Middle Devonian dendroid graptolites from the Brilon ...

Paläontologische Zeitschrift 2006, Vol. 80/3, p. 221–229, 30-09-2006

Middle Devonian dendroid graptolites from the Brilon Reef area (Rheinisches Schiefergebirge, Germany) JÖRG MALETZ, Buffalo with 3 figures MALETZ, J. 2006. Middle Devonian dendroid graptolites from the Brilon Reef area (Rheinisches Schiefergebirge, Germany). – Paläontologische Zeitschrift 80 (3): 221–229, 3 figs., Stuttgart, 30. 9. 2006. Abstract: Dendroid graptolites from the Middle Devonian (Givetian, middle Polygnathus varcus Biozone) of the Rheinisches Schiefergebirge are described for the first time. The fauna of the Burgberg locality south of the Brilon reef comes from a black shale layer in a succession of calcareous debris flows. The fauna includes five species of Dictyonema and Ruedemannograptus described in open nomenclature. The fragmentation of the material is considered as an indication of post-mortem transport. Keywords: Devonian • graptolites • Dendroidea • Germany • Rheinisches Schiefergebirge Kurzfassung: Dendroide Graptolithen aus dem Mittleren Devon (Givetium, mittlere Polygnathus varcus-Zone) des Rheinischen Schiefergebirges werden erstmalig beschrieben. Die Fauna aus der Lokalität Burgberg südlich des Briloner Riffs stammt aus einem dünnen Schwarzschieferband in einer Abfolge von Riffschuttkalken. Sie beinhaltet fünf Arten der Gattungen Dictyonema und Ruedemannograptus, die in offener Nomenklatur beschrieben werden. Schlüsselwörter: Devon • Graptolithen • Dendroidea • Deutschland • Rheinisches Schiefergebirge

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Introduction Graptolites are common in Palaeozoic strata and are established as one of the biostratigraphically most important fossil groups for this time interval. The roots of the Graptolithina, however, are to be found in the Middle Cambrian, while near the start of the Ordovician a tremendous radiation took place after the first planktic Graptoloidea were established with the genus Rhabdinopora (COOPER et al. 1998). The Graptoloidea died out with the Monograptus yukonensis lineage in the Emsian, Lower Devonian (JAEGER 1988), but the benthic forms of the Dendroidea survived for a considerable time after their planktic cousins extinction. They had never been as diverse and successful as the planktic graptolites, but had been a consistent even though inconspicuous component of benthic marine communities. The group flourished at least until the Carboniferous (UBAGHS 1941; RUEDEMANN 1947; ERDTMANN &

ADAMS 1975), but probably the latest Dendroidea got extinct only in the Permian (DENG 1985). The dendroid graptolites, thus, were more long-lived and successful in their own terms, but they are rarely found in the fossil record as their preservational potential is low due to the unstable environment in which they lived. This is obvious, if their palaeontological record is compared with the good record of the planktic graptoloids. In the geological history these quiet, shallow-water environments are rarely preserved and most dendroid graptolite faunas are found to be transported and not being preserved in situ with their colonies and holdfast structures intact and in growing position (compare ERDTMANN 1974, 1976). A rich collection of dendroid graptolites was made recently from black shales intercalated with fore-reef limestones of the locality Burgberg south of the Brilon reef complex of the Rheinisches Schiefergebirge. This material represents the first undoubted Middle Devonian

Address of the author: Jörg Maletz, Department of Geology, University at Buffalo, SUNY, 772 Natural Sciences Complex, Buffalo, N.Y. 14260-3050. U.S.A.; e-mail

