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The Silurian retiolitid graptolite Plectograptus:
New observations and new species
DENIS E.B. BATES, ANNA KOZŁOWSKA, JÖRG MALETZ, NANCY H. KIRK1, and ALFRED LENZ
Bates, D.E.B., Kozłowska, A., Maletz, J., Kirk, N.H., and Lenz, A. 2006. The Silurian retiolitid graptolite Plectograptus:
New observations and new species. Acta Palaeontologica Polonica 51 (3): 525–540.
The Ludlow genus Plectograptus, with the type species Retiolites macilentus Törnquist, 1887, collected from Thuringia
(Germany), has been a widely−identified, monospecific, but poorly understood taxon for almost one hundred years. This
was due to poor and incomplete preservation of the type material, and misidentification by subsequent authors up to 1995.
The original, and only, type specimen of P. macilentus collected by Törnquist being lost, a neotype is herein selected from
a small collection of Thuringian material. The genus has now been redefined and based on this, and SEM studies of iso−
lated material, the defining characteristics of the genus are (i) the possession of a simple ancora umbrella with five radial
lists with an incompletely developed rim; (ii) an ancora umbrella separated from lateral ancora sleeve walls by exception−
ally large lateral orifices; (iii) the possession of mid−ventral lists; (iv) simple, orderly zigzag lateral wall ancora sleeve
lists. Recently, two additional species, P. robustus and P. wimani, previously placed in different genera, were assigned to
Plectograptus. This study recognizes three new species: P. mobergi,P. toernquisti, and P. trijunctus, bringing the total
number of species to six. Species are distinguished by the presence or absence of genicular processes, inclination of the
thecal ventral walls and mid−ventral lists, presence or absence of reticular lists, and three−way or four−way sleeve/lateral
rod/apertural lip junctions.
Key w ords: Graptoloidea, Retiolitidae, Plectograptus, rhabdosome, Silurian, Ludlow.
Denis E.B. Bates [deb@aber.ac.uk], Nancy H. Kirk (deceased), Institute of Geography and Earth Sciences, Universityof
Wales, Aberystwyth Ceredigion SY23 3DB, UK;
Anna Kozłowska [akd@twarda.pan.pl], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL−00−818 Warszawa, Poland;
Alfred C. Lenz [aclenz@uwo.ca], Department of Earth Sciences, University of Western Ontario, London, Ontario N6A
5B7, Canada;
Jörg Maletz, [jorgm@buffalo.edu], Department of Geology, State University of New York at Buffalo, 772 Natural Sci−
ences and Mathematics Complex, Buffalo, New York 14260−3050, USA.
Introduction
The genus Plectograptus was erected by Moberg and Törn−
quist (1909) for the species Retiolites macilentus Törnquist,
1887. Although the genus was monospecific for a long time, it
became one of the best−known retiolitids of the Upper Silurian
(e.g., Bouček and Münch 1952). The first isolated material
was described and illustrated by Eisenack (1951), as Retiolites
(Plectograptus)tetracanthus from Baltic erratic boulders.
Kozłowska−Dawidziuk (1995, 2002) included two more spe−
cies in the genus: Plectograptus wimani (Eisenack, 1951) and
P. robustus (Obut and Zaslavskaya, 1983).
A serious problem for taxonomic stability, however, has
long been posed by the poor preservation and incomplete−
ness of the type material of P. macilentus. Only a schematic
diagram with no scale was used to illustrate the species
(Törnquist 1887: fig. 3; reproduced here as Fig. 1B) in the
original description, but a photograph of the type was later
provided by Moberg and Törnquist (1909: pl. 1: 10). This
specimen lacks the proximal (ancora) end.
The original Törnquist type specimen of 1887 from the
Wetterahammer area, Thuringia has presumably been lost.
However, a collection of several specimens, not part of the
original collection, but like the type, derived from the Lower
Graptolitic Shales of the Wetterahammer area, are present in
the collections of the Senckenberg Forschungsinstitut und
Naturmuseum (Frankfurt/Main, Germany). The best speci−
men of that collection is herein chosen as the neotype (Fig.
