Plectograptus toernquisti, 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

Plectograptus toernquisti sp. nov.

Figs. 3B–G , 6A–C, 7 .

1993 Plectograptus (Plectograptus) macilentus ( Törnquist, 1887) ; Lenz

1993: 13, pl. 1: 6–8.

Derivation of the name: In honour of the late Swedish paleontologist S.L. Törnquist who erected the species P. macilentus .

Holotype: NMW 91.52 G.1702 . Paratypes NMW 91.52 G.1686, NMW 91.52 G. 1688A, NMW 91.52 G. 1693, NMW 91.52 G. 1697, NMW 91.52 G. 1698, NMW 91.52 G. 1701.

Type locality: Bramsche, Mecklenburg−Vorpommern, Baltic erratic boulder No. B9/96, Germany.

Type horizon: Specimens are associated with specimens of Saetograptus chimaera, Gorstian , Lower Ludlow.

Material.—Baltic erratic boulder No. B9/96, Germany; Cornwallis Island, Arctic Canada field collection SB E− 68 m, Gorstian, Lobograptus progenitor Biozone.

Diagnosis.—Ancora sleeve with only primary lists. Junctions between ancora sleeve lists, pleural lists, thecal lips and lateral apertural rods quadruple. Mid−ventral lists and lateral apertural rods distinctly inclined inwards from genicular lists to thecal lips. No genicular structures.

Description.—Minimum calculated length of rhabdosome 9.5 mm, with nine thecal pairs. Dorso−ventral width gradually increasing from 1.26 mm at the ancora to 2.7 mm at about the 6th thecal pair, then gently narrowing. Scalariform width 1.7 mm. Width of thecal lips 0.84–0.98 mm, height of free ventral walls from geniculum to lip 0.38 mm and from lip to geniculum 0.46 mm. The lateral profile distinctly “saw−toothed” ( Fig. 7B ), with lateral apertural rods and mid−ventral lists inclined inwards at about 30–40 °, and pleural lists inclined outwards at about the same angle. Genicular lists longer than thecal lips, so that they are convex outwards, and the mid ventral lists therefore longer than the lateral apertural rods ( Figs. 4B , 7A, B ). Pleural lists join the lateral ancora sleeve lists at the junction with the lateral apertural rods and thecal lips, forming quadruple junctions ( Figs. 4B , 7B ). As a result the ancora sleeve meshes are hexagonal. Pleural lists are composed entirely of type B lists ( Fig. 4B ).

Discussion.—The material described by Lenz (1993) as Plectograptus macilentus belongs to this new species, having quadruple junctions at the thecal lips, and markedly inward inclined lateral apertural lists.

Geographic and stratigraphic range.—EEP and Arctic Canada, Gorstian, Neolobograptus nilssoni and Lobograptus progenitor biozones, respectively.

Figures

Fig. 3. Proximal end of Plectograptus toernquisti sp. nov. A. Schematic drawing with insertion seams on ancora umbrella in black, arrow indicates the change from ancora umbrella rim to pleural list (the small curved “twigs” are fusellar shards). The numbers 1–5 are indicative of radial lists reaching the rim only, the letters A–E shows five meshes of ancora umbrella, the solid color region indicating the extent of the ancora umbrella. B–G. SEM micrographs of fragments showing ancora umbrella. B. Ancora umbrella and pre−th 12 orifice, arrow indicates the change from ancora umbrella rim to pleural list, NMW 91.52G.1698, Bramsche, Baltic erratic boulder Nr. B9/96, Germany, Gorstian, Lower Ludlow. C. Ancora umbrella from the outside, NMW 91.52G.1701, Baltic erratic boulder, Stoltera, Germany, Gorstian. D. Outside view of pre−th12 orifice with complete list of ancora umbrella rim (arrow), NMW 91.52G.1697, Baltic erratic boulder B4/97, Nienhagen, Germany, Gorstian. E. Outside view of ancora umbrella ZPAL G.39/2, st. A92, Jarosławiec, Baltic erratic boulder Nr. 22, Poland, Ludlow; E, whole ancora umbrella; E, outside view of part of ancora umbrella region arrowed on A. + 1 2

Fig. 7. SEM micrographs of Plectograptus toernquisti sp. nov. mature rhabdosome, NMW 91.52G.1702, holotype, Baltic erratic boulder No. B9/96, Bramsche, Germany, Gorstian. A. Ventral view. B. Stereopair of the obverse view of specimen. C. Oblique view looking distally. D. Proximal view. Scale bars 1 mm.

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).