0031– 0220/06/0080–0221

$ 4.05

© 2006 E. Schweizerbart’sche Verlagsbuchhandlung, D–70176 Stuttgart

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graptolites found in the area. Despite the fairly poor preservation of the graptolite rhabdosomes, they yield important data for the palaeogeographic relationships and sedimentary environment from which they originated. In the Middle Devonian the clastic sedimentary input from the Old Red Continent to the NW into the NE– SW trending Rhenohercynian Basin of the Rheinisches Schiefergebirge stopped. The situation became favourable for the growth of reefs on the platform margin and on volcanic elevations, environments, in which also dendroid graptolites were able to settle and flourish. The Brilon reef developed and shed its debris into S–SE direction to form fore-reef limestones and turbidites. In the Brilon Anticline the reef growth started with biostromal structures in the Eifelian, Middle Devonian (STRITZKE 1989), and reached a thickness of about 800 m (KREBS 1974). Mid-Givetian volcanism to the south reached the Brilon fore-reef area and formed a strong morphological differentiation in the marginal basin region. Based on this relief, the thickness of the reef detritus varies strongly in the area. The reef detritus interfingers with the pelites of the basin facies. After the reef growth ended, the pelagic shales of the Cypridinenschiefer predominated in the basin areas and occasionally overlapped onto the reef in the Famennian, but left elevated parts of the fore-reef area free. On these the Cephalopod Limestones were deposited subsequently mainly south of the Brilon reef.

The locality The Burgberg locality (Fig. 1) on the northern flank of the Messinghausen Anticline (Messinghäuser Sattel), south of the main reef complex of the Brilon Anticline exposes a relatively complete suite of mid-Givetian to late Frasnian strata. The section is well exposed in the northern part of the quarry at the western slope of the Burgberg. The area is located about 1.5 km E of the village of Rösenbeck and 600 m NW of Niederhof near the main road L870 (sheet map 4518, Madfeld). The lithostratigraphic succession of the Burgberg locality was described by EDER et al. (1975), EDER et al. (1977) and STRITZKE (1990, 1991). The well bedded succession looks complete and undisturbed, even though it is folded, but especially the basal portion of the limestone succession shows significant tectonic stress with sheared and striated planes, fracture zones and many veins filled with milky calcite. The whole section including the metabasalts measures about 150 m in thickness and forms the central part of the steeply overturned beds (general strike 55˚, dip 72˚ SE). Pyroclastics such as spillitic tuffs (“Schalstein”), which form the upper part of the volcanic rocks of the Hauptgrünstein in the area, underlie the limestone series and were excavated in the southern part of the formerly bipartite quarry (Fig. 1). EDER et al. (1977: fig. A14) indicated the presence of tuffitic layers also in the lower part of the limestone section. The overlying nearly 70 m eschweizerbartxxx

of limestones are interpreted as debris flows and calcareous turbidites (STRITZKE 1991). The graptolite-bearing black shales are part of an about five meters thick shale-limestone interval which forms only a minute portion at the base of the approximately 70 m thick limestone succession overlying pyroclastics of the Hauptgrünstein Formation. The conodont biostratigraphy shows the presence of late Givetian to early Frasnian strata, ending in the early Palmatolepis gigas Biozone (STRITZKE 1991: fig. 2). Slightly earlier, in the Ancyrognathus asymmetricus Biozone, the first nodular limestones (Flaserkalke) appear in the section, intercalated with further debris flows. These higher parts of the succession were described by EDER et al. (1977). In the uppermost ca. 20 m of the section, from the upper Palmatolepis gigas Zone onward, exclusively nodular limestones are present until the lowermost Carboniferous. The typical Kulm succession of the Lower Carboniferous of the Rheinisches Schiefergebirge terminates the section with shales and alum shales still visible in the faulted and disrupted upper corner at the northern end of the quarry. The dendroid graptolites come from the basal part of the limestones of the Burgberg locality. The measured section starts at the top of the Hauptgrünstein. The uppermost unit of the Hauptgrünstein consists of a ca. 2 m thick layer of grey-yellowish, deeply weathered tuffs. The basal unit of the limestone succession contains a mixture of mostly pale grey crinoidal limestone and variably hue-stained tuff with a nodular appearance of the bedding. Above a thin layer of black shale it is followed by further dark bioclastic limestones with minor black shale interbeds. The interval at 2.5–3.0 m includes at least six dark limestone beds intercalated in silty black shale layers, but only the lowermost black shale band at about 2.5 m from the base of the section yielded a rich fauna of comparably well preserved graptolites. In the remaining black shale layers only small indeterminable fragments of dendroid graptolites can be found. The interval shows a lensoid character of the limestone layers with slumping features and differentiated compaction of the sediment. In the black shales only the graptolite fragments have been discovered, whereas the limestone layers bear a rich fauna of crinoids, corals, gastropods and brachiopods (STRITZKE 1991). They are deposited as organodetritic debris from the close-by Brilon reef to the north. Based on the included conodont faunas, the base of the Burgberg section can probably be localised in the middle Palmatolepis varcus conodont zone. The lower 1.7 m of the limestone section is not dated, however. Faunas of the middle Palmatolepis varcus Biozone appear in the 1.7–5.0 m interval (STRITZKE 1991).

