Triphyllozoon cornutum Silén, 1954

(Figs 36, 37; Table 33)

Triphyllozoon cornutum Silén, 1954: 28, fig. 12, pl. 2, fig. 9.

Material examined. Holotype by monotypy LUZM 54, off Castle Rock, near Cape Naturaliste, Western Australia. Leg. Prof. T. Gislén, Australia Expedition 1951–1952, collected 12.12.1951.

Description. Colony erect, rigid, reticulate; holotype specimen 2 cm high and 2.8 × 1.5 cm in diameter (Fig. 36A hand specimen); fenestrae oval, 0.5–0.7 × 0.25–0.45 mm, and trabeculae consisting of 2–7 alternating autozooidal series (Fig. 37A, I).

Autozooids flask-shaped, longer than wide (mean L/ W 1.80), boundaries indistinct (Fig. 37B, C); frontal shield tubercular, flat, imperforate except for a few sparse, small, round to elliptical pores along the lateral and proximal margins, 12–25 µm in diameter (Fig. 37C–E).

Peristome relatively well developed proximally, the two lobes not fusing, forming a deep (40–50 µm), teardropor funnel-shaped median pseudosinus (Fig. 37E, H); frequently a small, elliptical avicularium enclosed on the rim of one lobe; two oral spines placed laterally at level with the distal margin of the primary orifice, visible in some zooids (Fig. 37H), about 15 µm in diameter at the base; secondary orifice elliptical.

Avicularia adventitious, numerous. On the frontal side of the colony four types distinguishable by position, shape and size: type (1) small peristomial avicularium placed on the proximal rim on either side, frequent, almost on every zooid, elliptical to subcircular with finely denticulate rostrum directed distolaterally (Fig. 37C, D, H); type (2) medium-sized, figure-eight-shaped frontal avicularia, frequent, present in the majority of zooids, one or two per zooid, one placed horizontally at zooidal mid-length and directed laterally, the other centred proximally and proximally directed (Fig. 37C); type (3) large lozenge-shaped frontal avicularia placed obliquely and occupying almost the whole frontal shield of the zooid, rare (five observed in a portion of colony consisting of 100 zooids), with raised rostrum either acutely triangular (Fig. 37D, E) or slightly spatulate and spoon-shaped (Fig. 37F, G) directed distolaterally or proximolaterally; type (4) small ooecium-associated avicularia, rare (15 observed in a portion of colony having about 80 ooecia), oval, placed on a raised cystid and directed randomly (Fig. 37B, E). On the dorsal side of the colony two varieties distinguishable by shape and size: type (5) small to medium-sized figure-eight-shaped avicularia similar to those observed on the frontal shield, frequent, usually placed around the fenestrae (Fig. 37I), sometimes forming clusters (Fig. 37J), randomly directed; type (6) large lozenge-shaped avicularia with lanceolate rostrum (Fig. 37K), rare. All avicularia with complete crossbar.

Ovicell hyperstomial, prominent; ooecium slightly longer than wide (mean L/ W 1.16), frontal smooth with trifoliate dentate suture, the median suture longer (125–185 µm) than the lateral sutures (left suture 35–95 µm; right suture 60–100 µm) but similar in width (25–45 µm), mostly diverging at 160–180° (Fig. 37A–C); sometimes avicularia lodged in the two distolateral sectors of the ooecium outlined by the suture placed on pillar-like cystids (Fig. 37E). Labellum short and square.

Dorsal side coarsely granular (granules diameter 20–25 µm), with vertical and oblique vibices (25 µm wide), outlining sub-rectangular sectors, and numerous avicularia of two types as described above, most commonly surrounding the fenestrae and often in clusters (Fig. 37I–K).

Remarks. Out of 35 species of Triphyllozoon known to date, 15 were described from Australia (Bock 2023). In the thorough revision of the type material of Australian phidoloporids undertaken by Hayward (1999, 2000, 2004), the four species of Triphyllozoon described by Silén (1943, 1954) were not included. Specifically, Hayward (2004, p. 325) pointed out that T. cornutum, T. mauritzoni and T. microstigmatum were either not collected since their introduction or not recognized from Silén’s descriptions and figures, hence the need to re-examine the type material.

The “sometimes large, bicuspid, horizontal avicularium with acute mandible on the basal surface above or below a fenestra” described by Silén (1954, p. 28) was not observed, while the frontal avicularia described as interzooidal seem to be instead adventitious (Fig. 37D–G).