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Keywords:

  • autotheca;
  • bitheca;
  • Desmograptus;
  • dendroid graptolite;
  • stolon;
  • ultrastructure

Abstract

  1. Top of page
  2. Abstract
  3. Stratigraphical and Depositional Setting
  4. Systematic Palaeontology
  5. Internal Structure
  6. Discussion
  7. Acknowledgments
  8. References

Abstract: Desmograptus  micronematodes from Thornton quarry, Cook County, Illinois, USA, remarkably preserved in relief and encased in pyrite, is described. The internal details of the thecae, and of the stolon system, examined using the SEM, allow the reconstruction of the growth of a stipe. The stolons and stolonal nodes are formed of a dense crassal fabric, and are surrounded by a loose fabric of three-dimensional fibrils. The nodes have a complex structure of three boxes with proximal and distal nozzles. The base of a bitheca, and the base of each autothecal cup, has a central nozzle surrounded by a unique honeycomb fabric. The ultrastructure of the cortical fibrils, with their clockwise striations, is similar to that in Dendrograptus, and may be universal in the graptolites. The nodes of the stolon system appear identical to those of Acanthograptus and other dendroids, but differ from those of the extant hemichordate Rhabdopleura, which have only a diaphragm, and lack box structures.

Desmograptus is a long-ranging (Lower Ordovician to Carboniferous) dendroid graptolite with a widespread occurrence. It is characterized by having a conical rhabdosome with stipes which unite by regular anastomosis. Currently little is known about its stolonal structure and branching pattern. However, the discovery of material of the Silurian species Desmograptus  micronematodes (Spencer, 1884a), preserved in relief and outlined in pyrite, has enabled us to examine its internal structure, including the stolonal system, in detail.

Comparison of the internal structures with those of both early and late Ordovician dendroids illustrated by Crowther (1981) suggests that the stolon structure is a conservative feature of the order. Comparison with modern rhabdopleurans is less exact, with no box-like structure developed in their nodes.

Stratigraphical and Depositional Setting

  1. Top of page
  2. Abstract
  3. Stratigraphical and Depositional Setting
  4. Systematic Palaeontology
  5. Internal Structure
  6. Discussion
  7. Acknowledgments
  8. References

Desmograptus  micronematodes was collected from the lower graptolite bed (a Konservat-Lagerstätte) of the Racine Dolomite Formation (Silurian, Wenlock) at Thornton, Cook County, Illinois, USA (Text-figs 1–2). A large, well-known reef, the Thornton Reef is exposed in quarries owned by the Material Service Corporation, and in road cuts along interstate highways I-80 and I-294, 8 km west of the Illinois–Indiana border (Mikulic and Kluessendorf 1999).

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Figure TEXT-FIG. 1..  Geological map of north-east Illinois showing the location of Thornton quarry (modified after King and Beikman 1974).

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Figure TEXT-FIG. 2..  Section of Silurian strata in Thornton Quarry, showing the location of the graptolite horizons (modified from Loydell et al. 2002).

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Two dendroid graptolite-bearing beds comprise the Thornton Konservat-Lagerstätte, which is located in the inter-reef beds adjacent to and beneath distal reef flank strata (Kluessendorf et al. 1999; Loydell et al. 2002, fig. 2) exposed in the lowest level of the middle pit.

The lower graptolite bed is c. 2.74–3.05 m thick and consists of a very dark brown to black, pyrite-rich, thinly laminated (1–5 mm) and bedded argillaceous dolomite, with some chert nodules. The bedding is deformed and conforms to the irregularity of the bed below. As well as dendroid graptolites, Monograptus priodon-flemingii, retiolitids, conodonts, chitinozoans, disarticulated phyllocarids, xiphosurans, eurypterids(?), algae and worms(?) have been recorded (Kluessendorf et al. 1999; Loydell et al. 2002). Most of the bedding surfaces are covered with very small fragments of brown coloured algae and usually black dendroid graptolites.

The lower graptolite bed is of late Sheinwoodian (early Wenlock) age based on graptoloid graptolite, chitinozoan and conodont occurrences (Loydell et al. 2002).

The dendroid graptolites collected from the lower graptolite bed show a continuous range of preservational styles from full relief to completely diagenetically flattened (without tectonic distortion), with different degrees of peridermal fragmentation. Many of the flattened specimens contain disseminated pyrite and framboids scattered throughout the rhabdosome. A few specimens were found in full relief and these are mainly seen as small isolated fragments. Ultrastructural details can be seen on some isolated specimens but are commonly obscured by pitting produced by dolomite rhombs. However, some specimens of D. micronematodes are preserved with a 0.04–0.07 mm coating of pyrite surrounding the stipes (Text-fig. 3).

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Figure TEXT-FIG. 3.. Desmograptus micronematodes (Spencer, 1884a). SEM back-scatter image of the area of specimen PE60371 shown in Text-figures 4 and 11. A, limestone matrix. B, pyrite coating the rhabdosome. C. graptolite. Scale bar represents 100 μm.