Graptolite preservation The dendroid graptolites from the Burgberg locality are poorly preserved and the slabs often show a consider-

Devonian dendroid graptolites from the Brilon reef area (Germany)

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Fig. 1. Map of the area and the Burgberg quarry with indication of the section, based on EDER et al. (1977).

able degree of weathering. The graptolites are preserved mostly as light grey or white to slightly greenish pressure shadow minerals in which few remains of the original organic periderm are visible (Fig. 3F). The periderm material is preserved in the form of dark brown to black spots of organic particles swimming in the matrix of the silicate pressure shadow minerals. The light colouring of the pressure shadow minerals produces the strong contrast to the dark, silty shale in which the graptolites are preserved. Thus, the visible graptolite specimens do not represent the actual remains of the fossil organisms, but are in fact the pressure shadow minerals, that may have distorted the rhabdosomes considerably (UNDERWOOD 1992). Exact measurements of the rhabdosome dimensions and specific identifications may be unreliable, therefore. A similar preservation is generally seen in the Silurian to Lower Devonian graptolites of Thuringia and other regions of Germany (see for example JAEGER 1959; SCHAUER 1971), while pressure shadow minerals are not found in the well preserved Ordovician graptolites of the Rügen drill-cores (MALETZ 1998) and the G14 drill-core (MALETZ 1997). All graptolites are completely flattened and do not show any internal structure. Not even sclerotized stolons or pyrite fillings of the rhabdosomes are visible in the available specimens. A strong tectonic deformation is not obvious in the fragmented material, but this is probeschweizerbartxxx

ably due to the shapes of the dendroid colonies and to the poor preservation of the material. In many Ordovician to Devonian graptoloids the deformation is visible in parallel cracks in the periderm (UNDERWOOD 1992: fig. 3A; SCHAUER 1971) showing the orientation of tectonic shearing of the rhabdosomes. This is not visible in the material from the Burgberg as little periderm is left. However, this does not mean that the material is not tectonically sheared and distorted. Due to the poor preservation showing few rhabdosome details, no specific identifications are possible.

Palaeoecology All graptolites from the Burgberg locality belong to the order Dendroidea that lived as benthic faunal elements attached to the sea bottom or on hard substrates, on hardgrounds or larger detritus like brachiopod and trilobite shells. They are known to have flourished in restricted shelf environments, but are seldom preserved in these. A good example of the life style of dendroid graptolites and the preservation in their original environment was given by ERDTMANN (1974, 1976) from the Silurian Mississinewa Shale of North America. In this material the fan- or cone-shaped rhabdosomes of the dendroid graptolites are preserved in their original orientation and shape on the sea bottom, but were covered with sedi-

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Palaeodictyota JUX 1967

Thallogr., Desmogr. Dictyonema

Ophigraptus

KRAFT 1984

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Dictyonema

RUEDEMANN, 1947

Dictyonema Dendrogr., Desmogr.

Dictyonema, Callogr.

ˇ , 1957 BOUCEK Fig. 2. Biostratigraphic distribution of Devonian dendroid graptolites in Germany.

ment. Hold-fasts and root structures are still connected to the rhabdosomes and indicate the life orientation of the colonies. The more complete graptolite rhabdosomes are preserved in a single layer of dark grey to black, silty shale in the lower part of the Burgberg section. The surrounding lithological interval includes carbonatic debris flows with a strongly lensoid character of the individual beds. Rich faunas of crinoids, corals, gastropods and brachiopods were brought from the Brilon reef to the north into this somewhat deeper depositional environment. The strata are interpreted as organodetritic debris flows by STRITZKE (1991) in which the fossil components are largely broken. The debris flows are interbedded with thin black shale layers in which no fossils were found, except for the layer with the dendroid graptolites. The black shale layers may be interpreted as the background sedimentation or better to represent the uppermost, more fine grained material of the turbiditic flows. The graptolite rhabdosomes in the black shale layer are in most cases strongly fragmented. Often only small parts of the individual meshes have been found (Fig. 3). The larger fragments belong to more robust species as is seen from the rhabdosomes. However, parts of the shale layer also contain entangled meshes of the dendroids that cover larger surfaces (Fig. 3B). In these cases the specimens partly cover each other and the individual colonies are difficult to recognise. Complete rhabdosomes are rare and have been found only in very few specimens. In these, the basal disc of attachment is broken off and is not preserved (Figs. 3A, C). Thus, no indication for an in situ preservation is given in which the eschweizerbartxxx

