LITHELIIDAE Haeckel, 1862

Family LITHELIIDAE Haeckel, 1862 sensu Suzuki, emend. herein

Lithelida Haeckel, 1862: 240, 515-519 [as both family and tribe]; 1882: 464 [as a family]; 1884: 29 [as a family]; 1887: 604, 688-691 [as a family]. — Zittel 1876-1880: 124 [as a group]. — Mivart 1878: 176 [as a subsection]. — Bütschli 1889: 1968 [as a family]. — nec Rüst 1892: 175 [as a family]. — Schröder 1909: 4 [as a family]. — Anderson 1983: [as a family].

Spongocyclida Haeckel, 1862: 239, 452, 469 [as a tribe]. — Stöhr 1880: 119 [as a family].

Ommatodiscida Stöhr, 1880: 115 [as a family]. — Haeckel 1887: 484, 500 [as a subfamily]. — Dreyer 1889: 29 [as a subfamily]. — Schröder 1909: 42 [as a subfamily]. — Chen et al. 2017: 140 [as a subfamily].

Spireuma Haeckel, 1882: 464 [nomen dubium, as a subfamily].

Spiremida – Haeckel 1887: 691 [nomen dubium, as subfamily]. — Schröder 1909: 57 [as a subfamily].

Lithelidae [sic] – Popofsky 1908: 230 (= Litheliidae); 1912: 151. — Chediya 1959: 159. — Tan & Tchang 1976: 263. — Tan & Su 1982: 161. — Tochilina 1985: 101-102. — Chen & Tan 1996: 152. — Tan 1998: 274-275. — Kozlova 1999: 102. — Tan & Chen 1999: 260-261.

Litheliidae – Poche 1913: 210. — Campbell 1954: D99. — Riedel 1967b: 295; 1971: 655. — Nakaseko et al. 1975: 171. — Petrushevskaya 1975: 572; 1979: 109-110. — Nakaseko & Sugano 1976: 126. — Anderson 1983: 39. — Dumitrica 1984: 101. — Takahashi 1991: 91. — Hollis 1997: 44. — Boltovskoy 1998: 32. — Anderson et al. 2002: 1003. — De Wever et al. 2001: 164. — Afanasieva et al. 2005: S288. — Afanasieva & Amon 2006: 130. — Suzuki et al. 2009d: 248. — Chen et al. 2017: 157-158.

Ommatodiscinae – Campbell & Clark 1944a: 25; Frizzell & Middour 1951: 24. — Campbell 1954: D92. — Chediya 1959: 133. Litheliinae – Campbell 1954: D99. — Tan & Tchang 1976: 263.

Spireminae [sic] – Chediya 1959: 159 (= Spirematidae) [nomen dubium]. — Tan 1998: 275. — Tan & Chen 1999: 261.

Spongocycliidae – Kozur & Mostler 1978: 132-133.

Ommatodiscidae – Kozur & Mostler 1978: 134.

TYPE GENUS. — Lithelius Haeckel, 1861b: 843 [type species by monotypy: Lithelius haeckelspiralis Haeckel, 1861b: 843, nomen novum Matsuzaki & Suzuki in Matsuzaki et al., 2015].

INCLUDED GENERA. — Lithelius Haeckel, 1861b: 843 (= Lithospira with the same type species;? Azerbaidjanicus n. syn.). — Middourium Kozlova, 1999: 101 (= Monobrachium n. syn.). — Spiremaria Kozlova, 1960: 315 (= Spiromultitunica n. syn.). — Spongocyclia Haeckel, 1862: 469 (=? Lithocarpium n. syn., Ommatodiscinus n. syn., Ommatodiscus n. syn., Ommatodisculus n. syn.).

INVALID NAME. — Spirema.

NOMINA DUBIA. — Spiremarium, Spiremidium, Spireuma, Spongodruppium.

DIAGNOSIS. — Densely concentric or densely coiled shells, of spherical, ellipsoidal, disk-like and/or flattened lenticular shape, are found around a spherical microsphere. Straight robust radial beams emanate from the microsphere or are adjacent to the innermost shells. Pylome, when present, is found without robust walls. The protoplasm is documented for Lithelius. An opaque reddish-brown endoplasm occupies the shell. Hence, the endoplasm is invisible in living specimens. Pseudopodia are found radiating throughout the shell. Isolated skeletal fragments are found scattered in bundles of pseudopodia. Strongly cohesive pseudopodia appear to be immobile. A gelatinous matter is also present. No algal symbionts were observed.

