PLAGIACANTHOIDEA Hertwig, 1879

Suzuki, Noritoshi, Caulet, Jean-Pierre & Dumitrica, Paulian, 2021, A new integrated morpho- and molecular systematic classification of Cenozoic radiolarians (Class Polycystinea) - suprageneric taxonomy and logical nomenclatorial acts, Geodiversitas 43 (15), pp. 405-573 : 499-501

Superfamily PLAGIACANTHOIDEA Hertwig, 1879

Plagiacanthiden [sic] Hertwig, 1879: 200-202 (= Plagiacanthidae ) [as a family].

Cystidiicae [sic] – Campbell 1954: D103 (= Cystidioidea) [as a superfamily].

Plagoniicae [sic] – Campbell 1954: D103 [nomen dubium] (= Plagonioidea) [as a superfamily].

Sethopiliilae Campbell, 1954: D122 [nomen dubium, as a subsuperfamily]. — Nakaseko 1957: 27 [as a subsuperfamily]. — Dieci 1964: 187 [as a subsuperfamily].

Plagoniacea [sic] – Loeblich & Tappan 1961: 227 [nomen dubium] (= Plagonioidea) [as a superfamily].

Plagiacanthoidea – Petrushevskaya 1971a: 57-65; 1971b: 988; 1975: 589; 1981: 61-62; 1986: 132. — Petrushevskaya & Kozlova 1972: 534. — Goll 1979: 379 ( sensu emend. ). — Matsuzaki et al. 2015: 42. — Sandin & Suzuki in Sandin et al. 2019: 201 ( sensu emend. ).

DIAGNOSIS. — Plagiacanthoidea having one or two segments and several arches. The cephalic initial spicular system consists of MB, A-, V-, double L- and Ax-rods. The double l-rods are generally absent. The V-rod may be degraded in intra-genera or infra-ge-

nus variations. The presence or absence of other main rods of the cephalic initial spicular system, as well as the presence of arches is highly variable depends on taxa within this superfamily. These cephalic arches may be free inside the cephalic cavity, embedded in the cephalic wall, or both.

REMARKS

The Plagiacanthoidea consist of the Ceratocyrtidae , Dictyocryphalidae Suzuki , n. fam., Dimelissidae , Phaenocalpididae , Plagiacanthidae , Pseudodictyophimidae Suzuki , n. fam., Tripodisciidae and Ximolzidae Dumitrica , nom. nov. The taxonomic position of all families, except the Tripodisciidae , is based on molecular phylogeny analyses ofSandin et al. (2019). The taxon names at family-, genus- and species-levels for Plagiacanthoidea are the most difficult to determine among Nassellaria . This is due to several problems: a) the different published schematic drawings of the cephalic initial spicular system for the same genus, or even for the same species; b) the technical difficulties to identify the representatives of this superfamily under transmitted light microscopy; and c) the existence of many undescribed genera and species in this superfamily.

The evolution of nassellarians through time has shown the importance of the cephalic initial spicular system for the classification at the family rank. However, this principle does not apply to the Plagiacanthoidea . The principal distinguishing feature of Plagiacanthoidea at the genus level is the presence or absence of rods and/or arches of the cephalic initial spicular system ( Petrushevskaya 1971a; Sugiyama 1992a, 1993, 1994; O’Connor 1997b). Essentially, this difference requires that rods and/or arches must vary amongst genera, leading to the logical conclusion that the cephalic initial spicular system is unstable at the genus level in the Plagiacanthoidea . This was written in several papers (e.g., Funakawa 1995a, b; O’Connor 1997 a, b, 1999). Furthermore, detailed studies indicate that the architecture and combination of the arches are variable within the same genus ( Funakawa 1994, 2000).

Several papers published very different schematic drawings for the same genus or even the same species, proning the users to confusion. Sugiyama (1998) mentioned that several papers erroneously drew a combination of cephalic initial spicular systems. Thus, the evidence images must be carefully examined by the users. This discrepancy can be partially explained by the differences among major studies of Funakawa, O’Connor and Sugiyama. Sugiyama focused on identifying the commonalities in the cephalic initial spicular system at the genus level, whereas Funakawa concentrated on the differences at the species or intra-species level. The methodology followed by O’Connor is a combination of both approach of Japanese researchers. Nishimura (1990) occasionally observed nearly invisible rods of the cephalic initial spicular system and hypothesized that some of these were buried in the cephalic wall during the ontogenesis. This hypothesis should be treated carefully as few to no objective evidence was presented in many cases.

