Acropora russelli, WALLACE, 1994

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 : 749-750

ACROPORA RUSSELLI WALLACE, 1994

FIGURE 37

Acropora russelli Wallace, 1994: 983 , fig. 26; 1999: 316– 317, pl. 89 fig. A–G.

Diagnosis

Colonies with determinate growth, composed of delicate, slender, slightly flattened branches. Tubular to conical corallites, not touching, sparsely arranged on the lateral sides and central part of branches. Dense coenosteum ( Wallace & Wolstenholme, 1998; Wallace, 1999).

Material studied

NHMUK PI AZ 8834, one colony embedded into indurated matrix plus a few loose fragments.

Modern comparative material: Holotype, MTQ G40795, Cartier Reef , Timor Sea; MTQ G51487-51488, Halmahera Sea , Indonesia.

Skeletal characteristics

Corallum . Colony small arborescent, length 135 mm, width 110 mm, height 65 mm, preserved into an indurated silty clay matrix ( Fig. 37A ); slender and slightly flattened branches, branch length 23.89–33.25– 40.64 mm, basal branch diameter 7.45–7.78–8.11, mid branch diameter 3.54–4.34– 5.14 mm, branch tip diameter 2.84–2.91– 2.98 mm. Sparse branches separat- ed at least 9.5 mm, exert in angles 53.5–61.05– 78.56°; growth determinate; terete.

Corallites. A few axial corallites exposed protruding from indurated matrix ( Fig. 37 C-D), tubular, 2.3 mm exsert, outer diameter 1.58–1.67– 1.84 mm, inner diameter 1.03–1.06– 1.11 mm, costate wall, wall thickness 0.34–0.39– 0.45 mm, primary septa up to onequarter R, secondary septa observed as points and arranged as S1>>S2; radial corallites, not touching, sparsely alternate around the axis, tubular, appressed tubular or tubular conical, round to oval calices, profile length 1.57–2.18–2.72, exert in angles 45.89–54.42– 62.31°, outer diameter 0.60–0.73– 0.92 mm, inner diameter 0.39–0.44– 0.48 mm, distance between centres 4.17–4.39– 4.63 mm, septa S1>>S2. Corallite arrangement sequence 1–2–2–2– up to 4.

Coenosteum. Flattened spinules with elaborated tips aligned as costae throughout. Coenosteum amount 2.25– 2.87– 3.34 mm.

Occurrence

Late Miocene to Recent. This fossil colony represents the earliest occurrence of the species in outcrop TF500, Bontang, of Tortonian age, 9.4–9.8 Ma. On modern reefs, A. russelli has a scattered distribution with records from Mauritius, Sri Lanka, Halmahera , West Papua and north-west Australia ( Wallace et al., 2012).

Palaeoenvironment

A single colony of A. russelli was found partially recrystallized and embedded in a silty clay-rich sediment matrix. The specimen was collected in a bed dominated by thin platy corals of the genus Pachyseris , some Halimeda and fenestrate bryozoans. The outcrop TF500 is at the base of the Bontang Gardens outcrops, according to the stratigraphic correlation of Renema et al. (2015). On modern reefs, this species occurs on sandy slopes at depths of 15–25 m ( Wallace & Wolstenholme, 1998). In Indonesia, A. russelli has been found in Halmahera and Irian Jaya ( Table 4).

Remarks

Comparisons with modern specimens from Halmahera , Indonesia, such as MTQ G51487 and G51493, allowed the interpretation of the fossil material as A. russelli , based on the general organization of the colony, and morphology and dimensions of radial corallites. The fossil colony of A. russelli has been partially recrystallized resulting in a denser skeleton and partial loss of coenosteum ornamentation. The typical freeliving nature of modern specimens is difficult to determine in the fossil specimen, although the branches form a matted colony similar to modern conspecifics.

Figures

Figure 37. Acropora russelli. NHMUK PI AZ 8834, TF500, Bontang, East Kalimantan, Late Miocene (9.4–9.8 Ma). A, arborescent colony embedded into a fine sediment matrix. B, detail of a branch showing radial corallite arrangement. C and D, detail of axial corallites showing costate coenosteum. E, detail of tubular radial corallite.