TAXONOMY

Morphology and classification systems

Based on his previous monograph of 1862, and especially on the extensive collections of HMS Challenger, Haeckel (1887) produced the first comprehensive system of radiolarian classification, encompassing over 3,000 species, 2,400 of which were new to science. Although Haeckel's work is still a compulsory reference guide for anyone attempting to deal with the identification of these organisms, it has for some time been evident that it does not satisfactorily represent natural relationships. Indeed Haeckel’s groupings are only based on morphological similarities without the support of continuity in the fossil record, rather than on demonstrable evolutionary sequences. In addition, the rigidity of his diagnoses, based chiefly on strict geometric considerations (Figure 13), ignores the ample intraspecific variability of the polycystines. As a result, many of his described "species" are but slightly different morphotypes or even developmental stages of the same organism (see Figure 13 and Figure 14).

In spite of these shortcomings and the time elapsed, advances in the development of a better classification system have been very limited. Efforts to depart from and improve upon the classification schemes inherited from earlier workers have mainly followed two different approaches: cytological techniques and evolutionary studies.

Hollande and Enjumet (1960), Cachon and Cachon (1972), Petrushevskaya et al. (1976), Petrushevskaya (1981) proposed revisions which use not only the skeleton (as most other classifications), but also cytoplasmic features, in particular the "nucleoaxopodial complex" (sensu Petrushevskaya 1981). Although these schemes are probably sounder in biological terms, their applicability to fossil and subfossil materials lacking the protoplasm is problematic, which is one of the reasons for their very limited acceptance among radiolarian workers.

Analyses of evolutionary lineages in geological sequences were somewhat more succesful than cytological techniques in defining characters applicable to classification. Based on evolutionary evidence, Riedel and Sanfilippo (1986) produced an interesting critical review of the most important skeletal traits used by Haeckel. They concluded that some of them (e.g., number of segments, number of supplementary concentric spheres, number of feet, number of rays and of equatorial spines in discoidal Spumellaria, presence and nature of thoracic wings) have little or no suprageneric value. In contrast, several others (especially cephalic structure, but also pore arrangement, shell terminations in Nassellaria, etc.), traditionally considered as of minor value, are conservative through time, reveal evolutionary lineages and, therefore, are relevant for higher-rank divisions.

Riedel (1967b, 1971), Petrushevskaya (1965, 1971a), Goll (1968, 1969), Sanfilippo and Riedel (1970), Zhamoida and Kozlova (1971), Foreman (1973), Dumitrica (1988, 1989) based on skeletal features alone worked out alternative classifications, either for the entire order or for selected polycystine groups. Of these, Riedel's (1967b, 1971) suprageneric system has become the most widely accepted for extant and Cenozoic radiolarians, and is the one adopted herewith (with slight modifications; see also Kling 1978; Boltovskoy 1981e; Anderson et al. 1996). It should be stressed, however, that this system does not overcome many of the above-mentioned problems, and is therefore a compromise provisional classification. Several of the family-level definitions, especially in the Spumellaria, are rather vague and generally used as a lumping black box for the many forms with complex morphologies and poorly understood relationships (e.g., Litheliidae, Pyloniidae, Tholoniidae).

Specific identification of the polycystines is a time-consuming and frustrating task. With the exception of the few abundant and widespread species on whose names there is fairly good agreement, binomial nomenclature alone very often fails to pinpoint unequivocally a given morphotype because different names are applied to the same species and, conversely, identical organisms are reported under different specific and even generic names (see Boltovskoy and Jankilevich 1985). Because a very substantial proportion of the original species descriptions were published in old and often hard to get monographs, some authors find it faster and easier to create a "new species" for the unusual-looking skeleton in the slide, than to comb the dusty books in search of an adequate, already established name. Ecologically, paleoecologically and stratigraphically-oriented studies often underestimate the importance of a stable and consistent naming system; the lack of species illustrations in these reports allows the wrong designations to go undetected. This not only hinders buildup of useful information, but also significantly degrades the overall quality of radiolarian-based data for other applications. Recent literature has abundant examples of this bias, which introduces even more chaos into the already anarchic situation inherited from turn of the century works. Indeed, this may be a major reason for the waning use of radiolarians in stratigraphic and paleoecologic work.