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Systematic Palaeontology

  1. Top of page
  2. Abstract
  3. Stratigraphical and Depositional Setting
  4. Systematic Palaeontology
  5. Internal Structure
  6. Discussion
  7. Acknowledgments
  8. References

Class GRAPTOLITHINA Bronn, 1849 Order DENDROIDEA Nicholson, 1872 Family DENDROGRAPTIDAE Roemer, in Frech, 1897 Genus DESMOGRAPTUS Hopkinson, in Hopkinson and Lapworth, 1875

Type species. Dictyograptus  cancellatus Hopkinson, in Hopkinson and Lapworth, 1875, from the Lower Ordovician of south Wales, UK.

Emended diagnosis.  Rhabdosome conical, possibly fan-shaped; simple stipes; regular, dominant anastomosis; rare dissepiments; autothecae denticulate to isolate, opening at regular intervals with associated bithecae.

Discussion.  As noted by Rickards et al. (1990), Desmograptus forms a morphological and evolutionary series with Dictyonema and Callograptus, suggesting that these are probably form genera and would not stand up to rigorous phylogenetic analysis as valid separate entities. The genus Desmograptus is characterized by anastomosing stipes, which do not involve the transfer of thecae; therefore the stipes effectively remain independent. The distribution and number of dissepiments within the rhabdosomes of species of Desmograptus are variable. For example in Desmograptus  cumingsi and D. micronematodes the whole rhabdosome is dominated by anastomosis, and dissepiments are relatively uncommon, although distributed regularly throughout the rhabdosome. However, in other species, small parts of the rhabdosome (both proximally and distally) are dominated by dissepiments whereas the majority of the rhabdosome is connected by anastomosis with sporadically developed dissepiments.

Shrock (1928) divided specimens of D. micronematodes from the Mississinewa Shale Member (Wabash Formation), Indiana, into four groups because they showed considerable variation in shape, size and ‘pattern or structure’. It is possible that a morphological series exists between all species within the genus from specimens with large, to specimens with small, fenestellae. The development of dissepiments and fenestella size may be related to environmental parameters.

Desmograptus  micronematodes (Spencer, 1884a) Plates 1–3; Text-figures 3–15

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Figure EXPLANATION OF PLATE 1.   Figs 1–10. Desmograptus  micronematodes (Spencer, 1884a). 1, PE60376; longitudinal section of fusellar wall of autotheca, with external cortex on the left. 2, PE60373; longitudinal section of fusellar wall of autotheca; the internal surface of the theca on the left bears a thin endocortical bandage, with the cortical fibrils running longitudinally; F: fusellar layer; EC: external cortex. 3, PE60377; three bandages of the external cortex; each bandage is formed of parallel fibrils, covered with a granular sheet containing vesicles. 4, PE60372; fusellar fabric. 5, PE60377; internal cortex, with granular sheet fabric and the margin of a bandage (X – X); the two holes may be the result of fungal boring. 6, PE60377; successive layers of the periderm, looking towards the exterior, with growth upwards; A: fuselli, and B: cortex of an autothecal wall; C: spatulate bases of the fuselli of a second, and later, thecal wall attached to the earlier one; F: parallel fibrils underlying the bounding lamella of a fusellus. 7, PE60377; transverse section through an autothecal wall (A), and the spatulate bases (B) of later walls (Text-fig. 8). 8, PE60372; the spatulate bases of successive fuselli. 9–10, PE60371; anastomosis of two stipes, with extrathecal tissue formed between them. Scale bars represent 1 μm (figs 1, 3–5), 10 μm (figs 2, 6–8, 10) or 100 μm (fig. 9).

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Figure EXPLANATION OF PLATE 2.   Figs 1–7. Desmograptus  micronematodes (Spencer, 1884a). 1, PE60377; stolon; note the dense and fractured crassal fabric, and the fusellar fabric between the stolon and its enclosing thin-walled tube (T). 2, PE60372; thin-walled stolon. 3, PE60377; stereopair view, looking obliquely proximally, of a branching node (BN1 of Text-fig. 10); A: an earlier autothecal cup. 4, PE60377; stolon fractured longitudinally. 5, PE60377; close-up of part of the area of fig. 3, just distal to a branching node, with two stolons (S) of the proximal portions of two autothecae, a distal stolon (A), and thin internal thecal walls (W). 6, PE60377; anastomosing fibrils within an autotheca. 7, PE60377; stereopair view, looking obliquely proximally, just distal to a branching node. Scale bars represent 10 μm (figs 1–5, 7), or 1 μm (fig. 6).

