Excentrosphaerella sphaeroconcha Dumitrică , 1978

Dumitrica, Paulian, 2019, Cenozoic Spumellarian Radiolaria With Eccentric Microsphere, Acta Palaeontologica Romaniae 15 (1), pp. 39-60 : 55-58

Excentrosphaerella sphaeroconcha Dumitrică, 1978

Figures 10e-h; 11a-b

?1887 Actinomma (Actinommura) capillaceum Haeckel , p. 255, p,. 29, fig. 6.

1978 Excentrosphaerella sphaeroconcha Dumitrică , p. 238, pl. 5, figs. 17, 18, 22.

non 1992 Excentrosphaerella sphaeroconcha Dumitrică - Sugiyama & Furutani, p. 202, pl. 12, figs. 1, 2;?pl. 16, fig. 3.

Description. Microsphere very small and eccentric in a spherical or slightly prunoid outer medullary shell. Outer medullary shell with large circular or oval pores. Third shell spherical, rather thin-walled with polygonal to circular or subcircular pores and thin intervening bars. Cortical shell spherical, rather thin-walled and with numerous irregular polygonal pores and thin intervening bars. All shells interconnected by numerous filamentous beams, their number increasing with each shell so that between the third and the fourth shell their number is about one hundred or more. These beams are prolonged outside cortical shell into short and thin conical spines.

Dimensions. Diameter of microsphere 13-14 μm, of outer medullary shell 55-65 μm, of third shell 117-123 μm, of cortical shell 217-280 μm.

Remarks. The species illustrated by Sugiyama & Furutani (1992) as Excentrosphaerella sphaeroconcha does not belong to this species but to Excentrococcus annulatus Dumitrică. E. sphaeroconcha has all shells sphaerical whereas the specimen illustrated by them is flat as Excentrococcus and the shells outside outer medullary shell are ring-shaped. The best argument for this assignment is that the second, the third and the fourth shells are all focussed on their surface, whereas if they were spherical only the second shell could show its pores, the other two would have shown an optical section of their wall (see Dumitrică 1978, pl. 5, figs. 17, 18, 22, and Fig. 11 of the present paper). The number of shells is not an argument to assign this specimen to Excentrosphaerella , important is that they be spherical.

Excentrosphaerella sphaeroconcha Dumitrică , as illustrated by SEM ( Fig. 10f, g), resembles very much the type species of the subgenus Actinomma (Actinommura) capillaceum Haeckel (1887, p. 255 , pl. 29, fig. 6): the same morphology of the last two shells with polygonal pores, thin intervening bars, the same type of connection between them by numerous (around 100) filamentous beams that extend outside cortical shell into thin spines, and also rather similar diameters. The only difference is inside the middle shell. Haeckel illustrated a single microsphere, whereas the middle Miocene species from Romania has clearly a double medullary shell with an eccentric microsphere ( Fig. 11b ). The problem is: are the two species synonyms? If we agree that they are, we should consider that the specimen illustrated by Haeckel had also an eccentric microsphere inside the shell he considered as being microsphere but he could not see it. However, in the entire specimen illustrated by Dumitrică (1978, pl. 5, fig. 17) the eccentric microsphere seems to be rather well visible as a black area. There is also another argument in favour of the synonymy of the two species: the size of the microsphere. Haeckel considered the diameter of his so-called microsphere as being of 0.4 mm. The microspheric diameter of the Heliodiscidae is usually 13-20 μm but usually closer to the smallest value than to the greatest one. In this case, the diameter of the “microsphere” of A. capillaceum (0.4 mm) is more than double the measured diameter of the microsphere of the Heliodiscidae and closer to the diameter of the outer medullary shell of the members of this family. The conclusion would be that the two species could be synonym and that the genus Excentrosphaerella could be a junior synonym of Actinommura Haeckel 1887 . However, since a certain doubt of this synonymy exists, especially also because in the same subgenus Haeckel included Actinommura arcadophorum Haeckel 1887 , which is an entactinarian of the genus Rhizosphaera Haeckel ( Dumitrică, 2017) I prefer to consider the two genera valid and belonging to 2 independent families.

Range and occurrence. Rather frequent at some levels of the Radiolarian Shale Formation in both Subcarpathians and Getic Depressions, late middle Miocene (late Badenian), Romania.

Figures

Fig. 11 a, b. Excentrosphaerella sphaeroconcha Dumitrică, partly sectioned specimen showing the inner shells, focused on the third and second shell, respectively, sample 1315, Radiolarian Shale Formation, Păușești Otăsău, Romania, scale bar = 100 μm.