1G). That specimen clearly shows several of the significant
characteristics necessary for genus and species identifica−
tion, although it lacks a complete proximal end, so necessary
for the recognition of the “Plectograptus ancora” (see be−
low), and for the overall recognition of the subfamily Plecto−
graptinae Bouček and Münch, 1952.
Scanning electron microscopy (SEM) of three−dimen−
sionally preserved, isolated material is now routinely ap−
plied to study retiolitids in more detail. SEM study reveals
important new characters not normally shown in flattened
material. Material recently isolated from Baltic erratic boul−
ders and other localities shows a variety of rhabdosomal
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Acta Palaeontol. Pol. 51 (3): 525–540, 2006
1Nancy H. Kirk passed away on 4th September 2005.
characters sufficient to justify the recognition of a number of
distinct species and, thus, specimens shown as SEM micro−
graphs and assigned by Lenz (1993) and Kozłowska−Dawi−
dziuk (1995) to Plectograptus macilentus (sensu Bouček
and Münch 1952), are herein recognized as new species (see
below).
The purpose of this paper is to elucidate the morphology
of the genus Plectograptus, to redefine the existing species,
to designate a neotype for P. macilentus and to describe new
species.
Institutional abbreviations.—LO, Lund University Museum,
Lund, Sweden; MB.G, Museum für Naturkunde, Berlin, Ger−
many; NMW, National Museum of Wales, Cardiff, United
Kingdom; SMF, Forschungsinstitut Senckenberg, Frankfurt/
Main, Germany; SMD, Staatliches Museum für Mineralogie,
Dresden, Germany; ZPAL, Institute for Paleobiology, Polish
Academy of Sciences, Warsaw, Poland.
Other abbreviations.—EEP, East European Platform.
Terminology and morphology
General terminology for the retiolitids has been listed by
Bates et al. (2005). The term pleural list has been used for the
thickening of the sides of the aperture in the Glossograptidae
and Retiolitidae (e.g., Lenz 1993). However, with the identi−
fication in the retiolitids of the ancora sleeve as being an
outer extrathecal structure, it is clear that in most retiolitids
the apertural openings and, particularly those of all higher
forms are not homologous with the thecal apertures of other
graptolites (Bates and Kirk 1984, 1992). Instead, each open−
ing, or orifice, is bounded proximally by the thecal lip, dis−
tally by the next thecal lip, e.g., in Retiolites (Bates et al.
2005) or genicular list, e.g., in Spinograptus praerobustus
Lenz and Kozłowska−Dawidziuk, 2002 (Fig. 2A, C), and lat−
erally by longitudinal lists of the ancora sleeve, the pleural
lists (Fig. 2) (see Bates et al. 2005: 710). Proximal to the first
two thecae, the pre−th11and pre−th12orifices are bounded
proximally by portions of the rim of the ancora umbrella, and
laterally by the most proximal pleural lists (Figs. 2B, 3).
The lists of the ancora sleeve may be divided into primary
lists, which are formed at the growing end of the sleeve, mark−
ing stages in its extension, and secondary lists (reticular lists),
formed on the already extended sleeve. Two different types of
primary lists (A and B) are found in Plectograptus (Figs. 4–6).
In type A lists the initial list has a proximally−facing insertion
seam (Fig. 5A) which marks the extension of the initial (pre−
sumed) fusellum of the ancora sleeve. These lists run largely
transversely, and are usually convex distally, as in P. toern−
quisti sp. nov. (Fig. 4B). They also mark pauses in the exten−
sion of the sleeve, as it grew in synchronicity with the thecae.
Type B primary lists have an initial, first formed portion which
has a concentric structure (Fig. 5B). The core is similar to the
fusellar core of the spines found in both retiolite and other
graptolites, and is surrounded by concentric, but asymmetric,
layers of cortical appearance. Each list would appear to have
grown forwards, initially as a spine, towards the genicular re−
gion of the thecal wall, making contact with it at the end of the
genicular list (at the socket, see below, Fig. 6B5).Thesideof
the sleeve enwraps the thecal lateral apertural rods, being ex−
tended slightly later than the thecal wall. Beyond the thecal lip,
the sleeve was again extended forwards as a spine. The exten−
sion of the fusellum of the next portion of the sleeve is then
marked by an enwrapping seam on the list (Fig. 5B2)andfur
−
ther thickening of the list is then by bandaging, which no lon−
ger forms as concentric structures. In effect, the type A list
forms at the margin of a panel of fuselli; the type B list as a
spinose projection with a fusellar core.