The following illustrated glossary of most commonly used terms for the description of polycystine skeletons is chiefly based on the listing compiled by Petrushevskaya (1981). Capital letters (in parentheses) denote the group for which the term is used (N: Nassellaria; S: Spumellaria).

• Abdomen (N) (Figure 3O, 3P): the third segment of nassellarian multisegmented shells.

• Aboral, aboral pole (N): skeletal section located at the opposite extreme of the mouth or aperture. Nassellarian growth starts with this part of the skeleton.

• Adoral teeth: see teeth.

• Annular strictures: see strictures.

• Antecephalic chamber: see antecephalic lobe.

• Antecephalic lobe (N) (Figure 3M, 3N): Section of the cephalis at the base of which the dorsal spine is located. Separated from the eucephalic lobe by the apical spine. When this section is separated from the rest of the shell by a pored wall it is called the antecephalic chamber.

• Aperture: see mouth.

• Apical cupola: see galea.

• Apical horn (N) (Figure 3P): external extension of the apical spine.

• Apical pore (N): wall-pore located at the base of the apical spine.

• Apical spine (N) (Figure 3A, 3B): Internal cephalic spine which branches off the median bar close to the point of insertion of the dorsal spine. The apical spine can protrude outside of the cephalis, in which case its external section is called apical horn.

• Apical tube (N) (Figure 3J): Tube-like projection on the cephalis of the Artrostrobiidae, homologous to the vertical spine. Also termed lateral tube or tubule.

• Apophyses (N, S): any external or internal protruding outgrowth of the skeletal meshwork.

• Appendages (N): any external or internal protruding outgrowth of the skeletal meshwork. Riedel and Sanfilippo (1986) suggested that primary appendages (those directly connected to or having homologies with the internal spicule) are of higher phylogenetic significance than the secondary ones (not directly related to the internal spicule).

• Arches (N): anastomosed skeletal outgrowths of the main spines, such as the one forming the upper section of the sagittal ring (Figure 3A).

• Areolate (N, S): Referring to small, very regularly repeated wall perforations or pores.

• Arms (S) (Figure 2Q, 2R): elongate projections (usually 3) radiating from a central subcircular disc in the shells of some Spongodiscidae. Arms can consist of unstructured spongy meshwork (Figure 2Q), or of spongy meshwork with more or less clearly visible chambered rings (Figure 2R).

• Axial rod: see axial spine.

• Axial spine (N) (Figure 3A, 3B): spine projecting from the median bar and oriented toward the thorax and subsequent segments; can be simple or branched. Also called axobate.

• Axobate: see axial spine.

• Axoneme (N, S): the central shaft of parallel microtubules of the axopodia (cytoplasmic structure).

• Axoplast (N, S) (Figure 1A): central nucleus of the axopodial system (cytoplasmic structure).

• Bars (N, S) (Figure 2F): siliceous anastomosed beams that define the meshwork of polycystine shells, separated by pores.

• Basal feet: see feet.

• Basal plate: see collar plate.

• Basal pores (N): see collar pores.

• Basal ring (N) (Figure 3C): the subcircular structure formed by the arches that separate the wall of the cephalis from that of the thorax. In reduced skeletons, such as those of the Spyrida, these arches make an actual ring, whereas in other families the ring is embedded into the shell-wall.

• Basal segment (N): the last segment in multisegmented Nassellaria.

• Beams (S): siliceous rods joining contiguous shell structures. In some families (e.g., Actinommidae, Pyloniidae, Litheliidae) radial beams connect the successive, concentric spheres (Figure 2A) or consecutive whorls of the spiral (Figure 2L).