STRATIGRAPHIC OCCURRENCE. — Early Paleocene-Living.

REMARKS

The typical structure of the Litheliidae shows an extremely organized distribution of concentric inner walls, straight radial beams and openings on the walls (Spiremaria: Chen 1974: pl. 1, fig. 8; 1975: pl. 9, figs 4, 5; Weaver 1976: pl. 7, fig. 1). This framework produces very straight holes from the surface to the center of the shell (see also Lazarus et al. 2005: pl. 11, fig. 19). This characteristic is important in distinguishing the Litheliidae from other similar genera of different families such as Tholospira (Larcospiridae). This structure is also well observed in Lithelius (Petrushevskaya 1975: pl. 32, figs 1-3). The Litheliidae can be distinguished from the four-cornered Spongodiscidae (e.g., Spongaster) by the presence of concentric-type spongy structures on their corners. They are distinguished from the Euchitoniidae by the presence arms and the three distinctive innermost concentric shells called “margarita”.The Litheliidae can also be distinguished from the Spongopylidae in having the walled pylome that penetrates though the internal structure to the center and the lack of straight robust radial beams. The non-walled pylome is illustrated by Chen (1974: pl. 2, figs 1, 2). The Trematodiscidae are easily distinguishable from the Litheliidae by their particular decussated central part. The Spireuma -form of Lithelius exceptionally lacks the straight robust radial beams, making their differentiation from Larcopyle -, or the Stomatodiscus -form of Tholospira (Larcospiridae) extremely difficult. The former is only distinguishable from the latter by absence of the box- or corner shaped central structure observable under a light microscope. Spongocyclia is also sometimes confused with Flustrella (Trematodiscidae), Spongodiscus (Spongodiscidae) and Spongopylidium (Prunopylidae), but it differs from the latter three genera by its tight, very systematic, concentric structure with robust straight radial beams originating from the central part. The difference between Spiremaria and Middourium can be found by observing additional incomplete concentric walls or a spongy structure, on one or both pole sides. The aforementioned may be conspecific with each other. Internal skeletal structures were illustrated for Spiremaria with thin-section images (Popova 1991: pl. 2, figs 1, 2;1993: pl. 9, figs 1a-2b; Tochilina 1985: pl. 3, fig. 3) and broken specimens (Chen 1974: pl. 1, fig. 4; Weaver 1975: fig. 2.4; 1976: pl. 7, fig. 1; Sugiyama et al. 1992: pl. 7, fig. 7), for the Spirema -form of Lithelius (Popova 1991: pl. 2, fig. 6; Tochilina 1985: pl. 2, figs 1, 2) and for the Ommatodiscus - form of Spongocyclia (O’Connor 1999: fig. 6M; Ogane & Suzuki 2006: pl. 2, fig.7). This structure was also documented for Middourium (Tochilina 1985: pl. 2, figs 6, 9; Barwicz- Piskorz 1999: figs 2.J-2.L, 3.A-3.B; Jackett & Baumgartner 2007: pl. 1, fig. 35), the Spireuma -form of Lithelius (Barwicz- Piskorz 1999: figs 2.B-2.D; Nishimura 2001: pl. 2, fig. 16) and Spongocyclia (Li et al. 2018: figs 7.3, 7.4). Images of living specimens were obtained for Lithelius (Suzuki et al. 2013: figs 7.2, 7.4-7.6). Algal symbionts and protoplasm were documented using epi-fluorescent DAPI dyeing techniques and other dyeing methods for Lithelius (Ogane et al. 2014: pl. 1, figs 1-2). Undescribed genera still remain (Dumitrica 1973b: pl. 5, figs 4-6; Hollis 1997: pl. 10, fig. 9).

VALIDITY OF GENERA

Lithelius

The current concept of Lithelius is helpless to understand differences in the genera of the Litheliidae. This genus is characterized by its spherical to subspherical shape. No pylome in general is presented; but if the skeleton develops a “pylome-structure”, that always opens towards the outermost hard cortical shell (“crust” in the terminology of Ogane & Suzuki 2006). In other words, it never penetrates through any other internal structure. The drawing of the type species for Azerbaidjanicus appears to indicate a convex-lens shape but in the original description Mamedov (1973: 61) clearly wrote about “a regularly spherical form” and, thus, Azerbaidjanicus is synonymized with Lithelius. The oldest available name is Lithelius.

