Arachnoplecta architectonica, Dumitrica & Zügel, 2003

Dumitrica, Paulian & Zügel, Peter, 2003, Lower Tithonian mono- and dicyrtid Nassellaria (Radiolaria) from the Solnhofen area (southern Germany), Geodiversitas 25 (1), pp. 5-72 : 14-16

Arachnoplecta architectonica n. sp. ( Figs 7 ; 8 )

HOLOTYPE. — Photo No. 54908, 54909, 54912, 54915; stub Mue 22/17; Musée de Géologie , Lausanne, No. 74375 ( Figs 7 ; 8 ).

PARATYPE. — MNHN, bât. de Géologie, No. Gg 2001/2090.

ETYMOLOGY. — From the Latin architectonicus: architectonic, according to its architectural design.

MATERIAL EXAMINED. — Two specimens of which only the holotype is complete, both from the type horizon (sample Mue 22).

DIMENSIONS ( IN µM). — Length of spines 270-280, thickness of spines 5, maximum thickness of spines including blade-forming bars 17, span 490.

DESCRIPTION

Tetrahedral skeleton as with genus, formed by the apical, dorsal, and primary lateral spines, and by a system of net-like planes. Apical, dorsal, and primary lateral spines equal in length. Apical spine straight, and dorsal and primary lateral spines slightly curved downwards. In the central part of skeleton all spines simple, cylindrical and slender. In the proximal part, at about the same distance from the median bar, apical, dorsal, and both primary lateral spines give rise to four bars aligned to the spines. These bars diverge slightly over a short distance to run strictly parallel in the median part of the spines. Finally they converge to form a pseudo four-bladed structure in the distal part. Each of the four blade-forming bars connected to spines by short bars arising slightly obliquely at subequal distances from the main spines. This results in longitudinal rows of tetrahedral meshes proximally and medially, and small circular to elliptical pores distally. On the middle portion of main spines these short bars radiate slightly obliquely from the central axis and continue beyond the blade-forming bars to form part of the net-like planes. From the nodes of the blade-forming bars arises also bars parallel with the edges of the tetrahedron extending between spines in the plane of the radiating bars. Both types of bars intersect forming rhomboidal to tetrahedral meshes in each plane of the four blades. Spatial position of net-like planes in the skeletal architecture depends on the type of spine; on A it is different from that on D, and the two L. On the apical spine two net-like blades are in the latero-dorsal part forming an angle of 45° with the dorsal part of the sagittal plane. These blades join one blade of spine D and one blade of each primary lateral spine in two symmetrical places, noted here ADr and ADl, forming a triple junction. The other two blades of the apical spine form angles of 45° with the sagittal plane, but in the ventral part. Proximally they are supported by two proximal branches of the apical spine, and meet branches of the ventral spine directed latero-apically. Dorsal spine and the two primary lateral spines develop two of their four-bladed structures in three planes corresponding to the three lateral faces of the pyramid these spines outline. These structures leave in the basal part of the skeleton a wide pyramidal space. In the apical direction the other two blades of the dorsal spine and one blade of each primary lateral spine join two blades of the apical spine in ADr and ADl, whereas the other blade of the two primary lateral spines join the ventral spine. Ventral spine simple, without blade-forming bars but with straight branches radiating approximately in four perpendicular directions.

REMARKS

The planiform net-like structures of this species resemble those of the Tertiary or Quaternary species Plectaniscus cortiniscus Haeckel, 1887 , but the planiform structures of the latter are simpler, forming six diagonal planes at 120° between each pair of spines.

Figures

FIG. 7. — Structure of the skeleton of Arachnoplecta architectonica n. gen., n. sp. Abbreviations: A, apical spine; ADl, left junction area among one planiform structure of the apical spine, one of the dorsal spine, and one of the left primary spine; ADr, right junction area among one planiform structure of the apical spine, one of the dorsal spine, and one of the right primary lateral spine; D, dorsal spine; Lr, right primary lateral spine; Ll, left primary lateral spine; MB, median bar; V, ventral spine.

FIG. 8. — Arachnoplecta architectonica n. gen., n. sp., holotype; A, basal view (dorsal spine is directed towards the base of the figure and ventral spine towards the upper part); B, dorsal view (apical spine is directed towards the upper part of the figure); C, detail of A showing the median bar, the two primary lateral spines, and the ventral spine; D, detail of the left primary lateral spine with bars aligned to the central rod. Scale bars: A, B, 100 µm; C, D, 30 µm.