Plectograptus mobergi, Bates & Kozłowska & Maletz & Kirk & Lenz, 2006

Bates, Denis E. B., Kozłowska, Anna, Maletz, Jörg, Kirk, Nancy H. & Lenz, Alfred, 2006, The Silurian retiolitid graptolite Plectograptus: New observations and new species, Acta Palaeontologica Polonica 51 (3), pp. 525-540 : 534-538

Plectograptus mobergi sp. nov.

Fig. 8 .

1995 Plectograptus macilentus ( Törnquist, 1887) ; Kozłowska−Dawidziuk 1995: 317, fig. 33.

2002 Plectograptus macilentus ( Törnquist, 1887) ; Kozłowska−Dawidziuk 2002: 460–462 , fig. 2A.

Derivation of the name: In honour of the late Swedish paleontologist J.C.

Moberg who, with S.L. Törnquist, studied Plectograptus macilentus .

Holotype: MB. G 1091.6 ( Fig. 8B ).

Type locality: MB.G. 1091, Glacial erratic boulder, Stoltera, Mecklenburg−Vorpommern, northern Germany,coll. H. Jaeger (Orig. No. 353) .

Type horizon: Ludlow, sample includes fragments of a cucullograptid indet.

Material. — Nine specimens mounted on SEM stubs and additional fragments including juveniles preserved in glycerine, preserved in Museum fuer Naturkunde, Berlin under the type number MG. B 1091 . Other illustrated material from Baltic erratic boulders No. 22, Jarosławiec , Poland; Mielnik borehole, depth 978.9 m, Poland, Ludlow .

Diagnosis. —Ancora sleeve with only primary lists, mid−ventral lists vertical. The ancora sleeve list junctions with pleural lists are triple; otherwise like P. macilentus . Pleural lists slightly undulating. No genicular processes.

Description. —Maximum length with 12 pairs of thecae 10.1 mm, ancora width 1.2–1.8 mm. Rhabdosome gently widening from 1.74 mm at first thecal pair to 2.16 mm at fourth thecal pair (but parallel sided or tapering gently towards distal end from 2.5–2.0 mm. Scalariform width 0.66 mm proximally. Proximal obverse and reverse orifices 1.3 mm wide by 1.3 mm high. Thecal lips of th 11 0.83 mm distal from ancora hub, of th 12 1 mm distal. Lateral profile with thecal walls from genicular list to lips almost vertical. Pleural lists in two sections, the first inclined inwards from the thecal lips at about 30°and curving inwards to the centre of the ancora sleeve and bearing a proximally facing insertion seam; the second branching from the first about 0.18 mm from the lip junction, inclined outwards at about 6 °, running to the next genicular list.

Discussion.—The new species has the ancora sleeve lists junctions with pleural lists ( Figs. 4A , 8A 1, B, C, D 1) triple in contrast to those of Plectograptus toernquisti sp. nov.

One specimen shows aberrant growth in the distal part of rhabdosome at the level of th 51 ( Fig. 8D ), and at the level of the next theca it becomes regular again. Theca 51 is incomplete. The distal part of the rhabdosome is displaced towards the reverse side ( Fig. 8D 2 ), and is a little narrower than the proximal portion. The distal lists are thinner than the proximal ones suggesting regeneration of the colony.

Geographic and stratigraphic range.—EEP, Ludlow.

Plectograptus robustus ( Obut and Zaslavskaya, 1983) Figs. 1H, 9B, C .

1909 Plectograptus macilentus ( Törnquist, 1887) ; Moberg and Törnquist 1909: 13, pl. 1: 1.

1 2

1983 Agastograptus robustus Obut and Zaslavskaya, 1983: 108 . pl. 24: 1–3.

pars 2002 Plectograptus robustus ( Obut and Zaslavskaya, 1983) , Kozłowska−Dawidziuk 2002: 462, fig. 2D. non 2002 Plectograptus robustus ( Obut and Zaslavskaya, 1983) ; Kozłowska−Dawidziuk 2002: 462, fig. 2C.

2004 Plectograptus robustus ( Obut and Zaslavskaya, 1983) ; Lenz and Kozłowska−Dawidziuk 2004: 22, pl. 24: 1–3, 7; pl. 26: 3, 4, 8, 9, 11–14.

Emended diagnosis.—Ancora sleeve with only primary lists, mid−ventral lists vertical. Quadruple junctions among ancora sleeve lists, pleural lists, thecal lips and lateral apertural rods.

Pleural lists slightly undulating. Paired reticulofusellar genicular processes as in Plectograptus trijunctus sp. nov.