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Figure EXPLANATION OF PLATE 3.   Figs 1–10. Desmograptus  micronematodes (Spencer, 1884a). 1, PE60371; stolon within theca, looking distally towards the node. 2, PE60371; proximal face of node, looking obliquely distally. 3, PE60377; distal face of node, looking proximally; B: bitheca; S stolon. 4, PE60377; transverse cross-section of node looking proximally. 5, PE60377; fractured node, looking obliquely proximally; S: stolon; B: base of bitheca. 6, PE60377; stereopair view of node; distal to right; B: bitheca; S: stolon. 7, PE60377; distal face of triad node, looking proximally; S: the triangular autothecal stolon. 8, PE60371; distal face of node, at base of a bitheca; the central pore has a rim, which has a granular ornament, and extends in lobes. 9, PE60377; the rim of the distal pore, with a granular ornament. 10, PE60377; honeycomb fabric of the distal face of a node. Scale bars represent 10 μm (figs 1–7) or 1 μm (figs 8–10).

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Figure TEXT-FIG. 4.. Desmograptus micronematodes (Spencer, 1884a). Specimens with stipes coated with pyrite. A, specimen subsequently coated for examination by SEM; PE60377 (counterpart PE60371). B, specimen PE60378.

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Figure TEXT-FIG. 5..  Thecal labelling. A, the conventional labelling of dendroid graptolite thecae; s: stolotheca; b: bitheca; a: autotheca. B, revised labelling, after Kirk (1969), as adopted here; a0(s): ‘stolothecal’ part of first autotheca. On this scheme, the first autotheca (a0) produced by the sicula has no associated auto- or bithecae: at the first node autotheca a1 and bitheca b1 are produced.

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Figure TEXT-FIG. 6..  Sicula and initial thecae of Dendrograptus after Kozłowski (1949), relabelled as in Text-figure 5 (following Kirk 1969).

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Figure TEXT-FIG. 7..  A–D, Successive growth stages. The stolon system is shown in grey; PSa1: the proximal ‘stolothecal’ part of autotheca a1; CSa1 the central part, with the triad node giving rise to autotheca a2 and bitheca b2; DSa1: distal part (autothecal cup) of autotheca a1 from the end of the stolon to the final aperture.

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Figure TEXT-FIG. 8..  A, diagrammatic reconstruction of a thecal wall with a half tube attached to it. B, horizontal section indicated by H on A. C, vertical section indicated by V on A. Stippling indicates fusellar tissue filling fuselli.

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Figure TEXT-FIG. 9.. Desmograptus micronematodes (Spencer, 1884a). A, PE60377, base of bitheca. B, PE603777, bitheca of Text-figure 10, between diverging stipes. C, PE60377, oblique view of base of bitheca. Scale bars represent 10 μm (A, C), 50 μm (B).

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Figure TEXT-FIG. 10.. Desmograptus micronematodes (Spencer, 1884a). A–C, PE60377, small distal stolon (S) leading to the base of an autothecal cup (A). 3, PE60377, the small chamber (C) at the base of an autothecal cup, looking proximally; B, S: bitheca and stolon generated at the previous node. D, PE60377, distal face of node at the base of a bitheca; note the extensions of the rim of the pore, surrounded by honeycomb fabric. E, PE60377, stereopair view of the region of the base of an autothecal cup (A), with the small distal stolon (S) leading to it. F, PE60371, stereopair view of the base of an autothecal cup, looking obliquely distally; the succeeding authotheca (A) and bitheca (B) both curve around it. Scale bars represent 10 μm (A–D), 50 μm (E–F).

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Figure TEXT-FIG. 11.. Desmograptus  micronematodes (Spencer, 1884a), PE60377. Area of stipe branching; branching node BN1 is also illustrated in stereopair view in Plate 2, figure 3. A, autotheca. B, bitheca.

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Figure TEXT-FIG. 12.. Desmograptus micronematodes (Spencer, 1884a). A, PE60371, autothecal cup. B, PE60377, two authothecal cups: that on the left is fractured within the wall, exposing the fuselli; that on the right shows the smooth interior surface. B-B: bitheca. Scale bars represent 100 μm.

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Figure TEXT-FIG. 13.. Desmograptus micronematodes (Spencer, 1884a). A, PE60377, stereopair view of base of autothecal cup of Text-figure 12B, looking proximally. B, PE60377, stereopair view of the distal part of the area of Text-figure 10, looking obliquely proximally. C, PE60376, cortical tissue; the parallel cortical fibrils bear a spiral ornament; thinner cross-fibrils occur at intervals. D, PE60376, fusellar fabric; arrows indicate probable spiral ornament. Scale bars represent 100 μm (1A–B), 200 nm (C–D).

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Figure TEXT-FIG. 14..  Diagrammatic representation of stolon system. A, looking obliquely proximally at a non-branching node. B, looking obliquely distally at a branching node.

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Figure TEXT-FIG. 15..  A, diagrammatic vertical section through the stolon system. B, diagrammatic reconstruction of a normal node in Acanthograptus impar Bulman and Rickards (redrawn from Bulman and Rickards 1966, fig. 25). C, Rhabdopleura compacta diagrammatic longitudinal section of bifurcation of stolon and base of zooidal tube (after Urbanek and Dilly 2000, fig. 1D). D, diagrammatic longitudinal section through the stolon system of Rhabdopleura compacta (after Urbanek and Dilly 2000, fig. 10B).

