TAXONOMY

Outline classification

The following synopsis, based chiefly on the scheme proposed by Riedel (1967b, 1971), is restricted to extant families only. As pointed out above, it is a provisional classification very likely to change as more structural and evolutionary data are obtained. Taxa in bold characters are those treated in detail in this chapter.

Kingdom PROTISTA Haeckel, 1886
Phylum SARCODINA Hertwig and Lessser, 1876
Class ACTINOPODA Calkins, 1909
Subclass HELIOZOA Haeckel, 1886
Subclass ACANTHARIA Müller, 1858
Subclass RADIOLARIA Müller, 1858
Superorder PHAEODARIA Haeckel, 1879
Superorder POLYCYSTINA Ehrenberg, 1838, emend. Riedel, 1967
Order COLLODARIA Haeckel, 1881

Family THALASSICOLLIDA Haeckel, 1862
Family COLLOZOIDA Haeckel, 1862
Family THALASSOSPHAERIDA Haeckel, 1862
Family SPHAEROZOIDA Haeckel, 1862

Order SPUMELLARIA Ehrenberg, 1875

Family COLLOSPHAERIDAE Müller, 1858, emend. Strelkov and Reshetnjak, 1971
Family ACTINOMMIDAE Haeckel, 1862, emend. Sanfilippo and Riedel, 1980
Family COCCODISCIDAE Haeckel, 1862 emend. Sanfilippo and Riedel, 1980
Family PHACODISCIDAE Haeckel, 1881
Family SPONGODISCIDAE Haeckel, 1862, emend. Riedel 1967
Family LITHELIIDAE Haeckel, 1862
Family PYLONIIDAE Haeckel, 1881
Family THOLONIIDAE Haeckel, 1862

Order NASSELLARIA Ehrenberg, 1875

Family SPYRIDAE (=Trissocyclidae, Acanthodesmiidae) Ehrenberg, 1847, emend. Petrushevskaya, 1971
Family PLAGONIIDAE Haeckel, 1881, emend. Riedel, 1967
Family THEOPERIDAE Haeckel, 1881, emend. Riedel, 1967
Family CARPOCANIIDAE Haeckel, 1881, emend. Riedel, 1967
Family PTEROCORYTHIDAE Haeckel, 1881, emend. Riedel, 1967
Family ARTOSTROBIIDAE Riedel, 1967, emend. Foreman, 1973
Family CANNOBOTRYIDAE Haeckel, 1881, emend. Riedel, 1967

Order and family-level diagnoses

Order Collodaria. Solitary or colonial polycystines without a siliceous skeleton, or provided with simple or branched spicules scattered in the calymma. Due to their fragility, members of this group preserve poorly in net plankton samples, and either do not preserve at all or are represented only by their spicules in sedimentary materials. Partly because of these limitations, information on their classification and distribution is extremely scarce, and no further details are given herein. Detailed reviews of the colonial radiolarians, including several Collodaria, were produced by Hollande and Enjumet (1953), Strelkov and Reshetnjak (1971), and Swanberg (1979). Most of these species have tropical distribution ranges in the three major oceans. In the south Atlantic they are probably restricted to waters associated with the equatorial current system, the Tropics/Subtropics, and the oligotrophic Central Gyre (Figure 11). According to Haeckel (1887), this group comprises four families:

Family Thalassicollida: solitary cells, no skeletal elements; genera Actissa (Figure 15.1), Thalassocampe (Figure 15.2), Thalassopila, Thalassicolla, and Thalassophysa;

Genus Collozoida: colonial, no skeletal elements; genus Collozoum (Figure 15.6, 15.7, 15.8, 15.11);

Genus Thalassosphaerida: solitary, with siliceous spicules scattered in the calymma; genera Thalassosphaera, Thalassoxanthium (Figure 15.3), Physematium, Thalassoplancta, Lampoxanthium (Figure 15.5);

Genus Sphaerozoida: colonial, with siliceous spicules scattered in the calymma; genera Belonozoum, Sphaeorozoum (Figure 15.10), Raphidozoum (Figure 15.12).

Order Spumellaria. Solitary or colonial radiolarians with a well-developed shell of radial symmetry or one derived from the above. Variations in the type of symmetry include spiral shells (e.g., Figure 15.87), asymmetric, discoidal or lenticular (biconvex) (Figure 2H, 2I; 15.60, 15.61, 15.62, 15.63, 15.64, 15.65), triaxonic (Figure 2Q, 2R), quadrangular (Figure 15.66, 15.67), etc. In many cases two axes of symmetry can be clearly differentiated (Figure 2J) but, as opposed to the Nassellaria, the larger axis is homoaxonic. The central capsule (organic) of these cells has many small pores.