• Branched spines: see spines.

• By-spines: see spines.

• Calymma (N, S) (Figure 1A): extracapsular cytoplasm (see central capsule).

• Central capsule (N, S) (Figure 1A, 1B): an organic, perforated membrane within the radiolarian protoplasm which separates the intracapsular cytoplasm from the extracapsular one.

• Central chamber (S) (Figure 2R): the central, spherical structure of the skeleton of several Spongodiscidae.

• Cephalis (N): the first (uppermost) segment of the skeleton, which can be either undivided (Figure 3H, 3I, 3J), or divided into lobes or chambers (Figure 3K, 3M, 3N). The cephalis can be well differentiated from the thorax (Figure 3F, 3H, 3I, 3J, 3K, 3L, 3M, 3N, 3R), or it can be immersed into it partially (Figure 3D) or totally (Figure 3E).

• Cervical apophyses (N): lateral outgrowths of the dorsal spine.

• Cervical pores (N): paired pores in the base of the cephalis between the primary lateral and the dorsal spines.

• Cervical stricture (N) (Figure 3O): furrow or constriction which divides the cephalis from the thorax.

• Chamber (N): one of the several sections into which the cephalis of the Nassellaria can be divided (see antecephalic, lateral, eucephalic and postcefalic chambers). See also central chamber.

• Chambered rings (S) (Figure 2O, 2R): Concentric or spiral more or less visibly segmented rings around the central chamber in several Spongodiscidae (see arms).

• Club-like spines: see spines.

• Collar plate (N): the complex structure at the base of the cephalis formed by the spines and arches, all lying on approximately the same plane.

• Collar pores (N) (Figure 3C): pores in the collar plate.

• Collar stricture: see cervical stricture.

• Columella (N): free portion of the apical spine (Figure 3A) in the cephalis of the Nassellaria.

• Cortical shell (S) (Figure 2A, 2B): one of the outermost perforated spherical shells of the Spumellaria, located outside of the central capsule.

• Cupolae (S) (Figure 2C): dome-like protuberances of the skeleton of the Tholonidae.

• Cylindrical spines: see spines.

• Dorsal spine (N) (Figure 3A, 3B): Internal cephalic spine projecting from the median bar at the opposite end of the vertical spine, next to the apical and secondary lateral spines. The dorsal spine can protrude outside of the cephalis at the level of the cervical stricture.

• Equatorial plane (S): mainly applicable to lenticular, discoidal and biconvex spumellarian shells (Figure 2H, 2I); plane of maximum shell surface.

• Eucephalic lobe (N) (Figure 3N): in species with a divided cephalis, the section which hosts the median bar.

• Extracapsular cytoplasm: see calymma.

• Feet (N) (Figure 3Q): External projections of the wall of the thorax oriented down and sideways, usually in the number of 3. The feet are often associated with the dorsal and primary lateral spines.

• First shell (S) (Figure 2A, 2B): the innermost shell of the set of concentric spheres of an actinommid. Sometimes also applied to members of other families with several successively larger shells totally or partially enclosing one another. Shells are numbered from the inside of the skeleton toward the periphery; thus, the second shell is the one located next to- and outside of the first one, and so on.

• Fourth shell: see first shell.

• Frames (N, S) (Figure 2D): ridges on the surface of the shell surrounding the pores. Frames around pores can be roundish or polygonal (pentagonal, hexagonal, etc.).

• Fundamental spicule: see main spicule.

• Funnel-shaped pores (N, S): pores the external opening of which is conspicuously wider than the internal one.

• Galea (N): helmet-shaped portion of the shell of the Nassellaria associated with the apical horn; its base is formed by the top of the cephalis. Also called apical cupola.

• Gates (S) (Figure 2L, M): large openings in the skeleton's meshwork, usually conspicuously larger than the pores (Pyloniidae).

• Girdles (S) (Figure 2L, 2M): circular or ellipsoidal skeletal perforated plates arranged in three mutually perpendicular planes which form the skeleton of the Pyloniidae.

