Type species:Palacantholithus ? verticillatus  Dumitrica (OD)
Description:Skeleton formed exclusively of a very large spicule with five to seven, possibly more sturdy long cylindrical spines arising from a centre or from a scarcely distinguishable bar. Commonly four of them diverge obliquely downward and bear a verticil or a bunch of thick long centrifugal spinules; the others arise apically or obliquely downward and bear sometimes a verticil of short thin spinules.
Remarks:The species included in this genus have a common element: a large heteropolar spicule consisting of a basal group of four spines with spinules and an apical group of one to three possible more spines without spinules or with very thin ones. The median bar does not appear to be so important for systematic because it is missing not only in A. verticillatus but also in many specimens of A. heptaceros. Initially, the type species of the genus was provisionally included among the Pentactinocarpinae Dumitrica (1978b). It must be noticed that, despite the superficial resemblance of their spicule, the absence of any tendency toward secreting a shell distinguishes undoubtlessly Archaeothamnulus from the members of this subfamily and suggests other relationships. In the Triassic and, in fact, in the whole Mesozoic nothing similar was described so far but quite comparable spicules are known in the Paleozoic. Thus, the heteropolar spicule of Archaeothamnulus can be compared with Palaeoscenidium Deflandre that shows also an advanced heteropolarity and where the number of the basal spines is commonly four, and that of the apical is smaller. Close relationship is also suggested with Palaeothalomnus Deflandre which bears the same type of verticils and seems to have four basal spines similar to those of Archaeothamnulus. Unfortunately, the single specimen upon which Deflandre (1973a, 1973b) based this genus is partly broken off and does not let to see a differentiation of the spines and a heteropolarity of the spicule. A more or less advanced heteropolarity can be remarked in the spicule of Palacantholithus Deflandre, in the Silurian Palaeoscenidiidae illustrated by Holdsworth (1977, p. 172, pl. 1, figs. 4, 5) and in some Ordovician and Devonian species of Bissylentactinia Nazarov (1975b), Nazarov and Popov (1980). There is no doubt that a phylogenetic relationship with some of these taxa must exist. Archaeothamnulus differs, however, from all of them by its size, which is generally five to ten larger, and by other features. At last, Archaeothamnulus, and especially A. heptaceros, shows close morphological affinities with the huge spicule of the living Thalassothamnus Haecker. The differences between the spicules of the two genera are especially of quantitative order and do not appear to be so significant. They consist particularly in the greater number of spines and in the much larger size of the spicule of Thalassothamnus. Deflandre (1973a) rejected any phylogenetic connection between Palaeothalomnus and Thalassothamnus because of the large size differences. This objection, more or less justified for his genus and for almost all the Paleozoic spicular genera, is less important in the case of the large spicule of Archaeothamnulus, the size of which is commonly as large as 1-2 mm or larger. It must be noted, on the other hand, that Petrushevskaya (1979a) considered that between the spicule of the Thalassothamnidae and some Paleozoic spiculate Radiolaria such as Palaeoscenidium cladophorum Deflandre and Haplentactinia rhynophyusa Foreman is an evident morphological identity. Probably the main argument against considering Archaeothamnulus as a primitive Thalassothamnidae is its ancientness and the absence of some similar spicules in the post Triassic/pre-Quaternary sediments. Such an absence should not be a major objection if we think of the relative scarcity of these radiolarians in both Triassic and living plankton. On the other hand, the Thalassothamnidae are known only in living state. Or, although there is no fossil evidence, it is to be supposed that this family, so much different from all the Recent Radiolaria, had a rather long geological history. Otherwise it seems difficult to admit that the spicule once disappeared in the Upper Triassic or later could reappear in the Quaternary. The problem can be regarded in a similar manner for all Cenozoic spicule-bearing Spumellaria.
Stratigraphic range:Pelsonian to Fassanian [Middle Triassic, Mesozoic].
Etymology:The generic name intends to suggest a phylogenetic relationship with the living Thalassothamnus Haecker.
Subsequent descriptions:?