Family Collosphaeridae. Colonial polycystines, each individual has a single, thin-walled, spherical or subspherical latticed shell. The Collosphaeridae is the only group of colonial polycystines with complete latticed shells. Colonies consist of a gelatinous mass (which obviously disappears in the sedimentary record, as well as in many net-plankton samples where it breaks down) in which hundreds to thousands of shells are immersed (Figure 1C, 1D, 1E). The shape of the colony is not species-specific; it may be spherical, ellipsoidal, cylindrical, ribbon-shaped, etc., measuring up to several centimeters in length and a few millimeters in diameter. The siliceous shells are always represented by a single perforated sphere (internal spheres are never present), with or without centrifugal (external) or centripetal (internal) tubular projections and/or spines. Spines (when present) are conical (circular in cross-section). As with most other polycystines, specific assignments are based almost exclusively on the skeleton; however, studies of entire colonies, which allow investigating the intraspecific morphologic variability of the collosphaerids, indicate that quite dissimilar shell morphotypes can coexist within the same colony, thus stressing the assumption that at least some of the specific divisions based on the siliceous sphere alone are spurious (e.g., Kleijne 1987; Petrushevskaya and Swanberg 1990).

Family Actinommidae. Solitary species with latticed or spongy spherical, subspherical, or ovoid shells (not lenticular); with or without medullary shells. Surface of shell is often covered with spines, but not tubes. All actinommids posses either single or multiple, concentric spherical or ovoid shells. When several shells are present they are connected to each other by radial beams which pierce the cell. An enormous variety of forms was described in this family whose identification has traditionally been based on Haeckel's (1887) system. Haeckel based the classification of the actinommids (=suborder Sphaeroidea, exclusive of the Collosphaerida) on the following characters (in decreasing order of importance; see Figure 13): 1. Number of primary radial spines; 2. Number of concentric spheres; 3. Position of concentric spheres (intra- or extracapsular), type and relative size of spines, presence of by-spines, type of medullary shell, etc.). However, the number of primary spines varies intraspecifically, whereas the number of main concentric spheres, which within some bounds might indeed be species-specific (Riedel and Sanfilippo 1986), can only be used in the case of fully-grown individuals. It is quite obvious that, based on this trait, Haeckel (as well as many other authors) assigned new names to growth stages still missing the outermost sphere(s) (see Figs. 13 and 14). Furthermore, while growth of an actinommid as far as we know proceeds from the center toward the periphery (Figure 14, upper panel), dissolution works in the opposite direction, innermost, more delicate shells usually disappearing before the more robust cortical ones. Thus, materials from the sediments offer yet another suite of "new species", this time missing the medullary (rather than the cortical) shells.

Family Coccodiscidae (Figure 2H, 2J). Latticed discoidal or lenticular shell enclosing a single or double medullary shell, and surrounded by an equatorial zone of spongy or concentrically-chambered structures (Figure 2H), or forms with an ellipsoidal cortical shell equatorially constricted enclosing a single or double medullary shell (Figure 2J). The formerly actinommid subfamily Artiscinae was transferred to the Coccodiscidae by Sanfilippo and Riedel (1980) due to its phylogenetic affinities with extinct coccodiscids.

Family Phacodiscidae. Lenticular, biconvex, latticed cortical shell, not surrounded by spongy or chambered structures, within which a small, spherical single or double medullary shell is enclosed. The margin (but less commonly the surfaces) of the cortical shell may bear radial spines.

Family Spongodiscidae. Discoidal or cylindrical, spongy or finely chambered skeleton, with or without surficial pore-plate, often with radiating arms or marginal spines. The members of this family are characterized by possessing skeletons which are partly or entirely spongy in appearance. However, as opposed to the Actinommidae, which can also have spongy skeletons, the Spongodiscidae are not spherical. Their overall shape can be lenticular (biconvex discs, Figure 2I), cylindrical (Figure 15.74), quadrangular or subquadrangular in outline (Figure 15.67), or Y-shaped (Figure 2Q, 2R). With the exception of the cylinders, all others are depressed or flattened (rather than circular in cross-section, Figure 2I). Lenticular, quadrangular, and Y-shaped forms may be entirely composed of a spongy mass with no discernible structure (in which case the central part of the skeleton is often thicker and/or denser, and therefore appears darker in the light microscope; Figure 2O, 15.64), or may posses a small central chamber surrounded by concentric or spiral, continuous or interrupted bands (Figure 2R). The surface of some forms may be partly or totally covered with a very thin, porous sieve-plate, which in lenticular forms may extend beyond the central spongy mass forming a delicate equatorial girdle around the periphery of the shell (Figure 2P) (these morphotypes were formerly included in the family Porodiscidae).