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1878 Rhizograptus  bulbosus Spencer, p. 460.

1882 Calyptograptus  micronematodes Spencer, p. 165.

*1884a Calyptograptus  micronematodes Spencer, pp. 564, 579, 588, pl. 3, figs 4–4a.

1884a Rhizograptus  bulbosus Spencer, p. 580, pl. 4, fig. 4.

1884b Calyptograptus  micronematodes Spencer, pp. 14, 29, 38, pl. 3, figs 4–4a [copy of Spencer 1884a, pl. 3, figs 4–4a].

1884b Rhizograptus  bulbosus Spencer, p. 30, pl. 4, fig. 4 [copy of Spencer 1884a, pl. 4, fig. 4].

1889 Rhizograptus  bulbosus Spencer; Miller, p. 202, text-fig. 215 [copy of Spencer 1884a, pl. 4, fig. 4].

1896 Calyptograptus  micronematodes Spencer; Gurley, pp. 93, 308.

1896 Rhizograptus  bulbosus Spencer; Gurley, pp. 101, 308.

v.1909 Calyptograptus  micronematodes Spencer; Bassler, p. 39, text-fig. 49 [copy of Spencer 1884a, pl. 3, fig. 4–4a, text-fig. 50.

1909 Rhizograptus  bulbosus Spencer; Bassler, p. 41, text-fig. 52 [copy of Spencer 1884a, pl. 4, fig. 4].

1915 Calyptograptus  micronematodes Spencer; Bassler, p. 170.

1915 Rhizograptus  bulbosus Spencer; Bassler, p. 1113.

1925 Desmograptus  micronematodes Spencer; Ruedemann, p. 21, text-figs 7–11.

v.1928 Desmograptus  micronematodes Spencer; Shrock, p. 27, text-fig. 13c.

1928 Desmograptus  micronematodes Spencer; Cumings and Shrock, p. 216, text-fig. 13c [copy of Shrock 1928, text-fig. 13c].

1931 Desmograptus  micronematodes Spencer; Roy and Croneis, p. 244, pl. 45, figs 3–4, 6, 8.

1946 Calyptograptus  micronematodes Spencer; Fritz, p. 15.

1947 Desmograptus  micronematodes Spencer; Ruedemann, p. 223, pl. 23, fig. 1; pl. 23, figs 2–3, [copy of Ruedemann 1925, text-figs 7–8].

1954 Desmograptus  micronematodes Spencer; Kilfoyle, p. 95.

v.1978 Calyptograptus  micronematodes Spencer; Waddington et al., p. 141.

v.1974 Desmograptus  micronematodes Spencer; Erdtmann and Prezbindowski, p. 354.

v.1975 Desmograptus  micronematodes Spencer; Erdtmann, p. 46.

v.1976 Desmograptus  micronematodes Spencer; Erdtmann, p. 248.

1979 Desmograptus  micronematodes Spencer; Hansmann and Nitecki, p. 3.

1995 Desmograptus  micronematodes Spencer; LoDuca, p. 377.

1997 Desmograptus  micronematodes Spencer; LoDuca and Brett, pp. 379, 381, text-fig. 14.12e.

Emended diagnosis.  Rhabdosome small, broadly conical, originating from a bulbous proximal end and non-thecate stem; anastomosing stipes 0.2–0.3 mm wide branch dichotomously and laterally frequently and irregularly; 16–20 in 10 mm. Dissepiments usually sporadically developed but often more common in certain parts of the rhabdosome. Fenestellae of variable shape, typically 1.0–1.5 mm long and 0.2–0.4 mm wide. Autothecal spacing c. 40 in 10 mm.

Material.  Figured material is deposited in the Field Museum, University of Chicago, Illinois; accession G-7072, numbers PE60371–PE60378 and PE60414–PR60416.

Spencer’s (1884a) figured type material was from the Ancaster Member, Goat Island Formation (lower Wenlock, Sheinwoodian), Hamilton, Ontario, Canada; it was probably destroyed by a fire at the University of Missouri (see Bassler 1909, p. 4). A specimen figured by Bassler (1909, p. 40, fig. 50; ROM 21626) from this locality was purchased from the Spencer collection in 1919. Fritz (1946) listed this specimen as a neotype, a designation, which Waddington et al. (1978) stated was not valid since it appeared only in a catalogue. Additional material was described by Shrock (1928) from the Mississinewa Shale Member, Wabash Formation (?upper nilssoni–scanicus biozones, Gorstian, Ludlow), near Yorktown, Indiana and by Ruedemann (1925) from the Medusaegraptus epibole, Niagara Falls Member, Goat Island Formation (lower Wenlock, upper Sheinwoodian), Gasport, New York, USA. The species has also been described from the USA in the ‘Clinton sandstone’ (Llandovery), Niagara Falls, New York; the ‘Lecthaylus Shale’, Racine Dolomite Formation (lower Ludlow), Cook Co., Illinois; from the lower, main and top dendroid beds, Brandon Bridge Formation (spiralis Biozone, upper Telychian, upper Llandovery), Waukesha Lime and Stone west quarry, Waukesha, Wisconsin; and from the ‘Niagara Group’, Anticosti Island, Quebec, Canada. D. cf. micronematodes is recorded from the Mississinewa Shale Member, Wabash Formation (upper nilssoni–scanicus biozones, Gorstian, Ludlow), Huntington-west, Indiana, USA.