In species such as P. toernquisti there are quadruple (four−
way) list junctions at the ends of the thecal lips (Fig. 4B): the
fourth list of the junction being the succeeding pleural list of
type B, which extends to the next genicular list. In contrast, in
other species, these junctions are triple or three−way (Fig. 4A),
the succeeding ancora sleeve list being of type A, and growing
across the ancora sleeve. The succeeding type B list now has
its origin on the sleeve list, distal to the triple junction.
At the proximal end of the rhabdosome in Plectograptus
there are four initial type B pleural lists separating the proxi−
mal orifices. The lists extend from the rim of the ancora um−
brella (Figs. 2B, 3). These have no seams, as they separate
the dorsal obverse and reverse proximal orifices from the
ventral pre−th11and pre−th12orifices.
Secondary lists on the ancora sleeve are developed in most
retiolitids, but in only one species of Plectograptus,P. wi−
mani, except for the presence of the mid−ventral lists. They are
produced as accumulations of bandages on pre−existing
fusellar panels. The mid−ventral list of the thecal framework is
also a secondary list, and bears on its inner face the imprint of
the fusellar closures of the thecal wall, with some indications
of the mid−ventral zigzag suture (Bates 1987: fig. 7b, pl. 6: 2).
The sockets are depressions on the lists, and occur at the
junctions of the genicular lists with the lateral apertural rods
and the pleural sleeve lists (Figs. 2B, 6A, B) and, by defini−
tion, are absent if transverse rods are developed. They are
particularly prominent because the sleeve list is of type B.
526 ACTA PALAEONTOLOGICA POLONICA 51 (3), 2006
Fig. 1. A–G.Plectograptus macilentus Törnquist, 1887. A. Eisenack’s (1952) light photograph of isolated specimen, from Baltic erratic boulder, Kalinigrad
area, Russia, Ludlow. B. Original drawing of original size by Törnquist 1887. C,D. Bouček and Münch’s (1952) drawings of mature flattened material,
from Bohemia, Czech Republic. E. SEM picture of MB.G 1091, Stoltera, Baltic erratic boulder, Germany. F. Photograph of whole flattened specimen
LO7398 from Zeulenroda, Thuringia, Germany. G. Neotype SMF XXIV 433, from Wetterahammer, Thuringia, Germany; photograph of whole specimen
(G1), line drawing showing main skeletal lists (G2). H.Plectograptus robustus (Obut and Zaslavskaya, 1983); H1, light photograph of specimen LO2198
(Moberg and Törnquist 1909: fig. 1.1), originally identified as Plectograptus macilentus, Röddinge, Scania, Sweden; H2, enlargement showing genicular
processes (arrowed). Scale bars 1 mm.
®
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BATES ET AL.—SILURIAN RETIOLITID PLECTOGRAPTUS 527
Figs. 2, 4, and 6 illustrate the complex relationships between
the ancora sleeve and the thecal walls occurring at this junc−
ture, due to the linkage of these two approaching structures
(the lateral apertural rods are part of the thecal framework,
being lateral parts of the thecal aperture, whereas the pleural
lists, as here defined, are ancora sleeve lists).
The proliferating ventral and lateral walls of the thecae
meet the growing tips of the spines of the ancora sleeve, fus−
ing to produce the initial contacts between theca and sleeve.
At the same time, the next section of the ventral walls of the
thecae links across between the two socket areas. This must
have initially formed as a fusellar−cored spine, before ex−
tending forwards as the next section of ventral thecal wall. If
it was subsequently thickened, it would have formed a trans−
verse rod or genicular list, as in older retiolitids, and excep−
tionally in younger taxa such as Spinograptus praerobustus
Lenz and Kozłowska−Dawidziuk, 2002. For comparison, the
transverse rod found in Spinograptus with rarely preserved
membranes (Lenz 1994; Bates et. al. 2005) is shown dia−
grammatically in Fig. 2A, C.