Family Litheliidae. The lattice of the ellipsoidal, spherical or lenticular shell is totally or partially arranged along a bilaterally symmetrical spiral. Although very abundant, due to their complicated architecture the litheliids are poorly known, for which reason the morphotypes defined may include several different forms.

Family Pyloniidae. The major part of the shell is composed of a series of successively larger elliptical latticed girdles in three mutually perpendicular planes, with the major diameter of each girdle being the minor diameter of the next larger one (Figure 2L, 2M). The center is occupied by a small ellipsoidal structure - the microsphere (see Dumitrica 1989).

Family Tholoniidae. Completely latticed shell, without larger openings, and with constrictions that define several (typically 6) dome-shaped protuberances (Figure 2K).

Order Nassellaria. Solitary polycystines with a siliceous heteropolar shell, which can be represented by several fused spicules only, by a D-shaped ring and associated spines, or by more elaborate, mono- or multilocular latticed skeletons. With the exception of a few forms lacking a well developed skeleton (Figure 15.101, 15.119), the symmetry of this group is characterized by the fact that the two extremes of the major axis define two morphologically different poles of the shell. One of these, conventionally accepted as the top or anterior end, is where the cephalis is located. A widely recognized, albeit seldom utilized, feature of primary importance for the classification of the Nassellaria is the internal skeleton. The internal skeleton consists of a complex set of spines and connecting bars enclosed in the cephalis (Figure 3A, 3B, 3C), which allow comparison of homologous structures in forms differing widely in their external morphology. Unfortunately, analysis of these features requires dedicated efforts at understanding the complex spatial relationships involved. Furthermore, observation of this internal skeleton is only feasible with well preserved individuals oriented in the right position, which is seldom the case in specimens mounted in permanent slides. In addition to the small scattered perforations typical of the Spumellaria, the central capsule of the Nassellaria is usually provided with a single larger pore.

Family Spyridae (=Trissocyclidae). The skeleton is represented by a well-developed D-shaped sagittal ring (median bar and anastomosed vertical and apical spines), either free (Figure 15.101) or embedded into the latticed cephalic wall, in which case the cephalis is usually bilaterally lobed (Figure 15.93, 15.94, 15.100). Sometimes with thorax, abdomen always absent. The typical heteropolar nassellarian symmetry is often inconspicuous in the Spyridae.

Family Plagoniidae. Skeletons restricted to a simple tri- or tetraxonic nassellarian spicule (Figure 15.119), or a well developed system of main spines enclosed within a fully formed cephalis (Figure 3A, 3B). The degree of development of the cephalis may vary from a few anastomosed bars (Figure 15.120, 15.124) to a well developed, latticed or latticed/spongy chamber. Usually without postcephalic segments. In addition to several fairly well-defined species, the Plagoniidae comprise many probably related forms of obscure taxonomic status usually cited under various generic names (see below). The classification of these forms needs detailed ad hoc studies, for which reason many of them are provisionally lumped under the designation Plagoniidae group in the present chapter.

Family Theoperidae. Cephalis spherical or subspherical, relatively small, often poreless or sparsely perforate. It usually bears an apical horn. Internal spicule small and inconspicuous. With one or more, sometimes up to over 10, usually well-developed postcephalic segments. Generally, cap- or helmet-shaped, or conical in overall outline.

Family Carpocaniidae. The small, rudimentary cephalis is usually totally immersed in the large and well-developed thorax (Figure 3E). Abdomen absent or rudimentary.

Family Pterocorythidae. Cephalis large, divided into three lobes by two lateral furrows directed obliquely and downward from the apical spine to the base of the cephalis. The upper unpaired lobe is located above the two smaller paired ones (Figure 3K, 3L); these basal paired lobes are not always conspicuous. Many pterocorythids are two or three-segmented, lacking postabdominal segments.

Family Artostrobiidae. Spherical or subspherical cephalis, usually with an apical tube directed obliquely upwards (Figure 3J). The pores on all postcephalic segments, or at least on the last ones, are arranged in clearly defined transverse rows (Figure 3S). Usually elongated, multisegmented forms.

Family Cannobotryidae. Cephalis large, with several asymmetrical lobes (sometimes appearing as irregular bulges) (Figure 3M). Mostly 2-segmented forms (cephalis and thorax), but sometimes with post-thoracic segments.