Abdomen wall structure of Holocryptocanium barbui (Radiolaria)

The abdomen wall structure of the cryptothoracic Nassellaria Holocryptocanium barbui Dumitricǎ was studied using scanning electron microscopy. The abdomen wall consists of four layers, all of which have been observed as the ‘outer’ layer of specimens depending upon the different level of preservation. These different stages of abdominal wall preservation have been described previously as two separate subspecies of Holocryptocanium barbui.

All specimens described as H. barbici have their cephalis and thorax completely enclosed within a large spherical abdomen. Specific differences relate to different surticial pore frames on 1.he abdomen: some specimens of H. hurbui are described without pore frames or with a sparsely porate surface perforated by very small, rounded pores, with a thicker abdomen wall (e.g. Pessagno, 1977), other specimens described ;is H. harhui or H. jriponicum have hexagonal pore frames (e.g. Nakaseko et al., 1979: Schaaf, 1981. On the basis of those abdominal differences Nakascko & Nishimura (1981) divided the species H. barbiii into two subspecies: Holoctyptocriniicm hurhui harbici Dumitricii, having a thicker sparcely perforated or unperforated abdominal wall; and Holocryptocunium burbui japonicum Nakaseko & Nishimura, as the form with abdominal wall characterized by hexagonal pore frames. Both forms have identical structures of cephalis and thorax enclosed within the abdomen (Nakaseko RL Nishimura, 1981). Oivoldov,i (1990) has found from Albian deposits of the Vienna Basin both subspecies distinguished by Nakaseko & Nishimura (1981). However, some forms of H. harhui have a fractured sparsely porous layer on the surface of their abdomen which covered regular pore frames (similar to the abdominal surface of Holocryptocanium barhui japonicum as described by Nakaseko & Nishirnura).
The specks investigated herein originate from samples collected from Albian-Cenomanian marly deposits of the Branisko Succession ( Fig. 1: seven samples from Kietowy stream section (Ki) and thirteen samples from Stare Bystre section (SB)) in the Polish part of the Pieniny Klippen Belt, Carpathians (for detailed location see K. Bqk, 1992Bqk, , 1993. In these deposits H. barbui is abundant and is the most characteristic taxon within the radiolarian assemblages as described by M. Bqk (1995).
About 3000 specimens of H. barbui have been analysed.
The material is curated in the Institute of Geological Sciences, Jagiellonian University.

SYSTEMATIC DESCRIPTIONS
Class

THE CHARACTERISTICS OF ABDOMINAL WALL STRUCTURE
In the investigated material only the specimens in samples Ki-13 and Ki-14 are pyritized. Replacement of silica by pyrite has led to excellent preservation of Radiolaria with the finest details of ornamentation and other morphological features. As a consequence, it is impossible to observe the inner structure of the test under the light microscope. Fortunately the use of scanning electron microscopy of whole and broken specimens allows the observation of internal wall structures.
In the material investigated, four different forms of the surface of abdomen wall pore frames of H. barbi~i have been found: 1. with a smooth sparcely perforated surface of abdomen (Plate 1, Fig. 1): 2. with a smooth surface and circular pores bigger than in 3. with regular hexagonal pore frames and circular pores type 1 (Plate I , Fig. 4): (the same size as in type 2) which are slightly depressed into the mesh (Plate 1, Fig. 7); 4. with a surface mesh slightly coarse , with hexagonal pore frames and circular pores bigger than those in type 3 (Plate 1, Fig. 10).
Our samples also contain pyritized forms with broken abdominal walls in which the layers described as types 2, 3 and 4 overlie each other (Plate I , Figs 2, 3, 6). The abdominal layers defined as types 1 and 2 agree well with those described by Nakaseko & Nishimura (1981) as H. harhui barhui, whilst types 3 and 4 are similar to those described as H. harhui japoniciim. The material described herein contained broken forms in which all three layers are visible in cross-sections of single specimens (Plate 1, Fig. 3). Therefore, it is apparent that Nakaseko & Nishimura (1981) described these various layers as the external abdominal wall. All four layers of the abdominal wall have also previously been observed under the light microscope by Dumitricj (1970), he noted differences in thickness and luminosity. He also noted differences of external abdominal wall, but no definite structures were described.

CONCLUSIONS
Well-preserved forms of Holocryptocanium barbui Dumitrics have been found from the Albian to Cenomanian deposits of the Pieniny Klippen Belt (Poland). Pyritized specimens from samples Ki-13 and Ki-14 consist of three porous layers described above as types 2, 3 and 4.
Complete abdominal walls of our forms of H. barbui comprise four concentric layers: type 1 (outer layer) is smooth, thin and sparsely perforate with circular pores (Plate 1, Fig. 1): type 2 (the layer below) has larger circular pores than type 1 (Plate 1, Fig. 4): type 3 has hexagonal pore frames structure and circular pores (Plate 1, Fig. 7) and type 4 (the innermost layer) has hexagonal pore frames and the biggest pores which seem to be constricted in the inner side of the abdomen wall (Plate I , Fig. 10). The thin, most external, sparsely porous layer of type 1 has been observed only in a few siliceous specimens, therefore this layer is probably lost very quickly during the sinking of the skeleton. The forms with broken layer type 1 and situated below layer type 2 are absent in our pyritized material, but they were illustrated by Oivoldova (1990). Dumitrica (1970) has explained the differences in H.  -14. Fig. 8. Cephalis and fragments of broken thorax completely enclosed within the abdomen XSOO. Ki-14. Fig. 9. Transverse section through the abdominal wall ahowing porc structures X 9 0 0 . Ki-14. Fig. 10. Specimen of type 4 with a slightly coarse surface and larger hexagonal pore frames than fig. 7: lateral view showing sutural pore X12O. Ki-14. Fig. 11. Cephalis and fragments of broken thorax XIOOO. Ki-14. Fig. 12. lnncr side o f the ahdominal wall showing the aperture XSOO.  3 and 4), whereas the pores of the mature specimens have very narrow external openings (our types 1 and 2). Thurow (1988), who also observed four different surfaces of H. harhui, suggests a progresive trend in skeletal evolution from forms with a smooth surface and small pores (our type 1) to forms with hexagonal pore frames (our type 4). Our well-preserved and pyritized material shows that the different abdominal wall surfaces of H. harhui which have been described by previous authors as different species of Holocryptocaniurn or subspecies of Holocryptocaniurn harbui (Holocryptocanium japonicum Nakaseko et al., Holocryptocaniurn harhui barhui Dumitricg, Holocryptocanium barbui japonicum Nakaseko & Nishimura) are really the different stages of skeletal preservation (we can observe partly or almost completely broken layers of types 1 to 4 in a single specimen). We also could not make any suggestions about skeletal growth on the basis of our material.