hold-fast structures and roots should be preserved and the rhabdosomes seen in their original orientation to the substrate on which they were growing. The association of the graptolites and their mode of preservation indicates transport of the material from a possible near-by location in shallow water into the deeper water area, where this taphocoenosis is preserved. The true distance of transport is uncertain, but may not have been too far as otherwise the delicate rhabdosomes might have been destroyed completely. The dendroid graptolites certainly were growing close to the rim of the reef platform in shallow water and have been transported by a storm event into the final burial place. During this process most of the rhabdosomes have been broken into pieces. However, some rhabdosomes survived the harsh treatment, but also show indications of the transport as they are twisted and turned with their branches becoming entangled or partly disrupted (Figs. 3B, C). In places the rhabdosomes are washed together by currents. The material then was quickly buried in the mud before another shale- or limestone layer covered the whole. Due to the fast covering by sediment the delicate organic material of the rhabdosome periderm was preserved and was only altered by the younger tectonic movements and folding of the sediments. During these processes the pressure shadow minerals outlining the graptolites were growing.

Dendroid graptolites in Germany Dendroid graptolites have rarely been described from Germany and, thus, merit special interest when discovered. The record consists generally of isolated fragments


of rhabdosomes of dubious nature and taxonomic status. Mostly, the material cannot be identified to species or even genus level, as the preservation is very poor. Ordovician dendroids are unknown, except for a single specimen of Aspidograptus sp. from the Leimitz Shales (Tremadoc) of Bavaria (SDZUY 1961). SCHMIDT (1940) described Ptilograptus thuringicus as another poorly preserved species from the Upper Ore Horizon (Caradoc) of Thuringia. A few records of Silurian and Devonian material are published, but also represent special circumstances. JAEGER (1992) indicated that dendroid graptolites are totally missing in the Silurian of Thuringia due to the deposition of anoxic sediments, mainly black alum shales and lutites, of the Thuringian Facies. The author, when referring to further descriptions of dendroids from Germany, indicated the presence of dendroids in the Silurian (Upper Ludlow) of the Harz Mountain. The material, however, has not been described so far. STEIN (1965: 131) mentions a dendroid fragment as “Dendroidee (Gen. indet.)” from the Wenlock of the Boxmühle locality in the Frankenwald, but did not discuss or figure the material. JUX (1967) described a single specimen of a dendroid graptolite as ?Palaeodictyota montana from the Paffrath Syncline of the Rheinisches Schiefergebirge. The specimen comes from the Sand-Schichten of Late Frasnian age (RIBBERT 1998). The material clearly shows bundles of thecae forming the stipes of a typical acanthograptid. KOWALSKI (1987) described two specimens of Dictyonema from the Middle Devonian of the Eifel region. The smaller specimen from Gerolstein (Dictyonema sp. A in KOWALSKI 1987) consists of a rhabdosome fragment with 0.4 mm wide stipes that bear thin dissepiments. Thecae are difficult to observe, but indicate a thecal density of 10 th/10 mm. It is impossible to refer the specimen to a certain dendroid genus or species. It comes from the Loogh-Schichten of early Givetian age, but the exact locality was not given in the original description. The specimen from the Gees locality (Dictyonema sp. B of KOWALSKI 1987) also is a fragment of a much larger colony. It bears slender stipes and very thin dissepiments. The stipes reach a thickness of 0.15– 0.2 mm and count about 16–20 stipes in 10 mm. The dissepiments are thin, measuring 0.04–0.08 mm in thickness and are placed variably perpendicular or obliquely to the stipes. The fragment is more than 60 mm long and represents a considerable piece of the colony. However, it is not possible to reconstruct the colony shape from this fragment. The specimen comes from the Gees horizon of the Ahrdorf-Schichten of mid-Eifelian age. JAEGER (1992) described Ophigraptus hercyniae from a well preserved relief specimen found in the Harz Mountains. The specimen comes from the interval of the Lower/Middle Devonian boundary, probably from the Novakia sulcata antiqua Biozone of the uppermost Emsian. It is associated with the tentaculids Novakia cf. suleschweizerbartxxx

