Plectograptus Moberg and Törnquist, 1909

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 : 528-532

Genus Plectograptus Moberg and Törnquist, 1909 Type species: Retiolites macilentus Törnquist, 1887 , from Wetterahammer near Gräfenwarth, Thuringia, Unterer Graptolithenschiefer (Lower Graptolite Shale), Ludlow.

Biostratigaphic range.—Typically ranging through Gorstian, Lower Ludlow globally, but rare and geographically restricted in upper Homerian, upper Upper Wenlock.

Emended diagnosis.—Simple ancora umbrella with five radial lists and an incompletely developed rim. Ancora umbrella separated from lateral ancora sleeve walls by large lateral orifices. Nema free throughout the rhabdosome. Lateral walls of simple, orderly zigzag lists. Reticulum and genicular processes present in some species. Mid−ventral lists present.

Species included.— Plectograptus macilentus ( Törnquist, 1887) , Plectograptus robustus ( Obut and Zaslavskaya, 1983) , Plectograptus wimani ( Eisenack, 1951) , Plectograptus toernquisti sp. nov., Plectograptus mobergi sp. nov., Plectograptus trijunctus sp. nov.).

Description.—Virgella thin, rarely preserved as far as the prosicular rim; nema free, but probably was within the obverse thecal wall, and may be extended as a nematularium in mature specimens ( Figs. 1F, H, 9B ).

The Plectograptus type ancora umbrella is simple, with five radial lists (two near the center of origin being rudimentary) defining five meshes and an incompletely preserved rim ( Figs. 2B , 3A ). Primary ancora lists bifurcate, one branch to give a thin obverse list and a stronger secondary list meeting the ancora rim and the proximal side of the pre−th 11 ventral orifice; other primary list forking to give a thin reverse list and a stronger secondary list which divides to give two lists leading to the ancora rim. Apparent pre−th 12 ventral orifice is in part the fifth mesh of the ancora, the ancora rim being unthickened where it forms the proximal edge of the true pre−th 11 ventral orifice.

Thecal walls defined only by lateral apertural rods, horizontal thecal lips, genicular lists, and with mid−ventral lists formed of external bandaging running from geniculum to lip. Processes may be present on genicular lists. No transverse rods; instead sockets at junctions of pleural lists ( Fig. 6), lateral apertural rods and genicular lists ( Fig. 2 ). Nematularium may be present.

Large proximal orifices on the obverse and reverse sides of rhabdosome, extending beyond the lips of th 11 and th 12 ( Fig. 2B ).

Ancora sleeve defined by lists with a zig−zag pattern in the mid−dorsal part of the wall, formed of lists which become horizontal ventral−wards; the ventral edges of the meshes formed of pleural lists and lateral apertural rods ( Figs. 2 , 4 ). Bandages pustular ( Fig. 10E, G, H). Seams face inwards ( Fig. 10H).

Discussion.—When erecting the type species Törnquist (1887) provided only a schematic drawing, without a proximal end, reproduced here as Fig. 1B. This drawing is completely generalized and shows neither the genicular or mid−ventral lists. The first photograph, of flattened material without a proximal end, was provided by Moberg and Törnquist (1909: pl. 1: 10) with an annotation that this is the “original” of Plectograptus macilentus from Wetterahammer, Thuringia. This specimen has not been located in the Lund collection, and is presumed lost.

A single other specimen from the original Törnquist collection remains ( Fig. 1F), although that specimen, as suggested by the original labels written by Robert Eisel, was not part of Törnquist’s 1887 collection, and was collected from the Zeulenroda locality, near Gräfenwarth , Thuringia. The Zeulenroda specimen is not chosen as a neotype because it does not come from the type locality and does not show the main important characteristics of the species .

A neotype was selected from material collected at the original type locality by Maletz ( Fig. 1G). Justification for the selection of a neotype from that collection is as follows: the specimen has been collected from the same locality and the same strata as the original, and it clearly shows some of the morphology considered characteristic of the genus. These include the simple and very orderly zigzag pattern of lateral ancora sleeve lists, the distinctly climacograptid thecae, each with complete genicular, apertural and mid−ventral lists, and absence of genicular processes. The ancora umbrella, however, is only partly preserved in the specimen and a nema and nematularium is not visible.

The remainder of the material illustrated by Moberg and Törnquist (1909: figs. 1.1–1.9) comes from Röddinge in Scania, Sweden, and although bearing genicular processes, was originally identified as P. macilentus . All those specimens, including fig. 1.1, illustrated herein ( Fig. 1H) are here identified as belonging to P. robustus .

The first good illustrations, showing proximal end characters, were by Eisenack (1951), using isolated, but immature material from Baltic erratic boulders ( Fig. 1A). This material, described as a new species by Eisenack, is clearly assignable to P. macilentus . Bouček and Münch (1952) illustrated flattened, but well−preserved and mature specimens from Bohemia ( Fig. 1C, D) and used the genus to establish the subfamily Plectograptinae . Paradoxically, in taxonomic practice, it has been primarily from Bouček and Münch’s (1952) elucidation of the Bohemian material that the basic understanding of the morphological characteristics of the proximal end of the genus has been derived. The first SEM studies of isolated material were by Lenz (1993) and Kozłowska−Dawidziuk (1995), and these considerably enlarged the understanding of the genus although as noted above, the illustrated specimens described by them are herein recognized as new species.

