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Pentapylonium implicatum
Dumitrica, 1991
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Description - Add description
Description: The microsphere of this species consists of six triclades arranged according to the corners of a triangular prism and fused to one another in nine diclade junctions. These junctions are at the origin of nine primary spines which are equal in length, robust and bear five pairs of opposite bran- ches in successively perpendicular planes. The two branches of each pair may be perfectly symmetrical or strongly asymmetrical, in which case they are more or less displaced to one another along the spines. This microsphere may be imagined as deriving from a shell resembling a triangular prism stretched among the proximal ends of nine spines originated in the middle of each edge. By this process the six corners flatten becoming triclade nodes, whereas the edges of the prism bend outwards resulting in the nine diclade bars. The analogy with a triangular prism extends also to the elements of symmetry. As in any regular triangular prism whose bases are equilateral triangles, P. implicatum has an axis of triradial symmetry, three axes of bilateral symmetry, a principal plane of symmetry parallel with the base and perpendicular to the axis of triradial symmetry, and three auxiliary planes of symmetry normal to the base and bisecting the angles between the lateral faces. By analogy with such a prism, the skeleton of this species will have two diagnostic views: a view from the axis of triradial symmetry (PI. 6, fig. 4, 5, 7, 9; PI. 7, fig. 1, 2), when the triangular elements of the skeletal structure are emphasized, and a view (in fact three views) from one of the three axes of bilateral symmetry (PI. 5, fig. 1-7). These last axes correspond to the three main spines RL lying in the principal plane of symmetry, of which one is RLa, homologous with the apical spine of the genus Trimanicula or with the very short apical (upper) bar connecting S1b with S2a in Stauropylissa (PL 1). As these spines lie at approximately 120° and are equal in length and shape it is rather difficult to establish which one is the apical. Besides bars and spines the microsphere has five gates corresponding to the faces of the prism. Two of them (BG), corresponding to the two bases of the prism, are equal and hexagonal in shape when viewed from the main axis (PI. 1, A3, B3, C3; PI. 3, fig. 3; PL 6, fig. 4, 7, 9; PL 7, fig. 1, 2). Three corners of each hexagon (p, q, r and s, t, u) coincide with the diclade junctions in the base of six primary spines, while other three corners (a, b, c and d, e, f respectively) coincide with the tricalde junctions. The other three gates (LG), corresponding to the lateral faces of the prism, are rhombic when regarded from one of the axes of bilateral symmetry (PL 1, B1; PL 2, fig. 5; PL 7, fig. 6). The corners of these rhombs (m, p, n, s; m, q, o, and n, u, o, r) coincide with the places of origin of four primary spines, while the middle of their sides (a, b, e, d; a, d, f, c and b, e, f, c) with four triclade junctions. Due to their stretching in one direction or another neither the bars of the hexagonal gates nor those of the rhombic ones are coplanar but zigzagged, the corners nearer to the center corresponding to the triclade junctions and those farther to the diclade junctions. If we take into account the pyloniacean mode of growth and some peculiarities of the extra- microspheric girdles of the first system, it seems that the position of the primary spines with respect to the planes and axes of symmetry of the initial prism plays an important role in the order of shell construction. So, it seems that the first pair of branches of the spines RB is the only one to contribute to the construction of S1b, the first pair of branches of RL coming into action in a second stage to build the girdle S1c. This order is suggested by the skeletal structures situated in and on both sides of the principal plane of symmetry (PL 2, fig. 2). Between two spines RL it shows a zigzagged longitudinal bar in a wide W or M, situated in the principal plane, and three transverse bars. If we consider the position of each pair of bars with respect to the center of shell, then all this scaffolding may be imagined as resulting from three growth stages of a skeleton with two-pillared caps. In the first stage, a transverse two-pillared cap Sib arises from one branch of the first