From the sockets the geniculum runs across the ventral
face of the rhabdosome, and the thecal wall extends as a lateral
apertural rod, with an insertion seam for the thecal wall, to the
thecal lip. A mid−ventral list (the interpleural list of Lenz
1993) connects the mid−point of the geniculum with that of the
thecal lip. In contrast to the other lists, the mid−ventral list is
quite flat on its inner side, where it was emplaced on the outer
surface of the thecal wall. It is formed entirely of bandages
(Figs. 7, 8C).
Material and preparations.—New material described in
the paper is isolated and has been recovered from the erratic
boulders from northern Germany and Poland, and nodules
from Arctic Canada. The graptolites were recovered follow−
ing slow dissolution of the host carbonate in acid (1–10%
HCl, the strength of the acid varying with the chemistry of
the hosting carbonates). A fine hairbrush or eyedropper was
used to pick and transfer specimens. The material is stored in
glycerine in plastic containers, or on the SEM stubs.
Systematic part
Order Graptoloidea Lapworth, 1873
Family Retiolitidae Lapworth, 1873
Subfamily Plectograptinae Bouček and Münch, 1952
Genus Plectograptus Moberg and Törnquist, 1909
Type species:Retiolites macilentus Törnquist, 1887, from Wetteraham−
mer near Gräfenwarth, Thuringia, Unterer Graptolithenschiefer (Lower
Graptolite Shale), Ludlow.
Biostratigaphic range.—Typically ranging through Gors−
tian, Lower Ludlow globally, but rare and geographically re−
stricted in upper Homerian, upper Upper Wenlock.
Emended diagnosis.—Simple ancora umbrella with five ra−
dial lists and an incompletely developed rim. Ancora um−
brella separated from lateral ancora sleeve walls by large lat−
eral 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 toern−
quisti 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 ob−
verse 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 rudimen−
tary) 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 meet−
ing the ancora rim and the proximal side of the pre−th11ven−
tral 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−th12ventral ori−
fice is in part the fifth mesh of the ancora, the ancora rim be−
ing unthickened where it forms the proximal edge of the true
pre−th11ventral orifice.
Thecal walls defined only by lateral apertural rods, hori−
zontal 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), lat−
eral 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 th11and th12
(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 col−
lection remains (Fig. 1F), although that specimen, as sug−
gested by the original labels written by Robert Eisel, was not
part of Törnquist’s 1887 collection, and was collected from
528 ACTA PALAEONTOLOGICA POLONICA 51 (3), 2006
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 orig−
inal type locality by Maletz (Fig. 1G). Justification for the se−
lection 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 in−
clude 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, how−
ever, 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 speci−
mens, including fig. 1.1, illustrated herein (Fig. 1H) are here
identified as belonging to P. robustus.
The first good illustrations, showing proximal end charac−
ters, 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 flat−
tened, but well−preserved and mature specimens from Bohe−
mia (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 al−
though as noted above, the illustrated specimens described by
them are herein recognized as new species.
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BATES ET AL.—SILURIAN RETIOLITID PLECTOGRAPTUS 529
sockets
lip th1 lip th1
external common canals
internal
common canal
thecal aperture
pre-th1
orifice
pre-th1
orifice
ancora sleeve
membrane
Thecal framework
other ancora
sleeve lists
pleural lists
sicula
nema, virga, virgella,
ancora umbrella
ancora
membrane
umbrella
ventral walls
of first thecae
postulated position
of transverse rod
lateral apertural rod
thecal membrane
genicular lists
and processes
mid-ventral lists
thecal lip
Ancora sleeve
1
1
2
2
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.