Description.  See diagnosis above and Table 1. The anastomosing stipes branch dichotomously and laterally, frequently and irregularly (Text-fig. 4). The fenestellae are of variable shape depending on whether they are formed by anastomosis (Pl. 1, figs 9–10) and/or dissepiments.

Table 1.   Characters of Desmograptus micronematodes specimens examined.
SpecimenRhabdosome typeMesh typeMax. Rhabdosome L/W mmStipes/ 10 mmDissepiments/ 10 mmStipe width mmDissepiment width mmFenestellae L mm W mmAth/ 10 mm
IU 3660?ConicalSee below40/1715–16See below0.2–0.250.10.8–2.2 0.25–0.6?
ROM 21626?Fine regular25/4216–20?0.2–0.3?1.55–2.0 0.55–0.7?
PE60414?Coarse irregular19/9??Some0.2–0.30.30.5–1.0 0.4–1.0?
PE60415ConicalFine irregular28/2018None0.25–0.35?1.0–1.5 0.2–0.5?

Remarks. Shrock (1928) showed that D. micronematodes showed considerable variation in morphology. He divided the specimens collected in Indiana into four groups. Group 1 (see Table 2) is represented by nine specimens with a Desmograptus structure distally and a transitional Desmograptus/Dictyonema structure proximally, suggesting that the generic names Dictyonema and Desmograptus have little phyletic significance. Group 2 includes five specimens that match the description of Ruedemann (1925, p. 21). Group 3 is transitional between Dictyonema  polymorphum and D. micronematodes. Group 4 was described as a new subspecies, D. micronematodes magnus, characterized by broader stipes than D. micronematodes and differences ‘in shape and size of frond’ (Shrock 1928, p. 29). Given the considerable variation in stipe width (Table 1) and overall rhabdosome shape seen in D. micronematodes we do not consider this subspecies to be valid and D. micronematodes magnus should be subsumed within D. micronematodes. Desmograptus  micronematodes can be distinguished from D. cumingsi Shrock, 1928 by its much smaller fenestellae (compare Shrock 1928, fig. 13a and 13c).

Table 2.   Characters of the groups recognised by Shrock (1928).
GroupRhabdosome typeMesh typeDissepimentsStipes/ 10 mmStipe width mmProximal Fenestellae L/W mmDistal Fenestellae L/W mmAth/ 10 mm
1ConicalP: irregular D: regularP: yes D: no??1.0–2.5/0.4–1.00.8–1.2/0.5?
2Conical Yes?0.25–0.31.0–1.5/0.61.0–1.5/0.6?
3?IrregularYes?0.25–0.31.2–3.0/1+1.2–3.0/1+20–30
4 D. magnusConicalRegularYes?0.35–0.40.8–1.3/0.4–0.50.8–1.3/0.4–0.540

Internal Structure

  1. Top of page
  2. Abstract
  3. Stratigraphical and Depositional Setting
  4. Systematic Palaeontology
  5. Internal Structure
  6. Discussion
  7. Acknowledgments
  8. References

Growth pattern

Dendroid graptolites are characterized by having three types of theca (autotheca, bitheca and stolotheca), and a stolon system with regular triad budding (Bulman 1970, p. V25). The stolons are internal tubes which run the length of each stipe, budding at spaced nodes, with branches leading to the open cups of the autothecae and bithecae.

Stipe growth in Desmograptus follows the normal dendrograptid Wiman Rule (Bulman 1970, p. V26), with the stolon branching into three at each node (diaphragme vésiculaire of Kozłowski 1963; vesicular diaphragm of Bulman 1970, p. V26; stolonal node of Crowther 1981, p. 21). As Bulman pointed out, three different kinds of theca are not produced: the stolotheca is in fact the basal portion (equivalent to the protheca of the graptoloids) of the daughter autotheca. Text-figure 5 shows both the conventional labelling of thecae (Text-fig. 5A), and the revised labelling (Text-fig. 5B), as suggested by Kirk (1969), which is adopted here. On Kirk’s scheme, the first autotheca (a0) produced by the sicula has no associated auto- or bithecae (there is no node at the base of the stolothecal portion): at the first node autotheca a1 and bitheca b1 are produced (Text-fig. 6).

