Arthropoma iuncta, Martino & Rosso & Taylor & Chiu & Fujita & Kitamura & Yasuhara, 2025

Arthropoma iuncta sp. nov.

Di Martino, Rosso, and Taylor

Figure 48 View FIGURE 48

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Type material. Holotype PMC. B67. 29.7.2024 a, sample 19041 ( Figure 48A–C View FIGURE 48 ); paratype PMC. B67. 29.7.2024 b1, sample 19178 ( Figure 48D View FIGURE 48 ); paratype PMC. B67. 29.7.2024 b2, sample 19085 ( Figure 48E View FIGURE 48 ); paratype PMC. B67. 29.7.2024 b3, sample 19092 ( Figure 48F View FIGURE 48 ); paratype PMC. B67. 29.7.2024 b4, 19104 ( Figure 48G–H View FIGURE 48 ); Core 19, Daidokutsu cave, Okinawa, Japan, Holocene.

Etymology. Latin, meaning joined, referring to the orificial condyles joining over the sinus, thereby materially separating it from the orifice.

Diagnosis. Arthropoma with frontal shield evenly and densely pierced by circular pseudopores arranged in a regular pattern, except for a slender, imperforate, U-shaped strip below the orifice; orifice with robust condyles joined over the drop-shaped ‘sinus’, lacking spines; ooecium with mostly exposed, imperforate, minutely pitted endooecium marked by radial wrinkles, and smooth, calcified ectooecium reduced to a narrow band proximally; vicarious avicularia similar to autozooids but with an enlarged opesia and distal spines.

Description. Colony encrusting, multiserial, uni- to multilaminar. Autozooids distinct, delineated by fine furrows, quincuncially arranged, rounded hexagonal, slightly longer than wide (mean ZL/ZW 1.17). Frontal shield flat or slightly convex, densely pierced by pseudopores throughout except for a narrow, central, U-shaped strip below the orifice, 35–45 µm wide by 180–200 µm long; pseudopores circular, c. 14–20 µm in diameter, regularly arranged, with 1–2 rows distal to the orifice. Orifice semicircular, wider than long; the robust, rectangular, crenulated condyles (15 µm thick) bridging over the drop-shaped ‘sinus’, materially occupying the entire length of the proximal margin of the orifice and separating the two openings; peristomial rim and distal oral spines absent. Vicarious avicularia closely resembling autozooids in size, shape and frontal shield features, but with an enlarged, slightly spatulate opening/rostrum similarly characterized by joined condyles and drop-shaped ‘sinus’, and at least four distal spines; palatal shelf well developed distally, rapidly tapering laterally, absent proximally. Ovicell hyperstomial, globular; endooecium mostly exposed, imperforate, faintly marked by radial wrinkles, and minutely pitted; ectooecium partially calcified, covering the proximal periphery of the ooecium, smooth.

Measurements (µm). ZL 565±49, 457–671 (4, 20); ZW 484±46, 392–561 (4, 20); OL 122±5, 112–133 (4, 20); OW 170±8, 156–184 (20, 4); SinL 45±4, 39–53 (4, 20); SinW 32±3, 24–38 (4, 20); AvL 640±60, 572–713 (4, 4); AvW 481±67, 422–577 (4, 4); AvOpL 377±49, 326–426 (4, 4); AvOpW 303±27, 279–332 (3, 3); OvL 260±34, 206–300 (5, 13); OvW 322±16, 299–346 (5, 13).

Remarks. All species of Arthropoma described to date lack avicularia. Levinsen’s (1909) definition of the genus suggests that avicularia might be present, likely because Levinsen concurrently examined specimens belonging to both A. cecilii (Audouin, 1826) and Phonicosia circinata (MacGillivray, 1869) . However, an additional species of Arthropoma , identified as A. cecilii in Hirose (2010, pl. 189D), has vicarious avicularia with a few distal oral spines. It differs from A. iuncta in having an orifice with thinner condyles that do not connect over the sinus, thus leaving it distally open, and in the shape of the avicularian rostrum, which is horseshoe-shaped while in the new species it is slightly spatulate. Another unusual feature of this new species among Arthropoma is the presence of the condyles that separate the orifice from a functional ascopore in both autozooids and avicularia. This feature is exceptionally rare in avicularia, with the only known example being in the Late Cretaceous genus Dysnoetopora Canu and Bassler, 1926 ( Voigt, 1971).