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cata antiqua ALBERTI, 1981 and Styliolina sp. The genus Ophigraptus was indicated to differ from other dendroids through its zigzag shaped main stipe bearing branched secondary stipes. The thecae show the typical dendroid structure with associated bithecae positioned alternately on both sides of the stipes (see JAEGER 1992: figs. 1, 5). The shape of the rhabdosome and the proximal end of the colony is not known. Thus, the erection of a new genus for this material may be questionable. It could easily be included in the genus Dendrograptus, as it does not bear dissepiments. Dendroid graptolites are indeed rare in Germany, but have been found scattered through Middle and Upper Devonian strata of the Rheinisches Schiefergebirge and the Harz Mountains. The record shows that they may actually have had a wider distribution and were probably more common than indicated from the few records. They certainly were living in the shallow shelf areas, but have been preserved only in the few cases in which they were carried away from their original habitats into the deeper water regions in which they were embedded in the shales and siltstones. None of the specimens is preserved in life position in the environments in which it was living. This explains the rarity of the dendroids in the German Devonian and the poor and fragmentary preservation of the material.

Systematic palaeontology The taxonomy of the Dendroidea is based on BULMAN (1970) and CHAPMAN et al. (1996), who established a preliminary classification for the Dendroidea, based on morphological, stratigraphical and evolutionary data. The figured material is housed in the collection of Forschungsinstitut Senckenberg (SMF XXIV-425 to SMF XXIV-432), as is a small collection of additional material from the Burgberg locality. Order Dendroidea NICHOLSON, 1872 Family Dendrograptidae ROEMER in FRECH, 1897 Genus Dictyonema J. HALL, 1851 Dictyonema sp. A Fig. 3F Material: One small slab with a single fragment of a rhabdosome.

Description: The slab has a size of 40 × 20 mm and bears a fragment of the mesh covering the whole slab. The mesh clearly is a small distal fragment of a very large colony. The stipes are constantly about 0.3 mm wide and are oriented nearly parallel to each other, except for the branching divisions. The distance between the stipes is about 1.0–1.8 mm. Thus, there will be about 6 stipes in 10 mm of the mesh. Dissepiments are sparsely distributed in the rhabdosome. There are usually distances between 10 and 12 mm between the points of in-

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sertion of the dissepiments. The whole mesh looks very coarse compared to other species from the same locality. The dissepiments are 0.3 mm wide, thus, are as wide as the stipes. They are positioned strictly perpendicular to the stipes. Thecal apertures are not seen in the specimen and the thecal spacing is not possible to determine. Remarks: The single fragment of Dictyonema sp. A shows a very coarse mesh with comparably slender stipes, that has not been described before. Similarly coarse meshes are present in specimens of Rhabdinopora flabelliformis anglica (COOPER et al. 1998: figs. 5, 6), but this lower Tremadocian planktic species bears much wider stipes and is certainly not closely related to the here described dendroid species of the genus Dictyonema. Probably further discoveries will show that the fragment represents a fairly distal rhabdosome fragment of a species in which the more proximal mesh is much denser in composition.

fast. The colony has reached a length of at least 30 mm, but may have been much larger in mature specimens. The proximal end may have been overgrown with cortical tissue (Fig. 3C), as the number and orientation of the proximal stipes is obscured. Thus, the number of first order stipes and their origination from the sicula is not clear. The stipes are about 0.1 to 0.15 mm wide and do not change their width during growth. There are 8 to 9 stipes in 10 mm near the proximal end. Distally the rhabdosomes show about 7 stipes in 10 mm. The dissepiments are widely spaced and seem to be rarely preserved as they are fairly thin. The thickness in the few preserved dissepiments measures about 0.05 mm. They are oriented perpendicular to oblique to the stipes. The thecae are not seen on any of the stipes. Thus, their shape and development in unknown. The constant width of the stipes throughugh the rhabdosome indicates that little cortical overgrowth might be found in this interesting species during the growth of the colony.

Dictyonema sp. B Fig. 3G Material: Several distal rhabdosome fragments without proximal ends.