Most taxonomic studies on Plagiacanthoidea were based on scanning electron microscopy (SEM) images; hence the problem for identification of Plagiacanthoidea under transmitted light microscopy. Funakawa and Sugiyama, specialists of the Plagiacanthoidea , explained the process of identifying the specimens to one of the authors (N.S.). They stressed that the identification of the Plagiacanthoidea at species and genus levels is in fact possible under a light microscope (see for example the new taxa described in the Southern Ocean by Renaudie & Lazarus 2012; 2013; 2015; 2016).

Even if it is possible to identify Plagiacanthoidea under a light microscope, anatomical knowledge is essential in understanding their taxonomy. An accurate taxonomy for the Plagiacanthoidea should consider the following aspects: 1) the collar stricture between cephalis and thorax which is independently determined from the position of MB; 2) the presence of a cephalic wall; 3) a cephalic initial spicular system consisting of MB, A-, D-, V-, double L-, double l-, Ax-rods, and several arches; 4) the presence/absence of rods and arches, and their development, which are also important in identifying similar genera but not as critical in many cases due to preexisting knowledge of infra-generic variation; 5) the overall similarity among species that may lead to critical misidentifications; 6) the spinules on each rod that are coded as (a) “ a ”, “ m ” and “ g ” on A-rod from the near end of MB, (b) “ j ” and “ f ” on V-rod from the near end of MB, (c) “ p ” and “ d ” on L-rod from the near end of MB, (d) “ c ” on D-rod, and (e) “ t ” on l-rod; 7) the name of the arch can be coded with major rods names (e.g., AV-arch) when the exact position is not necessary to be signaled; however, if the exact position of the arch is needed, the arch must be coded with the code of spinules (e.g., aj -arch but not AV-arch); 8) the presence of additional arches, occasionally developed on other arches; and 9) stress the differences between “primary arch” (if both ends of the arch arise from the coded rods) from “secondary arch” (if only one end is arising from the coded spinules) and “third arch” (if neither of the two ends are arising from any coded spinules).

This superfamily was ranked at the family level ( Petrushevskaya 1971a; Sugiyama 1994; 1998; Funakawa 1994). This group was also raised at the superfamily level to include Mesozoic member ( Petrushevskaya 1981). Later, this superfamily was disassembled again ( De Wever et al. 2001). Molecular phylogenetic results ( Sandin et al. 2019) classified one distinctive group as Clade G (100% PhyML bootstrap values with 10000 replicates (BS) and>0.99 posterior probabilities) including Archiscenium (Phaenocalpididae) , Ceratocyrtis and Lipmanella (Ceratocyrtidae) , Archiperidium , Peromelissa and Lithomelissa (Dimelissidae) , Dictyocryphalus and Pseudodictyophimus ( Pseudodictyophimidae Suzuki , n. fam.), Protoscenium ( Ximolzidae Dumitrica , nom. nov.), and Pseudocubus (Plagiacanthidae) . The aforementioned genera cannot be separated within Clade G due to the small values in BS or PP. This indicates that the morphological differences are larger than the molecular difference for the complete 18S and partial 28S sequences (D1-D2 region). This suggests that (a) these genera should be regarded as a single group and that (b) an approach with morphological differences should be prioritized for the Clade G. Clade G is assigned to the superfamily level in consideration of taxonomic hierarchy consistency for Mesozoic families of Nassellaria .

Plagiacanthoidea are the most diversified Nassellaria in both environmental DNA ( Sandin et al. 2019) and relative year-round abundance ( Motoyama et al. 2005; Ikenoue et al. 2015) at every latitude ( Boltovskoy et al. 2010) and depth ( Boltovskoy 2017). Despite this diversity and abundance, the establishment of a taxonomic framework has not been completed yet.