|Dithecoid graptolites (Dithecoidea) - a taxonomic wastebasket
written by Piotr Mierzejewski
|Obut (1964) ascribed a specific role in early phylogenetic development of graptolites to the graptolite order Dithecoidea, proposed by him (Obut 1960). The order was characterized as comprising graptolites with bushy to tree-like colonies budding in diads, and dimorphism of thecae (bithecae missing). According to him, the Dithecoidea were the stem group for all the remaining graptolites, giving rise to the most primitive Dendroidea of the family Dendrograptidae in the late Cambrian. The transition from the Dithecoidea to the Dendroidea was, in this concept, connected with differentiation of thecae, leading to the origin of thecal trimorphism and budding in triads. Within the order Dithecoidea, Obut (1964, 1974) differentiated four families (Dithecodendridae, Siberiograptidae, Chaunograptidae and Bulmanidenridae), to which to which he assigned 7 genera (Dithecodendrum Obut, Siberiodendrum Obut, Aellograptus Obut, Chaunograptus Hall, Rhadinograptus Obut, and Mastigograptus Ruedemann) and 6 other (Archaeocryptolaria Chapman, Archaeolafoea Chapman et Thomas, Ceramograptus Hudson, Cactograptus Ruedemann, Ascograptus Ruedemann, Haplograptus Ruedemann) with reservation. It is worthy noting that all the genera, except for Mastigograptus Ruedemann, were known in 1964 only on the basis of imprints or carbonized remains of flattened rhabdosomes preserved at rock surface.
The above hypothesis concerning the origin of the Dendroidea and systematic position of the Dithecoidea did not get support from all graptolite workers. It was treated with marked scepticism by Bulman (1970) who treated the genera placed in the Dithecoidea by Obut as Dendroidea, Tuboidea, Crustoidea, Stolonoidea as taxonomically uncertain. He also doubted whether all of them represent graptolites.
|Bulman (1970, V54-V55) wrote:
"The gross morphology of all such organisms, when poorly preserved, is little guide to their true affinity; and in the absence of detail concerning such features as the stolon system and the presence of fusellar structure, it seems preferable to accept a large group of unclassified genera, some members of which may ultimately not to be Graptolithina at all."
"The Dithecoidea of Obut (1964) appear to comprise autothecae only, but no stolon system has been demonstarted and they compare superficially with such genera as Archaeocryptolaria and others which until now have been provisionally classed as hydroids. But among these latter, Archaeolafoea and Sphoenoecium exhibit inconclusive traces of what appears to be fusellar structure, which would suggest a generally "graptolitic affinity".
|Quilty (1971) described genera assigned to Dithecoidea by Obut (1964) from the Cambrian of Tasmania, treating them as either Hydroidea or graptolites of the order Dendroidea. Urbanek & Towe (1974) and Anfres (1980) treated the genus Mastigograptus Ruedemann as graptolite incertae sedis. A. Urbanek (1975, oral inf.) and Kozlowski (1975, oral inf.) held that it is possible that Dithecoidea (except for the genus Mastigograptus) do not belong to graptolites at all.|
|Sdzuy (1974) described some fossils from the Middle Cambrian of Spain whech he treated as dithecoid graptolites. He proposed seven new species allocated in three new genera and recogniyed the genus Archaeolafoea Chapman, previously known from Australia only. The affiliation of these forms with graptolites was assumed mainly with reference to alleged finding of traces of fuselli on some of them. The affiliation was highly questionable for both R. Kozłowski and A. Urbanek (oral inf., 1974).
In looking through Recent coelenterate collections in the Institute of Zoology of the Soviet Academy of Sciences, Leningrad, I noted some colonial scyphopolyps strikingly similar to the alleged graptolites of Sdzuy (1974). The forms, when found in Early Paleozoic deposits would be undoubtedly interpreted as corresponding to dithecoid graptolites of Obut and Sdzuy in morphology. Figure shows the striking similarity of "graptolite" colony of Sdzuy (1974) and that of Recent scyphopolyps of the order Coronata. Moreover, the alleged fuselii of the Spanish Dithecoidea may be easily compared with annulation of scyphopolyp periderm.