Material — Three specimens, glacial erratic boulder, Spandau bei Berlin, Germany, coll. H. Jaeger (Orig. No. 173), MB. G 1081 ; one flattened specimen from Grube “Frisch Glueck” near Zwickau , south of Stenn, Germany, Ludlow, Neolobograptus nilssoni Biozone .

Description.—Maximum length of rhabdosome with 6 pairs of thecae 5.5 mm, ancora width about 1.5 mm. Rhabdosome gently widening to about 2.5 mm distally. Pleural lists in two sections, similar to P. mobergi .

One flattened specimen from Grube “Frisch Glueck” near Zwickau represents a mature rhabdosome of P. robustus with 18 thecal pairs and a well−developed nematularium, which is preserved only inside the distal part of the rhabdosome ( Fig. 9B ). The rhabdosome distinctly tapers distally. Genicular processes are not developed in the distalmost thecae.

Discussion.—The material identified as Plectograptus robustus by Kozłowska−Dawidziuk (2002: 462, fig. 2C) is assigned to the new species Plectograptus trijunctus based on it having a triple junction and genicular processes ( Fig. 9A ).

The specimen from Central Asia illustrated in Koren’ (1994: pl. 2: 6) may represent P. robustus , since the quadruple junction is clear. There is no proximal end preserved and possible genicular processes are visible only in proximal thecae. The specimen of Plectograptus robustus illustrated in Fig. 9C has a quadruple junction in all thecae except th 11. There are also some distortions in the proximal and distal parts of the rhabdosome.

Geographic and stratigraphic range.—EEP, lower Gorstian, Neolobograptus nilssoni Biozone.

Figures

Fig. 8. SEM micrographs of Plectograptus mobergi sp. nov. A. Immature rhabdosome, ZPAL G.27/1, Baltic erratic boulder No. 46, Jarosławiec, Poland, Ludlow; A1, reverse view of proximal fragment, triple junctions of ancora sleeve with pleural lists indicated by arrows; A2, inside view showing shape of ancora umbrella and shape of rhabdosome in cross section; A, enlargement with part of ancora umbrella and theca 12; A, outside view of ancora umbrella 3 4 proximal to theca 11. B. Obverse view of rhabdosome with three pairs of thecae and long virgella preserved, holotype, Stoltera, Germany, MB.G 1091, Stoltera, Germany, Ludlow. C. Oblique view of theca 11 side of immature specimen, ZPAL G.39/3, Mielnik borehole, depth 978.9 m, Poland, Ludlow. Note that the obverse and reverse lists of the ancora umbrella are unusually long, and have portions of the ancora rim attached to them. D. Mature rhabdosome, with malformation and regeneration in the distal part of the rhabdosome, ZPAL G.39/4, Jarosławiec, Baltic erratic boulder No. 54, Poland, Ludlow: D, reverse view of rhabdosome—stereopair; D, ventral view of theca 11 side.

Fig. 4. Schematic diagrams of the ancora sleeve growth in Plectograptus Moberg and Törnquist, 1909. Numbers indicate the order of growth of ancora sleeve panels. A. P. mobergi sp. nov. with triple junctions of lists (arrows). B. P. toernquisti sp. nov. with quadruple junctions of lists (arrows).

Fig. 2. Schematic drawings of rhabdosomes showing coloured thecal framework and ancora sleeve structures with lists and postulated membrane of Spinograptus praerobustus (Lenz and Kozłowska−Dawidziuk, 2002) and Plectograptus Moberg and Törnquist, 1909. A. Fragment of medial part of rhabdosome of S. robustus: thecate part of rhabdosome (A1), thecate part and ancora sleeve (A2). B. Proximal fragment of illustrating the position of actual thecal aperture and proximal orifices. C. Fragment of Plectograptus rhabdosome with postulated membranes, common canals and thecal aperture.

Fig. 9. A. SEM micrographs of Plectograptus trijunctus sp. nov., holotype ZPAL G.27/2, Bartoszyce borehole, depth 1627 m, EEP, Poland, late Homerian; A1, whole rhabdosome, reverse view; A2, enlargement of apertural process. B, C. Plectograptus robustus (Obut and Zaslavskaya, 1983). B. Light photograph of reverse of mature rhabdosome with nematularium, SMD−SAS 217, Grube “Frisch Glueck” near Zwickau, south of Stenn, Germany, Ludlow; not to scale. C. SEM micrographs of a mature rhabdosome, MB.G 1081, Spandau bei Berlin, Germany, Ludlow: C1, obverse view; C2, paired apertural process; C3, ancora hub; C4, enlargement of ancora umbrella fragment.