises 0.2mm above the sicular aperture. The virgella is long (1.35 mm) and narrow (Fig. 9 D ).

Comparison. See under Pristiograptus dubius var. "A".

Association. Pristiograptus dubius var. "B" is found with Monograptus flemingii flemingii (Salter), M. f. compactus Elles et Wood, M. testis testis (Barrande), M. t. inornatus Elles, Pristiograptus pseudodubius (Bouček) (= P. parvus), Monoclimacis flumendosae (Gortani), Cyrtograptus lundgreni Tullberg, Paraplectograptus tenuis (Eisenack) and Eisenackograptus eisenacki (Obut et Sobolevskaya).

Age and geographic distribution. Pristiograptus dubius var. "B" is widespread throughout central and western Lithuania. It is found in the lundgreni Biozone of the Lower Homerian Stage within the Jaagarahu regional Stage which spans the upper part of the Riga Formation and the overlying Birštonas Formation.

Pristiograptus sp.

Fig. 10 A-E

Material. One specimen, no S.P9-219, from the Parovėja-9 borehole, depth 595.4m; rhabdosome with th_1-8.

Description. The rhabdosome is straight, massive. L is >11.1mm. The rhabdosome widens gradually. W is 0.8mm at th₁, 1mm at th₂, 1.1mm at th₃, 1.3mm at th₄, and 1.4mm at th₅. W_max is 1.6mm. The interapertural width is 0.57mm at th₁, 0.68mm at th₂, 0.73mm at th₃, 0.91mm at th₄, and 0.91mm at th₅. There are 5-5.5 cylindrical thecae in 5mm. Th₁ is 1.6mm long. The free portion of each theca is equal to half of its length. The thecal apertural lip is distinct (Fig. 10 B ), extending over the free part of the succeeding theca and connecting with its apertural lip (Fig. 10 A ). This connection is easily discernible in the medial and distal parts of the rhabdosome. Th₁ and th₂ either do not have such thecal apertural lip connections or they are covered by cortical tissue (Fig. 10 D ). The thecal apertural lip bifurcates (Fig. 10 D ) a feature well developed in the distal part of the rhabdosome. An intergrowth arch is well developed especially in the proximal part of the rhabdosome (Fig. 10 C ). The distance between the thecal apertural lip connection on the succeeding theca and the origin of its bifurcation is 0.25mm for th₁ and th₂, and 0.35mm for th_3-5. Thecae are inclined at 30º to the virgula. The sicula is short and narrow, 2mm long with an apertural width of 0.4mm. The apex of the sicula reaches the base of the th₂/th₃ interthecal septum. Th₁ arises 0.15mm above the sicular aperture. The sicular aperture is strongly concave in profile view (Fig. 10 E ). The virgella is thick and short (Fig. 10 E ), 0.74mm long.

Comparison. Pristiograptus sp. differs from P. pseudodubius (= P. parvus), P. dubius dubius, and Pristiograptus dubius var. "A" in the length and form of the thecal apertural lip in its connection with the succeeding thecal apertural lip. The thecal apertural lip of Pristiograptus sp. extends over the free part of the succeeding theca and connects with its apertural lip. In P. dubius dubius and P. dubius var. "B" the extension of the thecal apertural lip does not attain the middle of the free part of the succeeding theca. In P. pseudodubius the extension reaches the middle of the free portion of the succeeding theca. P. sp. differs from P. dubius var. "A" in that the thecal apertural lip bifurcates in the middle of the free part of the succeeding theca. P. dubius var. "A" lacks such a bifurcation.

Association. Pristiograptus sp. is found with Cyrtograptus perneri (Bouček), Monograptus flemingii flemingii (Salter), Monoclimacis flumendosae (Gortani), Pristiograptus pseudodubius (Bouček) (= P. parvus), and P. dubius var. "A".

Age and geographic distribution. Pristiograptus sp. is recorded only from the Parovėja-9 borehole, northern Lithuania, in the perneri Biozone of the Sheinwoodian Stage.

Pristiograptus lodenicensis was investigated by Radzevičius and Paškevičius (2000). The data in the accompanying table are unchanged from that presentation. But new pictures and photos improve the appreciation of their significance.

Pristiograptus lodenicensis group, Radzevičius et Paškevičius, 2000

Type species. Pristiograptus lodenicensis Přibyl, Přibyl, 1943, p. 21, pl. 1, figs. 4 & 6, Cyrtograptus lundgreni Biozone, Wenlock, Silurian of Lodenice-Budovice (Czech Republic).

Diagnosis. The rhabdosome is massive. The proximal end is straight or ventrally curved up to th₄ or th₅. Thecae are uniform and widen towards their apertures, which show small depressions centrally. Thecal apertures are perpendicular to the rhabdosomal axis. The sicula is small and conical with a slender, short virgella.