Obut (1974) has drawn too far-reaching conclusions with respect to possibilities of interpretation of his material from the Cambrian of Siberia and those described by some other authors. The major drawback of these materials is connected with the fact that the affiliation of these forms (except for those of the genus Mastigograptus Ruedemann which, actually, do not match the concept of Dithecoidea) with graptolites may be seriously questioned. Even if the fossils assigned to Dithecoidea were proved to be graptolites, their preservation precludes tracing the course of stolons and even finding their presence. The only solution to verify Obut's (1964) hypothesis is to study branching details of the stolonal system The course of stolons in the rhabdosome of Chaunograptus sphaericola (Öpik) as shown by Obut (1964, fig. 36) should not be treated as observation but rather hypothesis (see also Kozlowski 1962). The state of preservation of Soviet specimens as dithecoid graptolites is best shown by the fact that Obut (1974) found the alleged traces of fuselli only in those of the species Bulmanidendrum magnificum.
|Obut (1964) and Sdzuy (1974) accept the general shape of colony as one of features characterizing Dithecoidea, so I would like draw attention to Bulman's (1970) that a number of graptolites traditionally regarded as representatives of the order Dithecoidea may actually belong to the order Tuboidea. Dendroid and tuboid graptolites form very similar colonies so that, when flattened and carbonized in the course of fossilization, the nature of budding becomes inclear and, therofore, unequivocal identification is often impossible.|
|The majority of dendroids described on the basis of such material do not represent actual taxa but rather "form-genera" (Bulman 1970). Practically, the only criterion used by Obut (1964, 1974) to differentiate Dithecoidea from other graptolites is the general shape of the colony or rather the presence of one type of thecae as the presence of stolothecae was not proved. If representatives of pterobranch genus Rhabdopleuroides Kozłowski or hydroids Epallohydra Kozłowski or Desmohydra Kozłowski were preserved unflattened and carbonized among remains of other colonial organisms, the use of this criterion would result in their allocation in Dithecoidea "graptolites". It is not excluded that such assignation has already taken place. The genus Chaunograptus Ruedemann, interpreted as hydroid by Ruedemann (1947) and dithecoid graptolite by Obut (1964), is most probably congeneric with the above mentioned hydroids Epallohydra and Desmohydra (Mierzejewski 1986).|
|Similarly as some of the above mentioned authors, I consider Dithecoidea as an artificial group, comprising organisms of various, taxonomically distant groups. I am inlined to regard all forms described as graptolites from the Cambrian of Spain by Sdzuy (1974) as colonial scyphopolyps of the order Coronata. My hypothesis is undoubtedly much easier to accept than that of Sdzuy (1974) as it does not require any additional assumptions. Such additional assumption in Sdzuy's conception is the alleged existence of primitive graptolites, budding in diads and devoid of bithecae, unsubstantiated by any observations. Moreover, the presence of scyphoplyps Coronata in the Cambrian fully agrees with their modern interpretations as the most primitive polyps of coelenterates (Werner 1973).|
|The reinterpretation of the Sdzuy's (1974) "graptolites" as colonial scyphoplyps has far going nomenclatoric consequences. Differences between individual taxa of the "graptolites" are not significant when they are treated as scyphopolyps and, theoretically, they all may be placed in the genus Stephanoscyphus Allman. Taxonomy of recent Scyphozoa polyps from the order Coronata was not long ago solely based on the structure of skeleton, since the features connected with metageny were not used for taxonomical purposes prior to Werner (1971). It should be mentioned that, Stephanoscyphus, a formal genus, is used as a collective name in zoological practice. However, Stephanoscypus mirabilis Allman, 1874, the type species, appers to be the junior synonym of the Nausithoe punctata (Kolliker, 1853), so that the name Stephanoscyphus lost its validity as it appeared to be junior synonym of the name Nausithoe. This was not noted by zoologists who, instead of formal introduction of collective group name for sessile generation of Coronata, traditionally use the invalid name Stephanoscyphus Allman.|