• Internal spicule: see main spicule.

• Intracapsular cytoplasm: see central capsule.

• Jugal pores (N): paired pores in the base of the cephalis between the primary lateral and the vertical spines.

• Lateral lobes (N) (Figure 3K, 3L): lateral sections of the cephalis separated from the eucephalic portion by cephalic arches.

• Lateral spines (N) (Figure 3A, 3B): Paired spines projecting from the median bar. The main or primary lateral spines (right and left) are located in the vicinity of the vertical spine, while the secondary ones (right and left) are inserted close to the apical spine.

• Lateral tube: see apical tube.

• Latticed meshwork (N, S) (Figure 2D, 2E, 2J, 2L): siliceous meshwork of bars separated by regular or irregular pores of variable size, not spongy in appearance (see spongy meshwork).

• Lentelliptical shell (S) (Figure 2H, 2I): in the shape of a biconvex disc.

• Lumbar stricture (N) (Figure 3O): constriction between the thorax and the abdomen.

• Main lateral spines: see lateral spines.

• Main spicule: see main spines.

• Main spines (N) (Figure 3A, 3B, 3C): basic skeletal elements of most Nassellaria, composed of the median bar from which the apical, vertical dorsal and lateral spines arise.

• Mantle (N, S): usually thin, delicate, lace-like meshwork surrounding the main shell which appears in some fully grown polycystines.

• Median bar (N) (Figure 3A, 3B, 3C): the basic nassellarian internal skeletal element which supports the apical, vertical, dorsal and lateral spines. Its position defines the limit between cephalis and thorax.

• Medullary shell (S) (Figure 2A, 2B, 2J): one of the innermost perforated spherical shells of the Spumellaria, located inside the central capsule. The central smallest sphere (<30 µm) is also called microsphere by some authors.

• Meshwork (N, S): the combination of anastomosed rods and bars that form the external siliceous skeleton of the polycystines.

• Microsphere: see medullary shell.

• Mitral ring (N): in the Spyrida, the skeletal ring which lies in the plane parallel to that of the basal ring and perpendicular to that of the sagittal ring.

• Monolocular shell (N) (Figure 15.111): Nassellarian shell composed of a single segment.

• Monothalamous shell: see monolocular shell.

• Mouth (N): A large basal opening in the last segment of the Nassellaria (also called aperture), which can be open (Figure 3O, 3P) or obliterated by a porous plate or velum.

• Multilocular cephalis (N) (Figure 3K, 3M, 3N): cephalis provided with several chambers or lobes in addition to the eucephalic one.

• Multilocular shell (N): see multisegmented shell.

• Multisegmented shell (N) (Figure 3O): Nassellarian shell composed of several (as opposed to one) segments.

• Neck (N): elongated section of shell joining cephalis and thorax.

• Nodal points: see nodes.

• Nodes (N, S) (Figure 2F): area of the shell where two or more bars meet (=nodal points).

• Oral teeth: see teeth.

• Patagium (S) (Figure 2Q, 2R): a delicate, spongy meshwork in some Spongodiscidae which differs from the main shell by its looser structure.

• Peristome (N) (Figure 3P): differentiated rim around the mouth of the last segment, usually poreless, often bearing teeth or other structures.

• Phacoid shell (S): structure formed by two concentric spherical medullary shells enclosed in one lenticular cortical shell, characteristic of the families Coccodiscidae and Phacodiscidae.

• Polar caps (S) (Figure 2J): Additional skeletal growth in the form of latticed cupolae at both ends of some coccodiscid shells.

• Pore arrangement (N, S): spatial distribution of pores of the shell-wall; regular distributions can be checkered (Figure 3U) or in rows, either transversal (Figure 3S, 3T) or longitudinal (Figure 3V, 3W).

• Pores (N, S) (Figure 2D, 2E): perforations in the skeleton of the polycystines.

