Operculina? ammonoides (Schröter 1783) (Fig. 25:18–20)

1781 Nautilus ammonoides Gronovius, p. 282, pl. 19, figs 5, 6, nomen nudum.

1783 Nautilus ammonoides Schröter, p. 21.

?1884 Operculina ammonoides (Gronovius); Brady, pl. 112, figs 3–8.

1993 Assilina ammonoides (Gronovius); Hottinger et al., p. 154, pl. 222, figs 1–8; pl. 223, figs 1–14, pl. 224, figs 1–8; pl. 225, figs 1–9.

1994 Assilina ammonoides (Gronovius); Loeblich & Tappan, p. 170, pl. 387, figs 7–9; pl. 388, figs 1–4.

1999 Operculina ammonoides (Gronovius); Hohenegger et al., p. 153, fig. 28.

2002 Operculina ammonoides (Gronovius); Yordanova & Hohenegger, p. 199, pl. 33, figs 11–14.

2009 Assilina ammonoides (Schröter); Parker, p. 515, fig. 367a–j.

2012 Operculina ammonoides (Gronovius); Debenay, p. 228, pl. 20.

Description. See Hohenegger et al. (1999, p. 153, fig. 28); Hottinger et al. (1993, p. 154, pl. 222, figs 1–8; pl. 223, figs 1–14, pl. 224, figs 1–8; pl. 225, figs 1–9) and Parker (2009, p. 515, fig. 367a–j).

Remarks. Operculina? ammonoides (Schröter 1783) is characterised by a lamellar, planispiral, flat, discoidal test with raised sutures and thin peripheral carina. The ornament consists of small pustules that often cluster around the initial chambers and around the coil of the test along the sutures with more scattered occurrences across the later chambers. The test is finely perforated, but the sutures, initial chambers and apertural face are imperforate. The aperture is absent except for a tubular space extending from a single opening of the spiral canal over the marginal carina to the opposite opening of the spiral canal (Fig. 25:18–20).

Ellis & Messina (1940) in a 1998 supplement, stated that the taxonomic name established by Gronovius (1781) was rejected for nomenclature purposes by the ICZN (2000) and that Schröter (1783) therefore became the first published record of this species.

Substanial confusion surrounds the genus Operculina and this species in particular. Operculina? ammonoides has frequently been assigned to Assilina d’Orbigny 1826 (Hottinger et al. 1993; Loeblich & Tappan 1994; Parker 2009), but the genus Assilina is generally considered to have became extinct during the middle Eocene (Hallock et al. 1991; BouDagher-Fadel 2008). But detailed investigations of Operculina? ammonoides by Hottinger et al. (1993) established that this species does not have a folded septal flap nor trabeculae and is therefore more suited elsewhere. Further work is required to effectively establish the correct placement of this species.

Operculina? ammonoides displays intraspecific variation in the degree of test inflation, some tests are more lenticular than completely flat in profile and also in the extent that the pustulose ornament covers the test. This variation is observable in the specimens collected by Brady (1884) from Coral Reef, Australia (British Natural History Museum reg. no. 1955: 10:2, 813–841) and the CG specimens. Coral Reef specimens show a range in test thickness and degree of chamber inflation to the extent that specimens loose their flat shape to become distinctly lenticular in profile. The CG specimens are almost completely flat in profile and the pustulose ornament does not usually extend beyond the initial coiled chambers (Fig. 25:18–20).

Brady (1884) recorded this species from Amboyna and the Admiralty Islands in the Pacific from depths ranging from 27–46 m and Coral Reef, Australia. This species has a global distribution (Gulf of Aqaba, Red Sea from photic zone to 60 m—Hottinger et al. 1993; Timor Sea at 87–146 m—Loeblich & Tappan 1994; Sulawesi, Indonesia—Renema & Troelstra 2001; Sesoko Island, Japan at 20–50 m—Hohenegger et al. 1999, Yordanova & Hohenegger 2002; Exmouth Gulf—Haig 1997; Ningaloo Reef, Western Australia—Parker 2009; New Caledonia—Debenay 2012).

Distribution within study area. Forty of the 46 specimens of O.? ammonoides collected from the CG came from the relatively deep channel sample between Wistari and Heron Reefs. No more than two specimens per sample were collected from the rest of the sampled reefs. This distribution closely matches other published examples (Brady 1884; Hohenegger et al. 1999; Yordanova & Hohenegger 2002) with greatest abundance within the photic zone below 15 m.