FIGURE 31

Acropora derawanensis Wallace, 1997: 33, fig. 5

Diagnosis

Colonies arborescent to hispidose with indeterminate growth, composed of very slender and fragile branches. Axial and radials contribute equally to branch structure. Radial corallites appressed tubular, regularly distributed. Coenosteum composed of aligned simple spinules that can become costate throughout (Wallace & Wolstenholme, 1998; Wallace, 1999).

Material studied

Sumatra: RGM 299682, 1 specimen.

Modern comparative material: Holotype, MTQ G48963, Derawan Archipelago, East Kalimantan, Indonesia.

Skeletal characteristics

Corallum. One thin specimen from probably a middle section of a branch as tip is broken, length 17.84 mm, diameter 2.35–2.75– 4.10 mm, oval on transverse section, no colony form can be inferred from the single specimen (Fig. 31 A-C).

Corallites. Axial corallite only visible in the axis, inner diameter 1.25 mm, primary septa present up to threequarters R, secondary septa visible as points; radial corallites evenly sized, regularly distributed, mostly appressed tubular, round calices, not touching, arranged varies from two opposite or alternate corallites on the sides of the branches, profile length 1.31–1.50– 2.09 mm, angle 19.67–26.47–32.82° from the axis, outer diameter 0.84–0.87– 0.91 mm, inner diameter 0.44– 0.55– 0.68 mm, wall thickness 0.09–0.12– 0.19 mm, distance between centres 3.71–4.05– 4.48 mm, primary septa quite worn, secondary septa visible as points. Corallite arrangement sequence 1–?–2–[2–3]–?.

Coenosteum. Poorly preserved spinules arranged into costae throughout.

Occurrence

?Gelasian to Recent. The fossil specimen interpreted as A. aff. derawanensis is the earliest occurrence of the species and extends its distribution to the eastern Indian Ocean during the Early Pleistocene, Calabrian to Gelasian, 0.781 –2.588 Ma. Modern specimens of A. derawanensis are known from East Kalimantan, Sulawesi, Halmahera, Philippines and Micronesia (Table 4, Wallace et al., 2012).

Palaeoenvironment

The coral fauna from Nias beds comprises deepwater sediments overlaid by shallow-water reef terraces in which massive species are very abundant (Boekschoten et al., 1989). The current habitats of A. derawanensis are protected deep sandy slopes, in which flat-lying or upright colonies can form patches (Wallace, 1999).

Remarks

The thin, delicate, slender branch with typical appressed tubular corallites and costate coenosteum throughout compare well with the morphology of A. derawanensis exhibited by the holotype MTQ G48963, Derawan Archipelago. This fossil specimen was previously left in open nomenclature in the collections of the Naturalis museum identified by Oosterbaan (1985). Only one specimen with these characteristics has been recovered, so its identification remains to be confirmed when more fossil material becomes available.