530 ACTA PALAEONTOLOGICA POLONICA 51 (3), 2006
iInitial type B pleural lists
separating thecal orifices
lateral apertural rod
ancora umbrella membrane
genicular lists
mid-ventral lists
ancora umbrella lists
A
pre theca 1
orifice
B
C
D
E1
2
3
45
re
ver
se
lat
e
ra
l
or
i
f
i
c
e
o
b
v
e
r
s
e
a
l
ter
alorifice
1
pre theca 1
orifice
2
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 reachingthe
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 12orifice, 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−th12orifice 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; E1, whole ancora umbrella; E2, outside view of part of ancora umbrella region arrowed on A. ®
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 Semi−
plectograptus and Plectodinemagraptus (Kozłowska−Dawi−
dziuk 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−th12lists can be divided into two por−
tions: a distal portion (the true pleural list) without seams,
where they separate the pre−th12ventral orifice from the lat−
eral 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 lat−
eral apertural rods, thecal lips, genicular lists and mid−ventral
lists, similar to Spinograptus (Fig. 2A, C). Sockets on the in−
ner 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 con−
junctions with the pleural lists distal to the lips of the first two
thecae, although there is some variation in the level of con−
tact. The highest position noted is just below the lip of th31on
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 dor−
sal 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, H1). 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 prox−
imal end does not have large lateral orifices. Rickards rec−
ognized that it resembled Plectograptus?textor Bouček and
Münch, 1952, which was placed in Sokolovograptus by
Kozłowska−Dawidziuk (1995: 291). Plectograptus?carl−
steinensis Kozłowska−Dawidziuk et al. 2001 and Plecto−
graptus?ovatus Kozłowska−Dawidziuk et al. 2001 are ex−
cluded 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 recog−
nition of three new species, described below.
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BATES ET AL.—SILURIAN RETIOLITID PLECTOGRAPTUS 531
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 th11scar (arrowed), NMW
91.52G.1688B, Baltic erratic boulder No. B9/96, Bramsche, Germany, Gorstian. Scale bars: B–E, G 100 µm; F, 50 µm.
2
3
1
1
2
3
5
mid-ventral list
lateral
apertural rod
thecal lip
type A list
type B list
4
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 (ar−
rows).
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)
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 Törn−
quist 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−Da−
widziuk 1995: 317, fig. 33.
1995 Plectograptus macilentus (Törnquist 1887); Maletz et al. 1998:
pl. 1: 2, 5.
1998 Plectograptus macilentus (Törnquist 1887); Kozłowska−Dawi−
dziuk, Lenz and Štorch 1998: fig. 1E.
2004 Plectograptus macilentus (Törnquist 1887); Lenz and Kozłow−
ska−Dawidziuk 2004: 21, pl. 23: 1–5; pl. 24: 4–6, 8–12; pl. 26: 15.
Neotype: SMF XXIV 433, Wetterahammer, Thuringia, Germany, Gors−
tian, 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,Neo−
gothograptus balticus,Neolobograptus nilssoni (fragments), and Pri−
stiograptus 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 Zimmer−
mann 1912: 46). He remarked on the presence of Plectograptus maci−
lentus in Zone 19 at the locality. Jaeger (1991) described the bio−
stratigraphy of the remaining modern Wetterahammer locality, which
has been modified through road and railroad construction since Törn−
quist’s visit, showing its biostratigraphic extent crossing the Wen−
lock–Ludlow boundary and representing an important succession for
the documentation of the Cyrtograptus lundgreni extinction event.
Diagnosis.— Ancora sleeve with only primary lists. Junc−
tions between ancora sleeve lists, pleural lists, thecal lips and
lateral apertural rods forming a quadruple junction. Mid−ven−
tral lists vertical, pleural lists relatively vertical. No genicular
processes.
532 ACTA PALAEONTOLOGICA POLONICA 51 (3), 2006
fusellar walls
Fig. 5. Two types of ancora sleeve lists in representatives of Plectograptus
Moberg and Törnquist, 1909 shown in cross sections. A. Lists type A with
insertion seam, P. wimani (Eisenack, 1951), ZPAL G.39/1, Mielnik bore−
hole, depth 1405.0 m, EEP, Poland, Ludlow: SEM micrograph (A1), sche−
matic diagram (A2). B. Lists type B with spinose core, P. toernquisti sp.
nov., NMW 91.52G.1700, Bramsche, Mecklenburg−Vorpommern, Baltic
erratic boulder No. B9/96, Germany, Gorstian, Lower Ludlow: SEM mi−
crograph (B1) and schematic diagram (B2). Schematic diagrams of the lists
with lines suggest fusellar wall. Scale bars 10 µm.
