Buschmannia roeringi (Kaever & Richter, 1976) a so-called archaeocyatha, and the problem of the Precambrian or Cambrian age of the Nama System (S.W. Africa)

SIR The implication of archeocyathid occurrences in the Nama System are too important for the interpretation of South West African geology (especially the Precambrian or Cambrian age of the series) to let a doubt lie upon that problem. Until recently, the archaeocyatha reported were tubes or impressed bodies (Haughton, 1929, 1959; Germs, 1972) which are no longer interpreted as archaeocyatha but as worm traces or cribricyathids. On the other hand material collected in the Buschmann-Klippe Formation by Kaever & Richter seems to have a recognizable skeletal framework which, according to the original description, consists of radial tubes. Nevertheless their attribution to the archaeocyatha, even if there is no doubt in the authors' mind, was not convincing; most of the typical features, particularly the two porous walls and the porous intervallar elements were missing or said to have been worn out or not preserved by fossilization. The keg-like external form is known among the archaeocyatha. The comparison with Beticocyathus Simon is unfortunate as this genus, is represented only by a mould, the 'tubes' of which being only the infilling of intertaenial spaces, without any characteristic features preserved. There are no further reports of syringocnematida from Spain even after intensive revision of the type localities (Perejon, 1975, 1976; Zamarreno & Debrenne, 1977). Imperforate tubes as quoted in Buschmannia did not exist in archaeocyatha, but might have occurred in other groups such as Tabulata or Algae. Professor Dr Kaever and Professor Dr Oekentorp (Geol. Palaont. Inst. Universitat Miinster) kindly agreed to send us the material for examination. Petrographic slides and ultra-thin sections (Lafuste, 1970-1974) were made from their material. Observations included the reexamination of the original collections as well as the new preparations. Criticisms and comments of the original description and figures (Kaever & Richter, 1976, p. 29, pi. 4) proceed from these studies. Besides the keg-like or slightly flattened sphaeroids described by the authors (PI. 1, Fig. 1 a) bunches (PI. 1, Fig. 1 b) sometimes associated (PI. 1, Fig. 1 c), a great number of isolated elements similar to those described as tubes are scattered through the matrix (PI. 1, Fig. 1 d). The central part of the sphaeroid is not surrounded by any kind of perforate skeletal structure (PI. 2, Fig. 1 d). The intervallum could not be described as a scaffolding of hexagonal to polygonal 'tubuli' radiating from an 'inner wall': the radial structures in fact, spreading out of a microsparitic central part are made of a single crystal, lanceolate in shape with a rhombic to polygonal transverse section, depending upon their orientation (PI. 2, Figs. 1 and 2). They are often surrounded by dark material which could not correspond to the replacement or the remants of some calcareous skeleton. The sections are irregular in shape and size, the direction of the crystals may change abruptly, they often criss-crossed and may be cut by smaller independent crystals of different orientation (probably corresponding to the 'vesicular tissue' of the authors) (PI. 2, Fig. \d). The crystals may radiate from a double centre (PI. 1, Fig. 3) the two adjacent sphaeroids mingling their crystals; if they were colonial cups, as previously interpreted, they would have common tubuli, evolving into each other, and not superposition of independant crystals. This observation in addition to the relative angular position of the elements, does not correspond to any organic built up pattern known. Even in the most poorly preserved specimen the connections between the remaining elements should have reflected the original living structures, and should not have this geometric appearance and abrupt changes. A mineral origin was, therefore, suspected and the problem submitted to Drs Loreau and Rouchy, sedimentologists (National Museum of Natural History, Geology, Paris). They determined typical inorganic spherulitic structures corresponding to crystalline growth in situ spreading out on a support inside the sediment. These so-called spherulitic structures are not always spherical but also bundle-like, diverging from one point (PI. 1, Fig. 1 a, b). The crystals were determined, by optical microscopic characteristics confirmed by X-ray analysis, as calcite. The morphological section of the spherulitic structures, as well as the isolated or twinned crystals, are those of gypsum which was later pseudomorphed by calcite. This gypsum might not be itself primary but the result of a prior diagenetic transformation of previous mineral. The complete history of the mineralogic evolution is not known, and is not the purpose of the present article. It is only shown that an organic interpretation of these structures is no longer acceptable. According to M. Kaever & P. Richter, none of the fossils found before the discovery of Buschmannia were