From the material it can be shown that each autothecal tube, when fully grown, has within it three segments (Text-fig. 7):

  • 1
    A proximal segment (PS), which commences at the previous node, contains a stolon, and ends at the next node. The initial portion of this segment is within the parent autotheca; the second portion, distal to the base of the parent’s autothecal cup, is external.
  • 2
    A central segment (CS) which contains a narrow stolon leading to the thecal cup, and the internal portions of the daughter bitheca and autotheca (or two daughter autothecae at stipe branchings).
  • 3
    A distal segment (DS), the thecal cup, which has a dished base and is open at its distal end.

The PSs and CSs correspond to the ‘stolotheca’ of earlier authors, and the thecal cup to the autotheca as previously understood.

At the distal end of the CS the daughter thecae emerge through lateral pores in the wall of the parent autotheca, presumably by resorption. The structure of Dendrograptus, as described by Kozłowski (1949), is very similar.

Text-figure 7 shows successive stages in the growth of a stipe. In Stage 1 (Text-fig. 7A), autothecal tube a1 has grown forwards as a complete tube with a circular cross-section. Its DS is growing forwards, and the base of the thecal cup has already been secreted. The two succeeding daughter thecae, autotheca a2 and bitheca b2, have already formed as thin-walled internal tubes, and have just broken out of pores in the side of the parent theca, to become external tubes.

In Stage 2 (Text-fig. 7B), the parent autotheca a1 has reached its full length, and autotheca a2, extending beyond it, has become a complete tube. Bitheca b2 has also extended to its final aperture.

In Stage 3 (Text-fig. 7C), the zooid of autotheca a2 is presumed to have migrated up the growing tube, to allow the secretion of fusellar tissue within its PS. The large stolon (shown as broken lines in this diagram) may also have been secreted. Beneath the dished base of the zooid is an area of proliferation for its daughter thecae, autotheca a3 and bitheca b3.

In Stage 4 (Text-fig. 7D), the stolon of the PS of autotheca a2, and the node, have formed. The two zooids of autotheca a3 and bitheca b3 are already in their initial positions, and have started to secrete their thin-walled internal tubes (they will subsequently break out of the autothecal wall at the regions marked by X).

Although not found in our material, it is probable that the initial growth of thecae and stolons from the sicula followed a similar pattern to that of Dendrograptus (Text-fig. 6, after Kozłowski 1949). What he described as the initial stolotheca, growing at first within and then breaking out of the sicula, and possessing an internal stolon, would be the first autotheca (relabelled in this diagram as autotheca 0). The sicula, as the parent of this first autotheca, does not produce a bitheca.

Thecal walls

The thecal walls are constructed on the established graptolite pattern, with an initial primary fusellum and subsequent secondary cortex. The wall of an extending autotheca is composed of a series of growth increments, or fuselli, which form as half-rings, each a little more than a semicircle. Successive fuselli overlap one another along two zones, forming a zigzag suture. Each fusellar half-ring, or fusellus, has a core of fusellar fabric (Pl. 1, figs 1–2, 4; Text-fig. 8), with a bounding lamella of sheet fabric underlain by parallel fibrils (Pl. 1, fig. 6). Fuselli are typically spaced at c. 10 μm intervals. The external cortex is formed of bandages, which consist of parallel fibrils covered with sheet fabric with vesicles (Pl. 1, fig. 3). Internal cortex is much thinner, but is also formed of bandages with indistinct edges (Pl. 1, figs 2, 5). The bithecae, and the more proximal portions of the autothecae, are half tubes, formed on the outside of the parent autotheca, but still having alternating fuselli with a zigzag suture.

The fuselli of the half tubes characteristically swing forwards at their ends where they are attached to pre-existing thecal walls (Text-fig. 8). As they do so, they expand to form a broad spatulate base. This is commonly seen in longitudinal fractured sections (Pl. 1, figs 6, 8); in transversely fractured sections the base can be seen to be much wider than the rest of the fusellus (Pl. 1, fig. 7).

Autothecae

The basal segment of the autotheca, the so-called stolotheca, commences within the parent autotheca, at the distal side of the preceding node (Pl. 3, fig. 3). Its wall is extremely thin where it is internal to the parent autotheca (Pl. 2, figs 3, 5; Pl. 3, fig. 3), and is of normal fusellar construction where it has broken out. It is typically c. 40 μm in diameter, and 200–300 μm long. The stolon, c. 30 μm in diameter, occupies most of this space. Between the stolon and the thecal wall is a fabric of sparse fibrils, very similar in appearance to fusellar fabric (Pl. 2, figs 1, 5; Pl. 3, fig. 3; Text-fig. 14A, C). Crowther (1981, pl. 4, fig. 6) showed a similar fabric in Acanthograptus  musciformis (Wiman). The fibrils though, by contrast with those of typical fusellar fabric (see above), show bifurcation (Pl. 2, fig. 6). This fabric was also observed by Kozłowski (1949, p. 43), in light microscopy; he described it as a light material, and suggested that it held the stolon in place within the theca.