Table 1. Distinguishing characters of the species of Plectograptus.
Species characters P. macilentus P. toernquisti P. mobergi P. robustus P. trijunctus P. wimani
genicular processes absent absent absent paired paired single
mid−ventral inclination ±vertical inclined vertical vertical vertical inclined
sleeve/ lateral apertural rod/lip junctions 4 way 4 way 3 way 4 way 3 way 4 way
reticulum absent absent absent absent present present
Fig. 6. A–C.Plectograptus toernquisti sp. nov. lists showing sockets occurring at the junctions of the genicular lists with the lateral apertural rods and the
pleural sleeve lists. A. ZPAL G.27/2, Bartoszyce borehole, depth 1627.0 m, EEP, Poland, Ludlow. B. NMW 91.52G.1686, Bramsche, Baltic erratic boulder
No. B9/96, Germany, Gorstian: B1–B4, SEM micrographs; B5, schematic drawing of B4showing connections of lists and positions of sockets, insertion
seams are shown in grey. C. ZPAL G.39/2, Jarosławiec, Baltic erratic boulder No. 22, Poland, Ludlow. D.Plectograptus macilentus Törnquist, 1887 lists
with well developed fusellar increments, NMW 91.52G.1695, Stoltera, Baltic erratic boulder, Germany, Gorstian. All SEM micrographs.
®
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BATES ET AL.—SILURIAN RETIOLITID PLECTOGRAPTUS 533
Description.—Mature rhabdosome ³15–20 mm long, width
of lateral wall about 2 mm, total width including thecal loops
about 4 mm. Rhabdosome more or less parallel−sided through−
out, 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 maci−
lentus by Törnquist in 1887, from poor material from Thu−
ringia, Germany. Moberg and Törnquist erected the genus
Plectograptus in 1909, following the study of better−pre−
served material from Scania, Sweden. When erecting the
species in 1887, from Thuringia, Törnquist published only a
diagrammatic illustration, reproduced in Fig. 1B. Subse−
quently, when erecting the genus Plectograptus, Moberg and
Törnquist (1909) provided photographs of additional flat−
tened 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 the−
cae 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 origi−
nal type and thus, the very basis of the family Plectograptinae
and of the nature of the “Plectograptus ancora”. This prob−
lem has essentially been solved by the universal acceptance
of the morphological characteristics of mature specimens de−
scribed and illustrated in Bouček and Münch (1952) and,
subsequently, supplemented, elaborated on, and fully con−
firmed 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 iden−
tified 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).
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 91.52G.1702. Paratypes NMW 91.52G.1686, NMW
91.52G. 1688A, NMW 91.52G. 1693, NMW 91.52G. 1697, NMW
91.52G. 1698, NMW 91.52G. 1701.
Type locality: Bramsche, Mecklenburg−Vorpommern, Baltic erratic
boulder No. B9/96, Germany.
Type horizon: Specimens are associated with specimens of Saeto−
graptus chimaera, Gorstian, Lower Ludlow.
Material.—Baltic erratic boulder No. B9/96, Germany; Corn−
wallis Island, Arctic Canada field collection SB E−68 m,
Gorstian, Lobograptus progenitor Biozone.
Diagnosis.—Ancora sleeve with only primary lists. Junc−
tions 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 in−
creasing 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 geni−
culum 0.46 mm. The lateral profile distinctly “saw−toothed”
(Fig. 7B), with lateral apertural rods and mid−ventral lists in−
clined inwards at about 30–40°, and pleural lists inclined out−
wards at about the same angle. Genicular lists longer than
thecal lips, so that they are convex outwards, and the mid ven−
tral 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 com−
posed 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 in−
clined lateral apertural lists.
Geographic and stratigraphic range.—EEP and Arctic Can−
ada, Gorstian, Neolobograptus nilssoni and Lobograptus pro−
genitor biozones, respectively.