|In zoological practice, the name Stephanoscyphus is used for both colonial and individual forms. The species at present described under that generic name will undoubtedly be allocated to a few separate families in the future (Naumov 1961). In this respect, zoologists link the future prospects with the existing possibilities to reconstruct metageny of these forms. The lack of such prospects for those studying fossil Scyphozoa must have some effect on the taxonomic approach of paleontologists, as there is no hope of classifying fossil scyphoplyps at the family or generic level in the biological sense. In my work I use the generic name Byronia Matthew for all the solitary species of scyphoplyps as well as those forming encrusting colonies but I am treating it as a name for informal collective group. It is obvious that for me each of the species of Byronia listed in my paper belongs in a separate genus but there is no way in verifying that assumption. That is why I consider as justified, from the point of view if both taxonomy and usability of classifications, a description of all the fossil solitary and encrusting-colony-forming scyphopolyps under the name Byronia Matthew. In turn, dendroidal forms such as "graptolites" of Sdzuy (1974) should be allocated in the genus Archaeolafoea Chapman, and also treated as an informal collective group. When there appears a need to creat additional ranks in this classifications, it would be the best solution to use other informal units such as species group sor sections. This would prevent formation of a break between the taxonomy of modern forms and that of fossil ones.|
|One of the genera assigned to the Dithecoidea by Obut (1964), Mastigograptus Ruedemann, seems to require a separate discussion. This cosmopolitan genus comprises a signifacant number of species, mainly Ordovician ones, reported from the North and South Americas, North Africa, Europe and Australia. This is the only group form of Dithecoidea, whose affiliation with graptolites appears unquestionable. On the basis of material from Ordovician erratic boulders, Andres (1961) evidenced a distinct thecal trimorphism, i.e. differentiation of thecae into auto-, bi- and stolothecae, in Mastigograptus. When this is the case, the treatment of Mastigograptus as a dithecoid graptolite appears completely unsubstantiated. However, it should be noted that Obut and Sobolevskaya (1967) strongly question the affiliation of Andres' specimens with the genus Mastigograptus.|
|Summing up, I would like to note that the so-called dithecoid graptolites may be interpreted as proposed on the basis of superficial similarities to organic fossils, representing distantly related systematic groups. In the Dithecoidea were placed colonial scyphopolyps, encrusting hydroids (Chaunograptidae), graptolites with well-developed thecal trimorphism (Mastigograptus) and various incertae sedis organisms (e.g. Dithecodendrum Obut, Siberiodendrum Obut, Siberiograptus Obut). Some of them may represent green algae. For example, Rhadinograptus jurgaensonae Obut, 1960, interpreted by Obut (1964) as a dithecoid graptolite of the family Chaunograptidae, has nothing in common with graptolites (Mierzejewski 1991). Its "rhabdosome" reveals no traces of fusellar structure or other features of the graptolite morphology. Rhadinograptus appears to be an organic fossil, composed of a thick, irregularly annulated stem with bundle-like accumulations of numerous solid, slender appendices, devoid of cavities. One may suppose that it should be regarded as an algal fossil.|
Mierzejewski, P. 1986. Ultrastructure, taxonomy and affinities of some Ordovician and Silurian organic microfossils. - Palaeontologia Polonica 47, 129-220.
Mierzejewski, P. 1991. Estoniocaulis Obut et Rotsk, 1958 and Rhadinograptus Obut, 1960 are not graptolites. - Acta Palaeontologica Polonica 36, 77-81.
|Comparison of the colony morphology of Stephanoscyphus allmani (Kirkpatrick, Recent (a) and Tarnagraptus palma Sdzuy, Middle Cambrian. After Mierzejewski.|