Rhynchozoon lunifrons Dick and Grischenko, 2017

Rhynchozoon lunifrons Dick and Grischenko, 2017

Figure 60 View FIGURE 60

v. 2017 Rhynchozoon lunifrons Dick and Grischenko , p. 245, figs. 37–38.

Figured material. PMC EDM-Collection J.H.B.149a, sample 19033 ( Figure 60A–C View FIGURE 60 ), sample 19015 ( Figure 60D View FIGURE 60 ), sample 19031 ( Figure 60E View FIGURE 60 ), sample 19041 ( Figure 60F View FIGURE 60 ), and sample 19036 ( Figure 60G–I View FIGURE 60 ); Core 19, Daidokutsu cave, Okinawa, Japan, Holocene.

Description. Colony encrusting, subcircular, multiserial, uni- to multilaminar; second layer developing through frontal budding, originating from the centre of the colony. Two types of pseudoancestrular areas observed: first type, consisting of two small zooids budded in opposite directions, with a third, slightly larger zooid budded distally between them, surrounded by seven peripseudoancestrular autozooids ( Figure 60B View FIGURE 60 ); second type, consisting of two small zooids budded in opposite directions, each budding another zooid laterally, creating a rounded cavity in the centre, also surrounded by seven autozooids ( Figure 60F View FIGURE 60 ); pseudoancestrulae 215–275 µm in length by 150–230 µm in width, with an orifice 105 µm long by 115 µm wide; first zooids budded from pseudoancestrulae 250–315 µm long by 190–210 µm wide. Autozooids distinct only at the growing edge or in new, frontally budded laminae, separated by shallow furrows or a thin rim, hexagonal or irregularly polygonal, longer than wide (mean ZL/ZW 1.32), arranged radially or irregularly. Frontal shield convex, smooth to nodular and pitted, imperforate except for a row of 6–9 irregularly spaced, circular to elliptical, sometimes funnel-shaped or sloping outwards, conspicuous marginal pores, 10–70 µm in maximum diameter. Orifice deep and subcircular but slightly wider than long (mean OL/OW 0.85), with a finely denticulate anter (10–15 denticles), a broadly concave proximal margin, and rounded condyles. Four distolateral spine bases occasionally visible in some zooids, most commonly two, 16–25 µm in diameter, quickly becoming embedded in the frontal shield calcification of the distal autozooid. Peristome often obscuring the orifice, consisting of three mucrones: one suboral in the midline or slightly off-centre, developing first, and two paired laterally, developing later. Lateral mucrones often increasing to two on each side, forming a more complex structure of coalescent mucrones, sometimes bifurcating. Suboral mucro either slender and pointed or swollen accommodating an avicularium atop. Suboral avicularium sporadic, with triangular rostrum raised at a high angle to the frontal plane, directed frontolaterally; frontal avicularia rare, with the same shape and inclination as the suboral avicularia, the few examples observed placed proximally on the frontal shield, directed proximolaterally on either side; both types of avicularia with complete crossbar. Ovicell subimmersed, with extensive exposure of the smooth, faintly striated endooecium, the transversely elliptical to eight-shaped fenestra measuring 80–155 µm in length by 156–212 µm in width.

Measurements (µm). ZL 459±53, 390–605 (3, 20); ZW 348±67, 249–481 (3, 20); OL 107±8, 97–123 (4, 9); OW 125±7, 116–137 (4, 9); AvL (suboral) 89±21, 66–121 (4, 9); AvW (suboral) 61±5, 52–68 (4, 9); AvL (frontal) 85±30, 66–120 (2, 3); AvW (frontal) 65±8, 58–74 (2, 3); OvL 169±18, 134–214 (6, 18); OvW 236±17, 204–268 (6, 18).

Remarks. There are two main differences between the type specimens from Sesoko Island (Okinawa, Japan) and our specimens from Daidokutsu cave. First, the rostrum of the avicularia in the type specimens can become slightly hooked distally during late colony astogeny, a feature not observed in the specimens from Daidokutsu cave. This difference may be due to preservational issues or the fact that avicularia in our specimens (but also in general in this species) are rare and possibly from colonies that have not reached the appropriate age for this characteristic to develop. In the type specimens, oral spines are generally absent, except for one or two ephemeral distal oral spines observed in a socalled precocious form as defined by Dick and Grischenko (2017, figure 38b). In contrast, our specimens have up to four distal spines with robust bases. However, these spines quickly become embedded in the calcification spreading from the distal zooid. Despite these differences, other diagnostic characters and size measurements match, supporting the identification of the Daidokutsu colonies as R. lunifrons .