Plectograptus mobergi sp. nov.
Fig. 8.
1995 Plectograptus macilentus (Törnquist, 1887); Kozłowska−Dawi−
dziuk 1995: 317, fig. 33.
2002 Plectograptus macilentus (Törnquist, 1887); Kozłowska−Dawi−
dziuk 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, Mecklen−
burg−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 ad−
ditional fragments including juveniles preserved in glycer−
ine, preserved in Museum fuer Naturkunde, Berlin under the
534 ACTA PALAEONTOLOGICA POLONICA 51 (3), 2006
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 to−
wards 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 th110.83 mm
distal from ancora hub, of th121 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 inser−
tion 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, 8A1,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 51is incom−
plete. The distal part of the rhabdosome is displaced towards
the reverse side (Fig. 8D2), and is a little narrower than the
proximal portion. The distal lists are thinner than the proxi−
mal 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 Törn−
quist 1909: 13, pl. 1: 1.
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BATES ET AL.—SILURIAN RETIOLITID PLECTOGRAPTUS 535
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.
1983 Agastograptus robustus Obut and Zaslavskaya, 1983: 108. pl. 24:
1–3.
pars 2002 Plectograptus robustus (Obut and Zaslavskaya, 1983), Koz−
łowska−Dawidziuk 2002: 462, fig. 2D.
536 ACTA PALAEONTOLOGICA POLONICA 51 (3), 2006
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; A3, enlargement with part of ancora umbrella and theca 12;A
4, outside view of ancora umbrella
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 11side 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: D1, reverse view of rhabdosome—stereopair; D2, ventral view of theca 11side.
non 2002 Plectograptus robustus (Obut and Zaslavskaya, 1983); Koz−
łowska−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 geni−
cular 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, Neolobo−
graptus nilssoni Biozone.
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BATES ET AL.—SILURIAN RETIOLITID PLECTOGRAPTUS 537
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 photo−
graph 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.
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 ro−
bustus by Kozłowska−Dawidziuk (2002: 462, fig. 2C) is as−
signed 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 quadru−
ple 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 th11.
There are also some distortions in the proximal and distal
parts of the rhabdosome.
Geographic and stratigraphic range.—EEP, lower Gorstian,
Neolobograptus nilssoni Biozone.
Plectograptus trijunctus sp. nov.
Fig. 9A.
2002 Plectograptus robustus (Obut and Zaslavskaya, 1983); Kozłow−
ska−Dawidziuk 2002: 462, fig. 2c.
Derivation of the name: From the Latin tri and junctus, for a three junc−
tion.
Holotype: Holotype ZPAL G.27/2 (Fig. 9A).
Type locality: Bartoszyce borehole, 1627.0 m, Poland.
Type horizon: Gorstian, Neolobograptus nilssoni Biozone.
Material.—One immature specimen and several fragments
from Bartoszyce borehole, 1627.0 m, Poland.
Diagnosis.—Ancora sleeve only primary lists, mid−ventral
lists vertical. The ancora sleeve list junctions with pleural lists
triple. Thecal walls inclined inwards from genicular to aper−
tural lists, similar to Plectograptus mobergi and P. wimani.
Paired reticulofusellar genicular processes similar to those of
P. robustus.
Description.—Proximal obverse and reverse orifices as well
as thecal profiles similar to Plectograptus robustus. Lateral
profile with thecal walls from genicular list to lips slightly
undulating. Pleural lists in two sections, similar to P. mo−
bergi. Genicular processes on immature specimen with three
thecal pairs, developed only on third pair of thecae. See also
Kozłowska−Dawidziuk (2002) for additional description.
Discussion.—The new species is similar to Plectograptus
robustus in having similar apertural processes and thecal
profiles, but in contrast to Plectograptus robustus, the ancora
sleeve list junctions with the pleural lists are triple rather than
quadruple.
Geographic and stratigraphic range.—EEP, lower Gorstian,
Neolobograptus nilssoni Biozone.
Plectograptus wimani (Eisenack, 1951)
Fig. 10.