Description and dimensions:Large siliceous spicule consisting of a straight axial spine and four slightly curved basal spines arising from a centre and having a verticil of four spinules at a distance of 80-100µm from the spicule centre. Commonly, the verticil of the axial spine is weekly developed or absent. The four basal spines make an angle of 60 Ð100° within the close vicinity of the centre, then, before branching, they bend rather abruptly toward lateral direction. The spinules of the verticils are long and curved, changing their direction from normal or subnormal to the basal spines to radial; seldom one verticil has short, thin spinules approximately normal to spine. Both the spines and spinules of the whole skeleton are cylindrical and very gently tapered.       Maximum diameter of main spines 11-18µm, maximum preserved length of main spines 500µm, their total length reached probably 900-1000µm. Remarks:By its architecture this species appears to belong to the group of palaeoscenidiids discussed in this paper. However, its size, type of branching and general aspect cast some doubt on this relationship and make us to think of some possible relationship to the Paleozoic spicular radiolarians of Palaeothalomnus Deflandre, Palacantholithus Deflandre or Bissylentactinia Nazarov type. It is to be noted that associated or not with P. ( ?) verticillatus there is another species with a large spicular skeleton of the same type but consisting of seven verticillate spines arising from a centre in various directions. It is interesting that in spite of this undifferentiated disposition, the species displays also a slight polarity in that four spines have long spinules whereas the other three spines, generally opposite to the four, have short thin spinules.

Repository:[Holotype and paratype designated by the author in 1982 (Dumitrica, 1982a)]
Type locality:[No type locality designated in the original description of Dumitrica, 1978b. Designated in the revision of the species (Dumitrica, 1982a]
Original stratigraphic range:Late Illyrian - Fassanian, Middle Triassic, Mesozoic.
Etymology:[No information given]
Subsequent descriptions:YES

Literature cited:- Deflandre, G. (1973a). Sur quelques nouveaux types de Radiolaires Polycystines viséens, d'attribution systèmatique ambigue, certains évoquant à la fois des Plectellaires et des Spumellaires (New types of Visean Polycystin Radiolaria, of equivocal taxonomic position, some looking like Plectellaria and Spumellaria). Comptes rendus des séances de l'Académie des Sciences (Paris), Série II 276: 289-293.
- Deflandre, G. (1973b). Compléments historiques et taxinomiques sur les Radiolaires viséens. Remarques critiques sur les Plectellaires (Historic and taxonomic data on the Visean radiolarians; critical remarks on the Plectellaria) Plectallaria). Comptes rendus des séances de l'Académie des Sciences (Paris), Série II 276: 497-500.
- Dumitrica, P. (1978b). Triassic Palaeoscenidiidae and Entactiniidae from the Vicentinian Alps (Italy) and eastern Carpathians (Romania). Deri di Seama ale sedinfelo 64: 39-54.
- Dumitrica, P. (1982a). Middle Triassic spicular Radiolaria. Revista espanola de micropaleontologia 14(1-3): 401-428.
- Nazarov, B. B. (1975). Radiolyarii nizhnego-srednego Paleozoya Kazakhstana (Metody issledovanii, sistematika, stratigraficheskoe znachenie [Lower and Middle Paleozoic radiolarians of Kazakhstan (research methods, systematics, stratigraphic importance)]. Akademiya NAUK SSSR, Trudy (Academy of Sciences of the USSR, Transactions) 275: 203.
- Nazarov, B. B. and L. Popov (1980). Stratigrafiya i fauna kremnistro-karbonatnykh tolsch ordovika Kazakhstana (radiolyarii i bezzamkovye brakhiopody) (Stratigraphy and fauna of the siliceous-carbonate sequence ofthe Ordovician of Kazakhstan (Radiolaria and inarticulate brachiopods)). Transactions of the Geological Insitute of the Soviet Akademy of Sciences 331: 192.
- Petrushevskaya, M. G. (1979a). Novyi variant sistemy Polycystina (New variants of the system of polycystina). Iskopaemye i sovremennye radiolyarii; sbornik nauchnykh rabot (Fossil and Recent Radiolaria; symposium of scientific work). R. K. Lipman. Leningrad, USSR. Izdatelstvo Akademiya Nauk SSSR, Zoologicheskii Institut) (Academy of Science of the USSR, Zoological Institute) 101-118.