Acanthograptus Spencer, 1878

Rickards, R. B., Chapman, A. J., Wright, A. J. & Packham, G. H, 2003, Dendroid and Tuboid Graptolites from the Llandovery (Silurian) of the Four Mile Creek Area, New South Wales, Records of the Australian Museum 55 (3), pp. 305-330 : 310-312

Acanthograptus Spencer, 1878 and Thallograptus Ruedemann, 1925

The working distinctions between the two genera seem to be that Acanthograptus has short twig-like processes developed ventrolaterally from the main stipes, and placed alternatively along the stipe: each twig comprises several thecal tubes, most usually two autothecae (with two bithecae opening near the base of the twig). Thallograptus , on the other hand, has stipes and twigs which gradually become thinner as autothecae “peel off” to open as individual tubes projecting either from the sides of the stipe (and, therefore, are superficially twig-like) or projecting ventrally. In both genera the rhabdosomes are usually dendroid and both have compound stipes.

However, the type species Acanthograptus granti Spencer, 1878 , is less regular than the above generally held concept, although it undoubtedly has twigs amongst a dense branching pattern. Similarly A. praedeckeri n.sp. of this paper has a very frequent branching pattern (yet between branches has conspicuous twigs alternating along the stipe) and the stipes are clearly compound. In A. impar and A. murciformis , both described by Bulman & Rickards (1966), the stipes are compound and a fairly regular sequence of autothecae is maintained. In these species, however, a breakdown of regularity is apparent: not all twigs show the 2+2 pattern, and many autothecae and bithecae open adventitiously, with some autothecae opening erect and isolated—an incipient Thallograptus pattern. It would seem that there is a morphological continuum from the simplest of acanthograptids through the most complex, with compound stipes and breakdown of the twig structure, to thallograptids. However, individual evolutionary lineages have not been worked out; although one can conclude that because thallograptids are more common in the Silurian than acanthograptids, (the reverse being true in the Ordovician), then thallograptids probably evolved from acanthograptids. One can predict, therefore, that lineages of increasing stipe complexity may eventually be recognized. For the present most species fall fairly readily into the two genera, but in the present paper Acanthograptus praedeckeri n.sp. and Thallograptus christoffersonae n.sp. are not easy to distinguish because their rhabdosomes are so similar.

Simple stipes and compound stipes

The question of compound stipes raises problems elsewhere in the dendroid classification. For example Chapman et al. (1996) placed Pseudodictyonema , with its compound stipes, in a new family, the Pseudodictyonemiidae , which ranges from the Silurian to the Carboniferous. However, some care must be taken: many stipes of species of Dictyonema appear superficially to have complex stipe structure because all dendroid thecae are narrow and elongate and in dorsal view a ropy texture may be apparent in well preserved specimens. The complexity has to be such that there is a clear indication of more than one line of stolonal development in any one cross section of the stipe (see, for example, Bulman & Rickards, 1966, figs. 20–23). In this work Pseudodictyonema graptolithorum (Poěta) is such a case. We suspect that many more species will prove to have compound stipes when suitably preserved material is obtained. Indeed, compound stipes have evolved in a number of lineages, and Chapman et al. (1996) recognized four separate origins for this condition.

Dendroid bithecal morphology

Most dendroid species have bithecae comprising small, inconspicuous tubes, with unornamented apertures, which open inconspicuously on the lateral or, occasionally, other parts of the stipe wall. Some open inside the autothecal apertural region. There are, however, more unusual types of bithecae. These were first summarized by Bulman (1927– 1967) and he later (1933) gave a more detailed classification of them, defining five types ( Fig. 4 ): Type 1 consisting of the simpler form described above, opening externally just below or level with the autothecal aperture; Type 2 being those forms which open into the autothecal apertural region; Type 3 embracing all forms which open between the apertural region of autotheca n and the early free ventral wall of autotheca n+1; Type 4 which grows beyond the point of Type 3 and opens facing laterally; and Type 5 being a morphological step beyond Type 4 in which the bithecal apertural region curves proximally or distally.

The stratigraphy of these morphological types was investigated by Chapman & Rickards (1982) who concluded that the more complex types 4 and 5 ( Fig. 4E–G herein) were stratigraphically younger, appearing respectively in the Caradoc and Ashgill series (Ordovician) and being fairly common in Silurian species.

However, even today bithecae are known in relatively few dendroid species and the situation may be more complex than depicted in Fig. 4 . For example, Rickards et al. (2001) recorded an Iranian late Arenig species of Dictyonema in which the bithecae open as long narrow tubes in the spaces of the meshwork, and are isolated for almost one millimetre of their length. Such a bithecal form does not fall into Bulman’s (1933) classification of types, but could be a morphological extension of Types 1, 3, 4 or 5, but presumably not of Type 2.

Based on our present studies Dictyonema muirae n.sp. appears also to have aperturally isolated bithecae, but in this case they are conspicuous structures rather overshadowing and overhanging the autothecae ( Fig. 13C ). Such thecae could develop from the subtype of Type 5 where the bithecal tube curves distally ( Fig. 4F,G ). Rickards et al. (2001) described a comparable structure in their new species, Callograptus huckriedei , from the late Arenig of Iran.