1951 Retiolites wimani sp. nov.; Eisenack 1951: 145, pl. 25: 8.
1951 Retiolites sp. indet. Eisenack 1951: 146, figs. 6, 7.
1995 Plectograptus wimani (Eisenack, 1951); Kozłowska−Dawidziuk
1995: 318, fig. 27E.
Emended diagnosis.—Ancora sleeve with secondary lists,
quadruple list junctions at ends of apertural lists. Thecal walls
inclined inwards from genicular to apertural lists. Singular re−
ticulated genicular hood as wide as a length of genicular list.
Material.—Three mature specimens, partly damaged, and
one at ancora stage of growth (Kozłowska−Dawidziuk 1995),
Mielnik Borehole, Poland, depth 1044.9 m, Gorstian, Neo−
lobograptus nilssoni Biozone.
Description.—Largest specimen with 12 pairs of thecae, com−
pressed almost flat, resulting in outward bulging of the thecal
lips and genicular lists (Fig. 10). Ancora umbrella and distal
end broken off (Fig. 10A). Width of ancora sleeve gradually
increasing from 2.05 mm at the level of the first geniculum to
2.62 mm at the 7th thecal pair, gradually decreasing to 2.05
mm at the level of the 10th thecal pair. Total length estimated at
11.7 mm. Thecal orifices calculated as 1.00 mm wide by
0.36 mm from lip to geniculum, and cross−sectional obverse to
reverse width of rhabdosome 2.08 mm. Primary ancora sleeve
lists joining the lateral apertural rod/ thecal lip/pleural list
junction. Secondary (reticular) lists of the sleeve form a stout
meshwork between the primary lists. Genicular processes
complex and loop−shaped (Fig. 10F), inclined proximally at
about 30°from the horizontal (Fig. 10A).
The genicular processes are formed of primary curving
lists, each bearing an insertion seam on its inner face, and
secondary lists which are oblique to the primary lists, having
seams on their inner or outer sides.
Discussion.—The illustrated specimen (Fig. 10) represents a
more mature rhabdosome than the holotype (Eisenack 1951:
pl. 25: 8), which has thinner lists and only incipient genicular
processes. Two fragmentary rhabdosomes illustrated by Eise−
nack (1951: figs. 6, 7) represent more mature stages of growth,
with better developed genicular processes, similar to the Pol−
ish material.
The insertion seams on the inner face of the primary lists
making the genicular processes, show that the processes
538 ACTA PALAEONTOLOGICA POLONICA 51 (3), 2006
Fig. 10. SEM pictures of mature rhabdosome of Plectograptus wimani (Eisenack, 1951), ZPAL G.XVI/1607, Mielnik borehole 1044.0 m, Poland, Ludlow
in obverse (A) and ventral (B) views. C. Enlargement of proximal end. D. Oblique view of proximal end. E. Thecae showing sockets (arrowed) and
genicular hoods. F. Stereopair of two genicular hoods formed of primary lists and reticulum. G. Mid−ventral list and thecal lip. H. Ancora sleeve walls
showing surface of lists from outside and inside.
®
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BATES ET AL.—SILURIAN RETIOLITID PLECTOGRAPTUS 539
were formed initially with a (fusellar) membrane. The sec−
ondary lists with their inner or outer seams therefore were
formed on the outer and inner faces respectively of the
process.
Geographic and stratigraphic range.—EEP, lower Gorstian,
Neolobograptus nilssoni Biozone.
Acknowledgments
Authors thank Adam Urbanek (Institute of Palaeobiology Polish Acad−
emy of Sciences, Warsaw, Poland) for providing specimens from the
Mielenik borehole. AK, AL, and JM thank Ann Bates for hospitality
during visits to Aberystwyth. We thank the reviewers Michael Melchin
(St. Francis Xavier University, Canada) and Jan Zalasiewicz (Univer−
sity of Leicester, UK) for providing useful and thoughtful comments.
We are indebted to Kent Larsson (University of Lund, Sweden) for his
considerable efforts in finding, and making loans of, type specimens.
AL acknowledges long−term research funding support from the Natural
Sciences and Engineering Research Council (Canada). DB acknowl−
edges support from the Natural Environment Research Council (UK).
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