Description: The fragments do not show the complete shape of the rhabdosome. It is therefore not clear whether the rhabdosome is conical or wedge shaped. The fragments often show a wedge shaped appearance, but this could also be found in a fragment of a conical rhabdosome. The mesh is dense with about 1 mm wide stipes. There are about 5 to 6 stipes in 10 mm. The open spaces between the stipes are not wider than the stipes themselves. The dissepiments are between 2 and 5 mm apart. They are 1.0–1.1 mm wide and oriented perpendicular to oblique to the stipes. The openings in the mesh are elongated and strongly rounded due to the widening of the dissepiments at their attachment sites to the stipes. It is not possible to observe the thecae in the rhabdosomes due to inadequate preservation of the fragments. eschweizerbartxxx

Remarks: Dictyonema sp. B is characterised by its robust mesh with wide stipes, that easily differentiates the species from other material in the Burgberg locality. Dictyonema sp. C Figs. 3A–D Material: Many fragments and three more completely preserved specimens without attachment roots or disc.

Description: The rhabdosome shape in this species is a wide cone as is seen from the more complete specimens (Figs. 3A, C). It widens quickly from the proximal end on. A probably largely complete specimen preserved in lateral view (Fig. 3A) bears a rhabdosome angle of about 90˚, but seems to be broken immediately above the hold-

Remarks: Dictyonema sp. C is the most common species in the Burgberg locality. The thin stipes easily separates this species from others. The thin and irregularly distributed dissepiments are only rarely preserved and the rhabdosomes in many cases reminds of a species of the genus Dendrograptus. Dictyonema sp. C resembles strongly Dictyonema sp. A. Both possess a fairly wide mesh with slender stipes, but in Dictyonema sp. C the stipes are much more slender, whereas the spacing of the stipes is identical. Due to the robust stipes seen in Dictyonema sp. A it is unlikely that this species represents the distalmost parts of the rhabdosome of Dictyonema sp. C. There are no indications that the stipes widen distally in Dictyonema sp. C. Similarities to further described species from the Devonian are found only with Dictyonema (Dictyonema) elongatum BOUCˇ EK, 1957 from the Devonian of Bohemia. The species, however, has slightly wider stipes and more densely spaced dissepiments (BOUCˇEK 1957). Also Dictyonema (Dictyonema) subtile BOUCˇEK, 1957 bears more densely spaced stipes and dissepiments.

Fig. 3. Dendroid graptolites of the Burgberg section. – A: Dictyonema sp. C, SMF XXIV-425, laterally preserved rhabdosome. – B, D: Dictyonema sp. C, SMF XXIV-426, slab with many fragments, showing the thin mesh of the species. – C: Dictyonema sp. C, SMF XXIV-427, nearly complete colony with overturned proximal end, showing the conical rhabdosome shape. – E: ?Ruedemannograptus sp., SMF XXIV-428, fragment. – F: Dictyonema sp. A, SMF XXIV-429, the only fragment of this species, shows the coarse mesh. – G: Dictyonema sp. D, SMF XXIV430, distal fragment of the rhabdosome. – H: Dictyonema sp. B, SMF XXIV-431, fragment. I: ?Ruedemannograptus sp., SMF XXIV-432, proximal fragment. – Magnifications x2 for all specimens.


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Dictyonema sp. D Fig. 3H Material: Several robust distal fragments of the colonies.

Description: The rhabdosome fragments are too poorly preserved to show the colony shape of the complete rhabdosome. Fragments of 40 to 50 mm in length are present in the collection. They show a stipe density of 10 stipes in 10 mm. The stipe width is about 0.5–0.8 mm. Thus, the stipes are wider than the interstipe holes, giving a very dense appearance to the rhabdosome. For most of their length the stipes are parallel to each other. Branching occurs infrequently indicating that most fragments are fairly distal ones. The dissepiments are about 0.1 to 0.2 mm wide and spaced at 10 per 10 mm. They are often obliquely oriented to the stipes. The thecae are not seen in any of the rhabdosome fragments. Family Nephelograptidae BOUCˇEK, 1957 Genus Ruedemannograptus TERMIER & TERMIER, 1948 ?Ruedemannograptus sp. Figs. 3E, I Material: Two poor fragments of the rhabdosome.