These observations suggest that the broad evolution of dendroid bithecae suggested by Chapman & Rickards (1982) is oversimplified: bithecae may become more complicated through time, but Type 5 structures may have been present rather earlier than supposed.

Dendroid autothecal apertural variation

Possibly for reasons of serendipity most of the classic early work on well-preserved (and isolated) dendroids (e.g., Wiman, 1901; Bulman, 1927–1967, 1933; Kozlowski, 1938, 1949) has described species with relatively simple autothecal apertures. Specimens “in the rock” also suggest that most dendroid species have autothecal apertures with a short ventral denticle. Exceptions were described, such as Dictyonema peltatum Wiman, 1901 (see Bulman & Rickards, 1966) in which the ventral process develops a large shield-like plate, and in which adjacent plates may coalesce. Other species have the dorsal part of the aperture developed as a spine or process, as in Dictyonema rhinanthiforme Bulman, 1933 (see Chapman & Rickards, 1982 for detail). Dictyonema rhinanthiforme was originally recorded from the late Arenig or early Llanvirn of Sweden but the Chapman & Rickards (1982) material came from the early Llandovery of the Canadian Arctic region.

In some dendroids, especially Dictyonema and Callograptus , the autothecal apertures are slightly isolated from the main stipe. These forms may have a ventral denticle, a dorsal process, or both, as in the cases of D. rhinanthiforme and D. elegans Bulman, 1928 (see also Rickards & Wright, 1997, 1999).

The present collection has a species with isolated autothecal apertures, D. williamsae n.sp., although in this case there appear to be no apertural processes. On the other hand D. warrisi n.sp. has unusually long ventral apertural spines, a feature we have not seen recorded in dendroids before (except in Dictyonema sp. where the spines bifurcate; see Rickards et al., 1995). Further discussion is given under the description of D. warrisi n.sp.

In one of the few works on NSW dendroids, Sherrard (1956, table 1) tabulated some characters of NSW Ordovician and Silurian forms assigned to Dictyonema , but did not describe any dendroids from the Four Mile Creek area. She described material from Silurian strata at Spring-Quarry Creek, including an indeterminate Llandovery Dictyonema sp. and the new (? Ludlow) species Dictyonema favosum and Reticulograptus undulosum . From the Ludlow of Yass she described? Dictyonema sp. and the new species Dictyonema vinculosum , and noted that D. favosum was found with the graptolite from Yass that was then called Monograptus bohemicus (but see Rickards & Wright, 1999).

Systematic palaeontology

Material studied here has been collected by GHP over many years, by Dr Chris Jenkins and by RBR, AJW and GHP in 2000, with assistance from Dr Ian Percival and Ms Lucy Muir in 2000. Material described herein is deposited in the Australian Museum, Sydney and bears the prefix AM F. Localities prefixed by the letter W refer to collections made in November, 2000; those with the prefix F refer to collections made by Packham, and those with the prefix BF refer to collections made by Jenkins (1973). Although some of these localities are probably the same (e.g., W885, BF14 and F14) we have referred to them with their original prefixes or locality number in each case.

Class Graptolithina Bronn, 1849

Figures

Fig. 4. A–G, different types of bithecae: A, Bulman’s (1928, 1933) Type 1 with bitheca n+1 opening along the lateral wall of autotheca n. This is the most common of autothecal/bithecal relationships; B,C, Bulman’s (1928, 1933) Type 2 in which the bithecal aperture opens inside the autothecal tube or aperture; D, Bulman’s (1928, 1933) Type 3 where the n+1 bithecal aperture is located on the dorsal side of the aperture of autotheca n; E, Bulman’s (1928, 1933) Type 4 in which the bithecal tube n+1 passes between the dorsal wall of autotheca n and the ventral wall of autotheca n+ 1; F,G, Bulman’s (1928, 1933) Type 5 a modification of Type 4 in which the bithecal tube turns proximally or distally before the bithecal aperture is reached; H–M, autothecal variation in Dictyonema species; H, autothecal aperture is the termination of a simple tube; I, the ventral lip projects as a short, sharp denticle; J, the ventral lip projects as a spine which may or may not bifurcate; K, the ventral process is adorned with a large shield-like plate; L, the dorsal apertural process has plate-like process hanging more or less in front of the aperture; M, the apertural region is isolated from the ventral wall of the next autotheca; for the sake of simplicity in H–M the Type 1 bithecal arrangement is shown; fuselli omitted; a, autothecal aperture; b, bithecal aperture; these structures are described or commented upon in various places in the text.

Fig. 13. A–D, Dictyonema muirae n.sp., respectivelyAM F114763, BF14, AM F114775, F14, holotype AM F114762, F14, and AM F114764, BF14 proximal part of colony attached to pebble of phosphate; E–G, Dictyonema warrisi n.sp., respectively AM F114647, F14, holotype AM F114717, W885 and AM F114647, F14, with stipes more proximal than E; H, Dictyonema sp. 1,AM F11475, F14. Scale bars 1 mm; shading on C indicates badly preserved area.