• Porous sieve-plate (S) (Figure 2P): a thin, perforated plate that covers the spongy surface of some Spongodiscidae.

• Post-abdominal (N) (Figure 3O): refers to structures associated with the fourth and subsequent segments of multisegmented Nassellaria (segments, pores, ribs, etc.).

• Postcephalic lobe (N) (Figure 3N): one of the sections into which the cephalis can be divided, sometimes elongated into a tube.

• Primary lateral spines: see lateral spines.

• Primary spines: see spines.

• Pylome (S) (Figure 2N): A larger opening on the external shell of some Spumellaria, often extending into a tube or surrounded by larger spines.

• Radial beams: see beams.

• Ribs (N) (Figure 3Q): elongated, usually unperforated thickenings in the shell-wall. Sometimes the ribs can extend into wings (Figure 3R) or feet (Figure 3Q). Dorsal and primary lateral spines embedded in the wall of the thorax can also form ribs.

• Rosette-shaped (N, S): usually applicable to pores with a tri- or tetra-lobed outline (Figure 2E).

• Sagittal plane (N): the plane defined by the position of the median bar (and also the apical, vertical, dorsal and axial spines).

• Sagittal ring (N) (Figure 3A): a skeletal ring formed by the median bar at the base, the vertical and apical spines at the front and back, respectively, and an arch joining these two spines at the top. It can be either free or embedded into the skeletal meshwork.

• Second shell: see first shell.

• Secondary lateral spines: see lateral spines.

• Secondary spines (= by-spines): see spines.

• Segment (N) (Figure 3O): one of the several joints or sections which compose a Nassellarian skeleton, separated by the adjoining ones by an internal circular thickening of the shell-wall and/or by an external stricture.

• Septae (N): Well-developed internal divisions between the segments of nassellarian shells.

• Sieve-plate: see porous sieve-plate.

• Spines (N, S): any internal or external rod or needle with one free end. According to their relative size and number, spumellarian spines can be primary (larger, usually fewer in number; Figure 2B) and secondary (smaller, more numerous, sometimes bristle- or thorn-shaped; Figure 2B). In cross-section spines can be triangular (three-bladed) with smooth or serrated edges, or cylindrical; straight, or twisted (Figure 2G). The distal end of spines can be unbranched or branched (Figure 2G). Club-like spines have a swollen or thickened distal end (Figure 2G). Radial spines are those radiating centrifugally from the center of the shell (Figure 2B).

• Spongy meshwork (N, S) (Figure 2N, 2O, 2P, 2Q, 2R, 2S): sponge-like (as opposed to latticed) siliceous meshwork of thin, irregular, more or less densely packed bars, mainly in some spumellarian families (Actinommidae, Spongodiscidae).

• Strictures (N) (Figure 3O): joints between contiguous segments, often marked by conspicuous external constrictions.

• Teeth (N) (Figure 3P): protruding apophyses around the mouth, of variable length and thickness (=oral teeth, adoral teeth, terminal teeth, subterminal teeth).

• Terminal teeth: see teeth.

• Third shell: see first shell.

• Thorax (N) (Figure 3O, 3P): second segment of multisegmented nassellarians.

• Tripod (N): section of the internal skeleton formed by the dorsal and primary lateral spines (Figure 3A, 3B). In some shells the tripod protrudes externally forming the feet (Figure 3Q).

• Tubercles (N, S): pustule-like lumps (condyles, mamillae) on the surface of the siliceous shell.

• Unilocular shell: see monolocular shell.

• Velum (N): a porous plate obliterating the mouth.

• Ventral spine: same as axial spine.

• Vertical spine (N) (Figure 3A, 3B): Main unpaired spine projecting from the median bar in the vicinity of the primary lateral spines and the axial spine.

• Wings (N) (Figure 3R): lateral apophyses of the thorax usually connected to the shell-wall by siliceous meshwork, derived from external projections of the dorsal and primary lateral spines.