pair of branches of two spines RB. In a second stage, from the vault of this cap two bars arise in perpendicular directions, that is in the principal plane of symmetry. They fuse with the branches of the first node of two spines RL forming two-pillared caps. These caps would represent elements of Sic In a third stage, from the vaults of these caps other two pairs of bars arise in perpendicular directions. They represent pillars of the girdle S2a. Having established the order of skeletal construction in this species, let us come back to the girdle S1b. It develops, as I showed, from the first node of the spines RB (p1, q1, r1, s1, t1, u1), which is very close to the proximal end of the spines. The branches of each node directed to the principal plane of symmetry fuse two by two resulting in three curved bars that, in terms of pyloniacean structures (Dumitrica, 1989), represent three two-pillared caps (B, C, D). They border three pairs of gates open in the principal plane of symmetry. The other branches of this node are obliquely directed towards the axis of triradial symmetry of the prism forming two three-pillared caps (A, A') opposite and symmetrical with respect to the principal plane. The cupola of these caps is a hexagonal frame with a central pore (PI. 3, fig. 3; PI. 6, fig. 5, 8). The pillars of each cap border three gates open at 120°. Therefore the whole girdle has twelve gates. If we reduce the girdle S1b to its geometrical expression its fundamental shape reminds us of a triangular bipyramid with vertices cut off. The bipyramid is outlined by only its lateral edges, each edge consisting of one pillar of a two-pillared cap and one pillar of a three-pillared cap. The girdle S1c develops exclusively in the principal plane of symmetry, that is in the plane of the first pair of branches of the spines RL. It has the shape of a triangle with all corners cut off or of a hexagon (PI. 2, fig. 3; PI. 3, fig. 3; PL 6, fig. 4, 7, 9). The six corners of this polygon (Bn1 Bo1, Co1 Cm1, Dm1, Dn1) would represent vaults of the six two-pillared caps of which the girdle is composed. Each cap is a bent bar resulting from the fusion of a branch of one spine RL and a bar developed in the principal plane from a two-pillared cap of the girdle S1b (PI. 2, fig. 2, 3). The second system repeats on a larger and more complex scale the first one. Its complexity is primarily due to the appearance of additional pillars that divide the gates and that together with the branches of the spines unite the girdles to one another (PI. 2, fig. 1). According to the pyloniacean pattern of skeletal construction the girdle S2a should repeat the microsphere, its equivalent in the first system, that is it should have six three-pillared caps superposed on the six triclades of the latter. Such caps actually exist, but instead of three they have six pillars (PI. 2, fig. 1; PI. 3, fig. 1). Three of them, disposed at 120°, are branches of the second node of the primary spines. One of these branches come from a spine RL, the other two from two spines RB. It is to be mentioned that, unlike the other two girdles of each system, all spines contribute to the building of the first girdle, irrespective of their position on the initial prism. The other three pillars of these caps have an intermediate position and arise from the previous girdles. One of them arises from a three-pillared cupola of S1b in the manner described in another paper (Dumitrica, 1989, text-fig. 1E), the other two from two-pillared cupolas of S1c situated between two spines RL. The first pillar is usually represented by a single bar, exceptionally two. Together with the similar pillars of the other two caps from the same face and with the three pillars of a three-pillared cap Sib they form a characteristic rosette of six pores around the central pore of the cupola A or A' (PI. 6, fig. 5, 8; PI. 3, fig. 3; PI. 7, fig. 1). The other two pillars of a cap S2a form also a characteristic structure of four pairs of pores or gates on the faces corresponding to the lateral gates of the microsphere (PI. 2, fig. 5, 6; PI. 7, fig. 6). The cupolas of the caps S2a have 3-8 pores and give rise to three pillars according to the pattern illustrated in PI. 2, fig. 1, all of them contributing to the construction of the girdle S2b. On the whole, the fundamental shape of the girdle S2a reminds us of a triangular prism having for corners the cupolas of the six caps. It shows the triangular bases when viewed from the axis of