Remarks. All species not belonging to the dubius group, including P. lodenicensis, were previously assigned to the vulgaris group (Přibyl, 1943). Subsequently, P. vulgaris (Wood) was synonymised with P. ludensis (Murchison), so this group must be renamed. The taxonomic position of the lodenicensis group is not clear because P. lodenicensis is not a typical Pristiograptus. The thecae are similar to those of Colonograptus (e.g. Colonograptus gerhardi (Kühne)), differing only in the lesser curvature of the lip of the thecal aperture, as is also the case with Pseudomonoclimacis (e.g. Pseudomonoclimacis latilobus (Tsegelniuk)) and Monograptus massai Jaeger, the latter recorded from the Ludlow of NW Libya. This Libyan species can be assigned the lodenicensis group as it possesses a typical Pristiograptus sicula.

Age and geographic distribution. The lodenicensis group is known from the lundgreni Biozone of Lithuania, Latvia, the Czech Republic and Poland.

Pristiograptus lodenicensis Přibyl (1943)

Fig. 11 A-H , 12 A-F

1943 Pristiograptus lodenicensis Přibyl; p. 23-24, text-figs. C-D & J-K; pl. III, figs. 8-11;
1952 Pristiograptus lodenicensis Přibyl; Münch: p. 91, pl. 20, fig. 7;
1967 Pristiograptus lodenicensis Přibyl; Koren and Ulst in Gailitė et alii: p. 244, text-fig. 58; pl. XXIX, figs. 4-5;
1974 Pristiograptus lodenicensis Přibyl; Ulst: p. 115, pl. XIII, figs. 1-2.a-b; pl. XI, fig 7;
2000 Pristiograptus lodenicensis Přibyl; Radzevičius and Paškevičius: p. 97-98, pl. IV, figs. 1.a-c, 2.a-c; pl. VI, fig. D.

Holotype. Pristiograptus lodenicensis Přibyl, 1943. Plate I, fig. 2. Upper Wenlock (Homerian stage), Cyrtograptus lundgreni Biozone.

Material. About 50 rhabdosomes and fragments from the Kurtuvenai-161, Likėnai-396 and Parovėja-9 boreholes. Some specimens from Parovėja-9 were chemically prepared.

Description. The rhabdosome is straight, massive; the proximal end of some specimens is slightly curved ventrally as far as th₄ or th₅. L is at least 30mm. W is 0.7mm at th₁, 0.8mm at th₂, 0.9mm at th₃, 1mm at th₄, and 1.2 mm at th₅. W_max is normally 1.5mm but may reach 2mm by th_16-18. The inter-apertural width is 0.55mm at th₁, 0.6mm at th₂, 0.7mm at th₃, 0.85mm at th₄, and 0.9mm at th₅. Thecae are uniform, slightly curved, and widen towards the aperture. The apertural lips of symmetrically flattened thecae form an acute angle (ß) with the succeeding thecae. Thecae of specimens possessing relief are slightly different. They are slightly curved, widening towards the aperture, which shows (Fig. 11 ) a small ventral depression and curvature at the connection with the succeeding theca (Fig. 12 B & D-E ). Overall, thecal apertures are perpendicular to the rhabdosomal axis. Th₁ is 0.9-1mm long, and overlaps for 1/3-1/2 of its length. Thecae are inclined to the rhabdosomal axis at an angle 29-32º. The sicula is small, 1.2-1.5mm long, and 0.25-0.3mm wide aperturally (Fig. 12 F ). It is slightly curved ventrally. The sicular apex attains a level between the middle and the top of th₂.

Comparison. P. lodenicensis differs from all dubius types of Pristiograptus in the existence of a depression in the central part of the aperture and in the angle of the aperture that overall is perpendicular to the rhabdosomal axis, whereas in the dubius group this angle (ß) is always obtuse. The thecae of P. lodenicensis are conical while those of the dubius group are cylindrical. The thecae of P. virbalensis are square in cross-section. P. lodenicensis differs from P. deubeli in the greater curvature of the thecal apertures and in P. deubeli only the apertures of th₁-th₄ are curved.

Association. P. lodenicensis is found with Monograptus flemingii flemingii (Salter), M. f. compactus Elles et Wood, M. testis testis (Barrande), M. t. inornatus Elles, Pristiograptus pseudodubius (Bouček) (= P. parvus), Monoclimacis flumendosae (Gortani), Cyrtograptus lundgreni Tullberg, Paraplectograptus tenuis (Eisenack) and Eisenackograptus eisenacki (Obut et Sobolevskaya).

Age and geographic distribution. P. lodenicensis is widespread in the East Baltic area: (Lithuania, Latvia). It is found in the upper part of Riga Formation and in the Birštonas Formation, in the lundgreni Biozone of the Homerian Stage, Jaagarahu regional Stage. P. lodenicensis is also found in the Czech Republic, and Poland.

Acknowledgements

I am grateful to Juozas Paškevičius for providing the material and to Petras Musteikis for reading the manuscript and for critical remarks. I thank Jan Zalasiewicz, David Loydell, Alfred Lenz, Philippe Legrand and an "anonymous reviewer" for helpful suggestions, corrections and improvements. I am obliged to Jurga Stasevičiute, David Loydell and Nestor Sander for improvements in the English of the presentation.

References

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