The simple ancora umbrella ( Figs. 2 , 3 ), with a minimum of radial lists, and an incompletely thickened peripheral rim, is comparable to that in other late retiolitids, such as Semiplectograptus and Plectodinemagraptus ( Kozłowska−Dawidziuk 1995: fig. 34). This is the Plectograptus type ancora umbrella. It is joined to the rest of the rhabdosome by four pleural lists. These separate the four proximal orifices, and are unseamed. The pre−th 12 lists can be divided into two portions: a distal portion (the true pleural list) without seams, where they separate the pre−th 12 ventral orifice from the lateral orifices, and a proximal portion where they form part of the ancora rim. The ventral part of the rim is not normally thickened into a list, but can be traced in growth as seams along the lists, with fusellar increments projecting from them ( Fig. 3 ), culminating in a prominent “twig” (on the reverse list) marking the final growth of the ancora rim (shown by the arrows in Fig. 3A, B, F ). In other late Silurian genera, such as Spinograptus and Holoretiolites , the ancora umbrella is more complex, with more radial list forkings, and the pleural lists do not form the only lists linking the ancora umbrella to the rest of the rhabdosome (Bates et. al. 2005).

The thecal framework of Plectograptus is reduced to lateral apertural rods, thecal lips, genicular lists and mid−ventral lists, similar to Spinograptus ( Fig. 2A, C ). Sockets on the inner face of the junction between pleural lists, genicular lists and lateral apertural rods, mark the conjunction of the thecal walls with the ancora sleeve ( Figs. 2B , 6). These take the place of the transverse rods, as seen in genera such as Cometograptus , marking the proximal ends of the ventral thecal walls (Kozłowska−Dawidziuk 2001).

Very large proximal obverse and reverse orifices are prominent and unique features of Plectograptus , contrasting markedly with the smaller and more restricted orifices seen in all other genera of the plectograptine group (among the retiolitines, only the poorly known taxon Dabashanograptus chengkouensis Ge, 1990 appears to have large obverse and reverse orifices ( Bates et al. 2005: fig. 7B). On their distal sides the first primary lists of the ancora sleeve make conjunctions with the pleural lists distal to the lips of the first two thecae, although there is some variation in the level of contact. The highest position noted is just below the lip of th 31 on the first thecal series, and at the level of the thecal lip of th 22 on the others ( Fig. 1A). In contrast, in other genera such as Neogothograptus and Holoretiolites , the distal margins of the first lateral orifices make contact with lists proximal to the lateral apertural rods of the first thecae. Succeeding dorsal lists of the sleeve alternate from side to side, suggesting that the sleeve grew as a series of lobes ( Fig. 4 ). These were extended in parallel with the thecal walls, with which they make conjunctions only along the lateral apertural rods.

A nematularium is preserved in some flattened specimens of Plectograptus ( Moberg and Törnquist 1909; Bouček and Münch 1952; Tomczyk 1956) ( Fig. 1F, H 1). It has not been observed in any isolated material.

Plectograptus ? bouceki Rickards, 1967 is not assignable to Plectograptus , because it has sleeve walls which are formed of a reticulum without any major lists, and the proximal end does not have large lateral orifices. Rickards recognized that it resembled Plectograptus ? textor Bouček and Münch, 1952, which was placed in Sokolovograptus by Kozłowska−Dawidziuk (1995: 291). Plectograptus ? carlsteinensis Kozłowska−Dawidziuk et al. 2001 and Plectograptus ? ovatus Kozłowska−Dawidziuk et al. 2001 are excluded from Plectograptus , as they have different proximal end structures ( Kozłowska−Dawidziuk et al. 2001). Their generic assignment is unknown.

Recently acquired material of Plectograptus has some characters not seen in the type species, permitting the recognition of three new species, described below.

The pleural list is without a seam. F. Lateral view of the same region as on A, NMW 91.52G.1686, Baltic erratic boulder No. B9/96, Bramsche, Germany, Gorstian, arrow indicates the change from ancora umbrella rim to pleural list. G. Inside view of ancora hub region with th 11 scar (arrowed), NMW 91.52G.1688B, Baltic erratic boulder No. B9/96, Bramsche, Germany, Gorstian. Scale bars: B–E, G 100 µm; F, 50 µm.

Species distinguished here are:

– Plectograptus macilentus Törnquist, 1887

– Plectograptus toernquisti sp. nov.

– Plectograptus mobergi sp. nov.

– Plectograptus robustus ( Obut and Zaslavskaya, 1983) – Plectograptus trijunctus sp. nov.

– Plectograptus wimani ( Eisenack, 1951)

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.

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. 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. 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).