triradial symmetry (PI. 6, fig. 4, 5, 7-9; PI. 8, fig. 1, 2), and the squarish or triangular faces when viewed from any of the spines RL (PI. 3, fig. 2; PI. 5, fig. 3, 7; PI. 7, fig. 3-5). It is worth mentioning that due to the opposite, alternate position of their branches, between the first and the second node four cloverleaf pores appear around each spine (PI. 2, fig. 5, 6; PL 3, fig. 2; PL 7, fig. 3, 4, 7). Such pores will also appear along the spines between all pairs of branches (PL 5, fig. 1, 2; PL 6, fig. 4; PL 7, fig. 1, 2). The girdle S2b has the same position, fundamental shape and mode of growth as the girdle Sib which it repeats. It arises from the third node of the spines RB and, as well as S1b, has three two-pillared caps disposed at 120° between each pair of spines RB (PL 3, fig. 3; PL 6, fig. 4, 5, 7; PL 7, fig. 1, 2) and two opposite and symmetrical three-pillared caps in the axis of triradial symmetry (PL 5, fig. 3, 6, 7; PL 6, fig. 1, 6). Each two-pillared cap is a latticed band resembling a wide V when regarded from an axis of bilateral symmetry (PL 2, fig. 7; PL 6, fig. 1, 3). Either arm of this V-shaped cap has three pillars of which the central one corresponds to a branch of the third node of a spine RB, and the lateral ones arise in lateral direction from two cupolas of the girdle S2a (PL 2, fig. 7; PL 3, fig. 2, 3; PL 6, fig. 5, 7). The two three-pillared caps of the girdle S2b repeat on a larger scale the equivalent caps of the girdle S1b. They develop from the branches of the third node of RB directed towards the axis of triradial symmetry which constitute the main pillars (PL 6, fig. 6). Three other pillars arise from the cupolas of the girdle S2a on the border of the six-pored rosette discussed above (PL 3, fig. 3). The girdle S2c repeats the girdle S1c. It develops in the principal plane of symmetry, has the shape of a hexagon or of a triangle with corners cut off (PL 6, fig. 4, 5, 7; PL 7, fig. 1, 2) and consists of six two-pillared caps. Each cap has a pillar represented by a branch of the third node of one spine RL and another one developed in the principal plane from the vault of one of the two-pillared caps of the girdle S2b. This latter pillar may be either a bar (PL 5, fig. 4, 5) or a narrow band (PL 5, fig. 3; PL 6, fig. 4, 5, 7). The cupolas of this girdle give rise, in their turn, to a pair of opposite bars directed obliquely in planes passing through the axis of triradial symmetry, forming a characteristic wide V when regarded from one of the axes of bilateral symmetry (PL 5, fig. 3, 7). They represent pillars of the girdle S3a and have the same shape and position as their equivalent in the girdle S2a (PL 5, fig. 7). In the same view one can see the quadrangular shape of this girdle. Generally, the third system is built in the same manner as the previous ones, with the only difference that its girdles, and especially the last ones, become more irregular so as to give a more or less perfect spherical shape to the mature shell. The girdle S3a is still well outlined. As well as S2a it has two groups of three caps symmetricaly developed on both sides of the principal plane, each cap having six pillars of which three are represented by rod-like branches of the fourth node, and three by rod-like pillars developed from the cupolas of the previous two girdles : one from one three-pillared cap of S2b and two from two two-pillared caps of S2C. Surface of the mature shell is slightly undulate, with wide irregular meshes, and a distinct cortical shell is absent. Spines are equal in length, sturdy, with pointed distal ends.
Dimensions (in µm) : Diameter of microsphere 21-22, height of the triangular bipyramid of Sib 38-43, of S2b 88-93, of S.,b (diameter of shell) 168-195, radius of girdle Sic (measured along RL) 18-21, of S2c 45-50, of S3C (approximately radius of shell) 80-93.
Remarks : The description is based on the investigation of 20 specimens, most of them with a robust skeleton. Except for some minor individual variations, the regularity of the structure described and illustrated above is clearly emphasized when the shell is viewed from one of the mentioned axes of symmetry, which represent the only diagnostic positions of this species.
Occurrence: Generally, the species is a rare to very rare element of the Neogene radiolarian assemblages of the equatorial to subtropical Pacific.
Range : Lower Miocene - Holocene. | | Dumitrica 1991 |
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