Description: The rhabdosome fragments are up to 25 mm long and 30 m wide, but represent only minor parts of the complete colonies. The exact shape of the dendroid colonies therefore cannot be estimated. The material is poorly preserved and few characters are visible. One fragment (Fig. 3E) indicates infrequent branching, but denser branching occurs in the second fragment (Fig. 3I). The stipes are about 0.5 mm wide, but seem wider as they are very irregular. This may be due to the thecal apertures that are arranged to be shown on both sides of the stipes. The thecal apertures, however, cannot be verified as the preservation is too poor. There are 6 to 7 stipes in 10 mm of the rhabdosome. The dissepiments are difficult to observe. They may be spaced at 8–10 dissepiments in 10 mm. The dissepiments are thin, up to 0.15 mm wide and often curved. They seem to be originating from the thecal apertures. eschweizerbartxxx

Remarks: The material is too poorly preserved to show the thecal arrangement more clearly. The highly irregular shape of the stipes, however, indicates that the thecae are positioned on both sides. Therefore the material is preliminarily included in Ruedemannograptus. The genus Ruedemannograptus TERMIER & TERMIER, 1948 is known from the Ordovician of North America, where species of the genus were included in Streptograptus RUEDEMANN, 1947 (a junior synonym of Streptograptus YIN, 1937). BOUCˇ EK (1957) described Ruedemannograptus prantli from the Lochkovian (Lower Devonian) of Bohemia as the only species of the

genus known from Europe. Ruedemannograptus prantli is more robust with widely spaced stipes and differs strongly from the fragments described herein.

Comparison There are very few faunas of dendroid graptolites described from the Devonian worldwide and a taxonomical comparison therefore is difficult. The most comprehensive overview on North American dendroid graptolites from the Devonian was given by RUEDEMANN (1947). He listed several Dictyonema species from Lower and Middle Devonian strata, but the material is not comparable in detail with the Burgberg material as it consists of quite different species. The faunas from the Onondaga Limestones of New York State include at least four species, Dictyonema hamiltoniae, Dictyonema leroyense, Dictyonema megadictyon, and Dictyonema perradiatum. These need to be reinvestigated before a comparison can be made. None of them reminds of the slender stiped Dictyonema sp. A and Dictyonema sp. C with their typical wide meshes from the Burgberg locality. Especially Dictyonema sp. C with its very thin stipes and delicate rhabdosome differs considerably from all described species. It may be similar to Dictyonema lochmannae RUEDEMANN, 1942 from the Carboniferous (Mississippian) Englewood Formation, but the latter, much younger species has wider stipes (up to 0.3 mm) and densely spaced dissepiments. The material from the Givetian of Iowa, Illinois and Michigan described by BERRY (1969) includes material of the genera Desmograptus and Dictyonema, but is also not comparable with the material from the Burgberg locality in which Desmograptus is not represented at all. Devonian graptolites are well known from Bohemia and a number of different species have been described. The rich dendroid graptolite faunas of Lochkovian age were discussed and illustrated by BOUCˇEK (1957), whereas younger material was documented by KRAFT (1984) in some detail. BOUCˇ EK (1957) described rich dendroid graptolite faunas from the Monograptus uniformis and Monograptus hercynicus Biozones of Bohemia. The faunas include as the most common genera Palaeodictyota, Thallograptus and Coremagraptus, but also Dictyonema, Reticulograptus, Acanthograptus, Aspidograptus and Ruedemannograptus are present. In the Monograptus uniformis Biozone alone, the author found 24 species of dendroid graptolites. None of these is identical with a species from the Burgberg locality. Only very few specimens from the Eifelian and Givetian strata of Bohemia are known and these are generally in a fragmentary state of preservation. KRAFT (1984) described Thallograptus jahni (PRANTL, 1950), Desmograptus prantli KRAFT, 1984, and two species of Dictyonema (not figured). Except for one species of Dictyonema, all material comes from the Givetian.


None of the species is comparable with the material from the Burgberg locality. The fauna of UBAGHS (1941) from the Lower Visean (Carboniferous) of Belgium is much younger, but is well preserved. The genera Dictyonema, Callograptus, Desmograptus, Ptiograptus and ?Dendrograptus are differentiated. The material is not comparable with the dendroids from the Burgberg locality.

Acknowledgements Frank Langenstrassen (Univ. Göttingen) and B.-D. Erdtmann (TU Berlin) provided some of the material, as well as additional help with literature reseach.

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