Acropora grandis (Brook, 1892)

Santodomingo, Nadiezhda, Wallace, Carden C. & Johnson, Kenneth G., 2015, Fossils reveal a high diversity of the staghorn coral genera Acropora and Isopora (Scleractinia: Acroporidae) in the Neogene of Indonesia, Zoological Journal of the Linnean Society 175 (4), pp. 677-763 : 705-706

ACROPORA GRANDIS ( BROOK, 1892)

FIGURE 12

Madrepora grandis Brook, 1892: 457 ; 1893: 42, pl.1 figs A, B

Acropora dispar Nemenzo, 1967: 55 , pl.19 figs 3, 4

Diagnosis

Arborescent with sturdy branches. Radial corallites irregularly distributed, various shapes, pointing out in several directions. Coenosteum reticulate, simple or laterally flattened spinules ( Wallace & Wolstenholme, 1998; Wallace, 1999).

Material studied

East Kalimantan: RGM 41756, Pulau Tarakan, Mandul Formation, 2 specimens .

Modern comparative material: Lectotype, NHMUK 1892.6 .8.60, Palm Island , Great Barrier Reef, Australia.

Skeletal characteristics

Corallum . Two branches, terete; larger branch: length 122.63 mm, middle branch diameter 13.76 mm; shorter branch: length 64.48 mm, middle branch diameter 10.58 mm. In the larger branch, scars of broken secondary branches at angles 66.76–68.52–70.29°, distance between branches at least 87.22 mm.

Corallites. Axial corallite visible in transverse section and one incipient branch, 2.02 mm exsert, outer diameter 1.61–1.89– 2.17 mm, inner diameter 1.1 mm, primary and secondary septa poorly developed, up to one-quarter R; radial corallites in various shapes, predominantly in two sizes, larger radials mostly tubular to nariform, round to oval calices, profile length 1.11– 1.59– 2.19 mm, angles in all directions 41.33–73.47– 124.55°, outer diameter 1.48–1.63– 1.83 mm, inner diameter 0.73–0.89– 1.24 mm, wall thickness 0.22– 0.28– 0.33 mm, smaller radials interspersed, tubular to subimmersed, profile length 0.62–0.73– 0.96 mm, outer diameter 0.88–1.05– 1.21 mm, inner diameter 0.55– 0.80– 0.85 mm, wall thickness 0.20–0.28– 0.45 mm, mostly not touching, closely arranged, distance between centres 2.20–2.82– 3.49 mm, primary septa up to one-quarter R, secondary septa visible as points. Corallite arrangement sequence 1–4–[6–8]–[14–16].

Coenosteum. Costate on radial corallite walls, reticulate with simple or laterally flattened spinules between radial corallites. Coenosteum amount 0.60– 1.19– 1.99 mm.

Occurrence

Pliocene to Recent. The two specimens from Pulau Tarakan, East Kalimantan, are the earliest occurrence of A. grandis reported from the Pliocene. On modern reefs, A. grandis occurs mostly in sheltered habitats such as enclosed lagoons ( Wallace, 1999) and its distribution extends throughout the entire Indo- Pacific ( Wallace et al., 2012). In Indonesian reefs, A. grandis has a wide distribution and it has been recorded from adjacent waters to East Kalimantan from Sabah ( Malaysia) and Philippines ( Table 4).

Palaeoenvironment

The Mandul Formation is characterized by soft sandy marls and clays with impure limestones, containing corals, larger benthic foraminifera and a rich mollusc fauna ( Beets, 1984). Other coral species of this locality include Antillia constricta , Pavona decussata , Porites nigrescens and Goniopora planulata ( Gerth, 1923) . Similar mixed mollusc–coral faunas of Miocene age in East Kalimantan have been interpreted as coral carpets that developed in sheltered habitats ( Kusworo et al., 2015). This suggests that A. grandis occupied a similar habitat in the Miocene as it does on modern reefs. Four barnacle incrustations with diameters ranging from 9 to 16 mm were observed at the base of the larger specimen ( Fig. 12A ).

Remarks

The sturdy branches suggest that the fossil specimens belong to a tall arborescent colony. The high di- versity of radial corallites pointing out in all directions gives the appearance of a chaotic surface that is characteristic of A. grandis as observed in the lectotype NHMUK 1892.6.8.60. The specimens deposited at Naturalis museum were previously identified by Gerth (1923: 47) as Acropora pharaonis ( Milne Edwards, 1860) . However, A. pharaonis is characterized by abundant incipient axials and short branchlets, and considered to be restricted to the Red Sea ( Wallace, 1999).

Figures

Figure 12. Acropora grandis. RGM 41756, Tarakan Island, East Kalimantan, Pliocene (2.6–5.3 Ma). A, the two branches, showing barnacle incrustations at the base of the larger one (RGM 41756a). B, RGM 41756b, detail of radial corallites of diverse shapes with calices pointing in different directions. C–E, RGM 41756a: C, detail of a short branchlet showing axial corallite. D, detail of nariform radial corallite with costate wall and tubular to subimmersed radials. E, detail of simple to lateral flattened spinules on intercorallite coenosteum.