Plectograptus macilentus ( Törnquist, 1887 )

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 : 532-534

Plectograptus macilentus ( Törnquist, 1887)

Figs. 1A–G, 6D.

1887 Retiolites macilentus sp. nov.; Törnquist 1887: 491, fig. 3.?1908 Retiolites (Gothograptus) spinosus (Wood) ; Elles and Wood

1908: 345, text−fig. 226d (non a–c).

1909 Plectograptus macilentus ( Törnquist, 1887) ; Moberg and Törnquist 1909: 13, fig. 10. (non figs. 2–9).

1951 Plectograptus tetracanthus Eisenack 1951: 140 , pl. 23: 6–8; pl. 24: 8; pl. 25: 9; text−figs. 4, 5.

1952 Plectograptus macilentus ( Törnquist 1887) ; Bouček and Münch 1952: 22, fig. 7a–f; pl. 1–4.

1956 Plectograptus macilentus ( Törnquist1887) ; Tomczyk 1956: 44, pl. 1: 2a, b; text−fig. 9a–c.

1971 Retiolites (Plectograptus) macilentus Törnquist 1887 ; Schauer 1971: 85, pl. 39: 11,12; pl. 40: 7, 8.

?1889 Plectograptus macilentus ( Törnquist 1887) ; Pashko 1989: 117, fig. 2.

non 1993 Plectograptus (Plectograptus) macilentus ( Törnquist 1887) ; Lenz 1993: 13, pl. 1: 6–8.

1994 Plectograptus macilentus ( Törnquist 1887) ; Koren’ 1994: 140, pl. 2: 6.

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

1995 Plectograptus macilentus ( Törnquist 1887) ; Maletz et al. 1998: pl. 1: 2, 5.

1998 Plectograptus macilentus ( Törnquist 1887) ; Kozłowska−Dawidziuk, Lenz and Štorch 1998: fig. 1E.

2004 Plectograptus macilentus ( Törnquist 1887) ; Lenz and Kozłowska−Dawidziuk 2004: 21, pl. 23: 1–5; pl. 24: 4–6, 8–12; pl. 26: 15.

Neotype: SMF XXIV 433, Wetterahammer, Thuringia, Germany, Gorstian, Neolobograptus nilssoni Zone ( Fig. 1G). The slabs associated with the neotype specimen (SMF XXIV 433 to SMF XXIV 442) include a number of specimens of P. macilentus , Spinograptus spinosus , Neogothograptus balticus , Neolobograptus nilssoni (fragments), and Pristiograptus dubius . The graptolites are preserved as greenish pressure shadow minerals with some remaining parts of the periderm in a tectonized black shale.

Type locality: The original Wetterahammer or Wetterhammer locality was discussed by Hundt (1910) as his locality No. 6 (see also Zimmermann 1912: 46). He remarked on the presence of Plectograptus macilentus in Zone 19 at the locality. Jaeger (1991) described the biostratigraphy of the remaining modern Wetterahammer locality, which has been modified through road and railroad construction since Törnquist’s visit, showing its biostratigraphic extent crossing the Wenlock–Ludlow boundary and representing an important succession for the documentation of the Cyrtograptus lundgreni extinction event.

Diagnosis.— Ancora sleeve with only primary lists. Junctions between ancora sleeve lists, pleural lists, thecal lips and lateral apertural rods forming a quadruple junction. Mid−ventral lists vertical, pleural lists relatively vertical. No genicular processes.

Description.—Mature rhabdosome ± 15–20 mm long, width of lateral wall about 2 mm, total width including thecal loops about 4 mm. Rhabdosome more or less parallel−sided throughout, but may taper slightly towards the distal end. Thecae 10–12 in 10 mm. Genicular lips and thecal lists about same size throughout, joined by mid−ventral lists that are more or less vertically oriented. Fairly robust nematularium developed in larger specimens, beginning well inside rhabdosome and projecting well beyond. Without thecal (genicular) processes. Quadruple junctions, as noted in diagnosis.

Discussion.—The species was described as Retiolites macilentus by Törnquist in 1887, from poor material from Thuringia, Germany. Moberg and Törnquist erected the genus Plectograptus in 1909, following the study of better−preserved material from Scania, Sweden. When erecting the species in 1887, from Thuringia, Törnquist published only a diagrammatic illustration, reproduced in Fig. 1B. Subsequently, when erecting the genus Plectograptus, Moberg and Törnquist (1909) provided photographs of additional flattened material of P. macilentus from Sweden and the type from Thuringia. Most of the material, which was incorrectly identified as P. macilentus by Moberg and Törnquist in 1909 (e.g., fig. 1.1; illustrated herein as Fig. 1H), is stored in the Lund University Museum ( LO 2198–2208).

The simple and orderly zig−zag nature of lateral walls of the rhabdosome, and the “climacograptid” profile of the thecae were clearly evident in the original studies of the type material by Törnquist (1887) and Moberg and Törnquist (1909), and these features alone are sufficient to recognize the genus Plectograptus , and most of its species, including P. macilentus . However, one important problem is the absence of the proximal (ancora) end of the rhabdosome in the original type and thus, the very basis of the family Plectograptinae and of the nature of the “ Plectograptus ancora”. This problem has essentially been solved by the universal acceptance of the morphological characteristics of mature specimens described and illustrated in Bouček and Münch (1952) and, subsequently, supplemented, elaborated on, and fully confirmed by the recent SEM studies of isolated material. The second problem, concerns the presence or absence of the thecal processes, and an error involving material studied by the original authors. Moberg and Törnquist (1909) illustrated 10 specimens (including the original type, fig. 1.10) all identified as P. macilentus . Our examination of the nine other specimens, including the best specimen ( Fig. 1H), shows them to possess thecal (genicular) processes (see Fig. 9B ), and thus are assignable to P. robustus (see below).

Figures

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.