Idiotrochus alatus, Cairns, 2004

Cairns, S. D., 2004, The Azooxanthellate Scleractinia (Coelenterata: Anthozoa) of Australia, Records of the Australian Museum 56, pp. 259-329 : 296

Idiotrochus alatus n.sp.

Figs. 7I–K, 8A–C

Records/Types. Holotype: Franklin 05/89/40, AM G16699 . Paratypes: Franklin 03/99/D11 (Marion Plateau, Queensland), 3, USNM 1008840 ; Bathus 4–883 (southwest of New Caledonia), 1, USNM 1008841 . Type Locality: 26°45.27'S 159°30.59'E (Gifford Guyot, LHSMC), 315–360 m.

Description. Corallum (anthocyathus) compressed-conical, having rounded thecal faces and edges, the latter diverging at an angle of about 25°, although this measurement is masked by the prominent edge spines. Largest specimen (holotype) 4.21× 3.45 mm in CD and 4.85 mm in height. Calice elliptical, the GCD:LCD of larger specimens 1.2– 1.3, whereas smaller coralla are more circular (GCD:LCD 1.1–1.2). Base of corallum terminates in a crescent-shaped scar, measuring 1.5–2.0× 1.2–1.3 mm, which, in one paratype was overgrown by theca. Costae flat to slightly convex, smooth, often porcellaneous, 0.40–0.50 mm in width, alternating in position with the septa. Intercostal grooves narrow (0.04–0.06 mm) and fairly shallow, one corresponding to the midline of each septum. Prominent thecal edge spines occur on each thecal edge just above the basal scar, projecting perpendicular to the corallum as much as 3.4 mm in length and 1.5 mm in basal diameter. These spines appear to be a composite of 2 spines, a smaller lower spine having a distal diameter of about 0.25 mm and an upper larger spine having a diameter of about 0.5 mm, both having a common base and thus bifurcating distally. Each of these large spines is covered by 6 costae, the 3 on each side of a principal septum: the pair of costae that flank a principal septum cover the upper part of the spine; the costal pair adjacent to that cover the sides of the spine; and the costal pair adjacent to those fuse and cover the lower part of the spine. In the 2 larger coralla examined, just distal (0.5 mm up) to these basal thecal spines is an indication of another, much smaller spine, but in both specimens this spine was broken and occurred on only one side of the corallum. Corallum white. Anthocaulus unknown.

Septa hexamerally arranged in 3 complete cycles (24 septa) according to the formula: S1–2>S3. S1 have vertical, extremely sinuous axial edges that reach about half way to the columella; rounded upper edges that rise about 0.4 mm above the uppermost calicular edge; and outer edges that curve downward before meeting the theca, resulting in a thin, very delicate thecal rim extending only about 0.15 mm above the point at which the septa join the theca. S3 less exsert, about three-quarters width but much thinner than S1–2, also having sinuous axial edges. All septa bear prominent horizontal carinae on their faces, sometimes corresponding to the summits of the septal undulations, but sometimes occurring on opposite sides of a septum and wrapping around the axial edge, thus producing a small platform around the septum which usually overlaps with the platform of adjacent septa at a slightly different level. Paliform lobes of 3 size classes occur before the first 2 cycles of septa, forming an elliptical crown of 12 elements. The 2 smallest paliform lobes occur before the principal S1, and are about as wide as they are thick. The other 4 P1 are about 3 times the width of a principal P1. The 6 P2 are about 1.5 times as wide as the larger P1 and rise slightly higher in the fossa. All paliform lobes highly sinuous and ridged, like the septa, the characteristic horizontal septal platforms also present on the faces of the paliform lobes, often continuous with those of the septa. Fossa absent, the paliform lobes and columella rising to the calicular edge. Columella consists of 4 or 5 linearly arranged, twisted papillae.

Remarks. Three previously described species of Idiotrochus are recognized (Cairns, 1997): I. emarciatus Duncan, 1865 (Oligocene–Recent, Victoria and South Australia), I. australis ( Duncan, 1865) (Middle Miocene, Victoria); and I. kikutii (Yabe & Eguchi, 1941) (western Pacific). The calicular (septa, paliform lobes, columella) and costal characteristics of these three species, as well as I. alatus , are remarkably similar, the species being differentiated essentially on the shape of their coralla. Idiotrochus alatus is most similar to I. australis ( Figs. 8D,E) in corallum shape, both species having prominent thecal spines and a similar corallum size. Idiotrochus alatus differs in a variety of small ways, including having: two pairs of spines that are horizontally oriented (vs one pair oriented obliquely downward in I. australis ), a thin thecal rim, thinner intercostal grooves, and platform-like septal carinae. Furthermore, the costae of I. australis are slightly granular, and large specimens have a vestigial costa associated with each principal S1, which continues down to the basal spine.

Etymology. alatus, Latin for “winged”, an allusion to the prominent thecal edge spines.

Distribution. Marion Plateau, Queensland; Gifford Guyot; southwest of New Caledonia; 315– 450 m.