The succeeding CS commences at the node, and contains within it the internal portions of the two following thecae (Text-fig. 7). Distal to the node, the autothecal stolon, with a triangular cross-section, is much narrower and shorter than the ‘stolothecal’ portion of the stolon. Within this segment are packed the two daughter thecae: an autotheca and a bitheca, or where branching occurs, two autothecae.

With the breakout of the daughter thecae, the autothecal stolon ends at a small chamber beneath the base of the thecal cup (Text-fig. 10C). This chamber was recognized by Kozłowski (1963, fig. 16), and called by him the diaphragme vésiculaire. The base of the cup (Text-fig. 10D) is similar in structure to that of the ‘stolothecal’ and bithecal distal sides of the nodes (described below).

The cup segment of the autotheca (Text-figs 10A–B, 11, 12A–B, 13A–B) is c. 100 μm in diameter and 600 μm or more in length, lined with internal cortex. The form of the aperture is not known. From the internal structure, particularly the spacing of the nodes, it appears that the autothecae are spaced at intervals of c. 250 μm, i.e. 40 per 10 mm.

Bithecae

The bithecae are much smaller than the autothecae, c. 30 μm in diameter and 400 μm or more in length. A bitheca originates at the distal side of a node, with a base on the triad node similar to those of the autothecal cup or ‘stolothecal’ autotheca; it forms a short tube within the parent autotheca, and breaks out of its wall to secrete its own half-tube (Text-figs 9B, 10, 11E, 13A–B). Its walls, when internal to the parent autotheca, are thin and smooth, but the external portion is formed of fuselli, with external cortex.

Stolons

The basal portions of each autothecal stolon (the ‘stolothecal’ stolon; Text-fig. 14) are c. 20–30 μm in external diameter. The walls are normally about 2 μm thick [e.g. in specimen PE60377 (Pl. 2, figs 1, 7)]; although only 0.3 μm thick in specimen PE60372 (Pl. 2, fig. 2). The distal portions, between the node and the base of the cup, are much narrower: less than 10 μm in diameter, with walls, in PE60377, c. 1 μm thick. The distal autothecal stolon emerges from the node through a small pore (Pl. 3, fig. 7). Close to the node, it is triangular in section: it appears to reflect the constraints of space within the autotheca, shared with the daughter autotheca and bitheca (or two autothecae) (Pl. 3, figs 3, 7; Text-fig. 10E). Towards the base of the autothecal cup it widens, and becomes more circular (Pl. 2, fig. 7; Text-fig. 10F) before ending in the diaphragme vésiculaire (of Kozłowski 1963).

Nodes

The nodes have a complex box form, with separate chambers for the parent autotheca, and the daughter autotheca and bitheca (or two autothece in the case of a branching node; Text-figs 14–15A). As it approaches the base of the node, the autothecal stolon becomes wider, and oval in outline. It may already have within it three canals (cf. Dendrograptus; Bates 2003, fig. 1.7).

The proximal face of the node is concave, with three pores leading to the three chambers (Pl. 3, figs 1–2). Its surface is almost smooth, with a subdued granulate pattern. The three pores are arranged in a triangular pattern, each about 4 μm in diameter, and each may have a slightly raised rim.

Each chamber appears to have its own walls, which are closely pressed together, leaving triangular spaces between them, filled with small fibrils (Pl. 3, fig. 4). The proximal and distal internal faces of the larger chambers have again a subdued pattern on them (Pl. 3, figs 4–5). The smaller chamber is only a little larger than the pore leading to it, and is triangular in section (Pl. 3, fig. 4).

The external distal faces of the larger chambers (and also that at the base of the autothecal cup), have a distinctive appearance, with the central pores surrounded by an ornamented area. The pores have collars pointing distally (Pl. 3, figs 3, 6–9; seen in section in Pl. 3, fig. 6). Raised areas radiate from the collar; they bear a granular ornament (Pl. 3, figs 8–9; Text-fig. 10D). Between the raised areas, and sometimes extending a little beyond them, is a region bearing a honeycomb fabric (Pl. 3, figs 7–8, 10; Text-fig. 10D). This consists of walls, less than 1 μm high, separating polygonal spaces each c. 0.5 μm wide. This fabric is almost certainly that described by Urbanek and Towe (1974, p. 17, pl. 30), in Acanthograptus, as a ‘distinct spongy structure’. Their transmission electron microscope (TEM) section is probably cut close to the distal face of either a node, or at the base of an autothecal cup.

Ultrastructure

The fusellar fabric (Pl. 1, fig. 4; Text-fig. 13D) is composed of fibrils c. 100 nm in diameter. They do not appear to branch, but are held together by a glue-like material where they cross one another. Traces of spiral striations, similar to those on cortical fibrils, may be present.