Middourium

Both Middourium and Monobrachium were simultaneously established by Kozlova (1999: 101 for Middourium and 102 for Monobrachium). The translated description of Middourium from Russian follows. “ Sponguridae with a regular elliptical shell slightly truncated near polar areas; 10 or more lattice internal shells distributed in tight spirals separated by intervals no larger than 7-10 µm. Conical pylomes at each area. Shell sometimes enveloped by a thin porous plate. ” That of Monobrachium fol- lows. “ Sponguridae with a shell elongated along a single axis composed of a sub-spherical or plainly ellipsoidal part and a single large appendage of also ellipsoidal shape. The thickly spongious tissue of the internal ellipsoidal part seems to form concentrical of closed ellipsoidal envelopes which are very closely distributed. Pylome-shaped aperture may be located at both pole of the shell, or a single one, and the whole shell may be enveloped by a finely porous envelope.” The major difference written in the original description is the presence of a single large appendage only in Monobrachium. Hetero-coverage on one pole of the ellipsoidal shell commonly occurs during ontogeny so this difference does not correspond to a genus level. Both these genera have also a conical pylome at each pole. Presence of the pylome may be recognized with tiny spinules around the pylome. Some confusion may occur when there is a single appendage at one polar end, so that Middourium is selected as valid name. The taxonomic position of Middourium at the family level needs to be reexamined because the support image of this genus in the Atlas has a walled pylome.

Spiremaria

Spiremaria is characterized by a highly dense concentric structure and an ellipsoidal to ovoidal shape. Almost all outer concentric shells cover throughout the shell. The original definition of Spiromultitunica specifies an ellipsoidal shape and a dense convolution (Tochilina & Popova in Tochilina 1985: 105). As referred to a thin sectioned specimen of Spiromultitunica (Popova 1993: pl. 9, fig. 2), this genus has the same internal structure and shape as Spiremaria. Kozlova (1960: 315) does not comment the occurrence of a pylome in Spiremaria whereas Tochilina & Popova (in Tochilina 1985) described a pylome at one pole. However, this “pylome-structure” opens on the outermost hard cortical shell (“crust” in the terminology of Ogane & Suzuki 2006) and it never penetrates through any other internal structure. Such ambiguous pylome is insufficient to clearly establish a division into two genera, and thus Spiromultitunica is synonymized with Spiremaria. Although the independency of Spiremaria from Lithelius needs a phylogenetic study of the Litheliidae, typical Lithelius always lack pylome or pylome-like structures on the crust.

Spongocyclia

Spongocyclia is distinguished from any other genera in the Litheliidae by a convex-lens discoidal shape. The independency of Spongocyclia from Spongodiscus has long been in debate among authors of this paper. The view perpendicular to the equatorial plane of the disk-shaped shell is clearly different from that of the typical Spongodiscus as shown in the lower photo of the supporting image for Spongocyclia in the Atlas. This photo shows obvious concentric structures but no-spongy structure. Ommatodiscus has the same type species as Ommatodisculus. Campbell (1954: D92) indicated an elliptical disc with two pylomes for Ommatodiscus and a circular disk with two pylomes for Ommatodiscinus, but one opening only is recognizable on the type-illustrations of both these genera. The translated definition of Lithocarpium by Stöhr (1880: 97) from German mentioned “ an elliptical shell with a tubular peristome, and a circular opening with a corona of small teeth ”, and this explanation was insufficient to specify this genus. Worse, Campbell (1954: D119) wrongly classified Lithocarpium into the Nassellaria with a complete mismatched illustration of the nassellarian Carpocanium species on fig. 59.5. As any taxonomic act must be based on name-bearing specimens under ICZN (1999), the type-illustration prioritized the description.The type-illustration is surely different with a circular disk for Ommatodiscinus and an elliptical disk for Ommatodiscus, but this difference is too small to separate them independently. Lithocarpium looks to have a densely spiral concentric structure and a lobate shell according to Petrushevskaya (1975: 572). These three genera have one opening on one side but this opening does not form a true pylome (See the supporting image for Ommatodiscus in the Atlas). Such “pylome-structure” always opens on the outermost hard cortical shell (“crust” in the terminology of Ogane & Suzuki 2006) and it never penetrates inside the skeleton. In consideration of this character, no obvious differences can be found among Spongocyclia, Ommatodiscus, Ommatodiscinus and Lithocarpium. The oldest available name is Spongocyclia among them.