The cortical fabric consists of cortical fibrils packed together quite tightly, and linked by thin cross-fibrils. The cortical fibrils (Text-fig. 13C) are c. 120 nm in diameter, and have clear striations on their surface, spiralling clockwise, and which make an angle of about 20 degrees with the fibrils. These are comparable with similar fibrils described by Bates (2003), and it may be that spiralling is characteristic of all graptolite fibrils. The cross-fibrils, which adhere to the surface of the cortical fibrils, are about 20 nm in diameter. They tend to be spaced at intervals of c. 80–100 nm, but the pattern is not completely regular (Text-fig. 13C). This pattern of parallel cortical fibrils with spaced cross-fibrils has been illustrated in TEM photographs of Dictyonema sp. by Urbanek and Towe (1974, pls 2–3, 10, 14) and in Dictyonema  rarum Wiman by Rickards and Dumican (1984, figs 2–4). The latter authors interpreted their TEM observations as showing the cortical fibrils to have spaced, concentric flanges.

The stolons are formed of a dense material, crassal fabric, as defined by Urbanek and Towe (1974, p. 4). The material is extremely uniform, and even under high power in the TEM it shows only vague banding (Urbanek and Towe 1974, pl. 28). Where broken, a stolon has a conchoidal fracture (Pl. 2, figs 1, 4).

Discussion

  1. Top of page
  2. Abstract
  3. Stratigraphical and Depositional Setting
  4. Systematic Palaeontology
  5. Internal Structure
  6. Discussion
  7. Acknowledgments
  8. References

Comparison with other dendroids

The construction of the stolon system and its nodes appears to be very similar to that in other dendroids. Crowther (1981, pl. 4, fig. 3) illustrated and described part of the stolon system of Acanthograptus  musciformis (Wiman). His illustration, looking distally, shows a ‘stolonal triad’, with, on the left, part of the distal face of one chamber [described by him as being ‘thick walled, elliptical (27 μm by 20 μm) [with] a small axial cavity (7.5 μm by 4 μm)’]. He interpreted this as being the ‘expanded distal termination of a short bithecal stolon’, just distal to a node. However, comparison with the nodes of Desmograptus  nematodes shows that it is the internal distal face of a chamber, his axial cavity being the pore through the face. Just distal to the node, the smaller triangular autothecal stolon and larger circular ‘stolothecal’ stolon can be seen. Also present in his illustration is the thin wall (described by him as a thin tail) separating the internal portions of the two daughter thecae (cf. his pl. 2, figs 3, 7). Crowther (1981, pl. 2, figs 4–5) also illustrated part of a node in Acanthograptus  divergens Skevington. This would appear to be the distal face of a node, with the central pore surrounded by a raised collar. On this face (Crowther 1981, pl. 2, fig. 6) is a three-dimensional meshwork of short rods, termed by him a macular fabric. This may be the fabric described here as a honeycomb fabric; however, there is a distinct difference between its rods, and the walls of material in Desmograptus (Pl. 3, fig. 10).

The structure of the nodes has also been illustrated by Bulman and Rickards (1966, fig. 25), redrawn here in Text-fig. 15B), in Acanthograptus  impar Bulman and Rickards, 1966. The diagram has presumably been reconstructed from the serial sections made by Wiman. The chambers appear circular in their longitudinal section, however, in contrast to the box-shape of those in Desmograptus. They also show the narrow bore distal portion of the autothecal stolon, but its course from the proximal ‘stolothecal’ portion, narrowing through the node, seems quite different from that in Desmograptus.

Comparison with Rhabdopleura

Urbanek and Dilly (2000) illustrated and described diaphragm complexes in the extant hemichordate Rhabdopleura  compacta Hincks, 1880. The stolon, about 20 μm in diameter, widens slightly towards a diaphragm (Text-fig. 15C). The diaphragm has a slightly convex upper or distal surface, and is provided with a central pore. Distal to the diaphragm is a widening collar, which extends into a cone: together these form the inner wall of the zooidal tube, enclosed within the fusellar one. This complex differs from that in Desmograptus (Text-fig. 15A), in which the node at the base of an autothecal cup forms a small chamber (the diaphragme vésiculaire). They do not describe in detail the diaphragm at the branching of a stolon, but their illustration (reproduced diagrammatically here as Text-fig. 15D) shows that it appears to be a single structure. Hence it appears that the box-like nodes of Desmograptus are not present in Rhabdopleura.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Stratigraphical and Depositional Setting
  4. Systematic Palaeontology
  5. Internal Structure
  6. Discussion
  7. Acknowledgments
  8. References

Acknowledgements.  We thank the Material Service Corporation for access to their quarries, and Dr Stephen Wade, of the bio-imaging unit of Aberystwyth University for assistance in electron microscopy. KMS thanks technical staff at the School of Earth and Environmental Sciences, University of Portsmouth for their assistance.

References

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  2. Abstract
  3. Stratigraphical and Depositional Setting
  4. Systematic Palaeontology
  5. Internal Structure
  6. Discussion
  7. Acknowledgments
  8. References
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