Acropora samoensis

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 : 697-700

ACROPORA SAMOENSIS ( BROOK, 1891)

FIGURE 8

Madrepora samoensis Brook, 1891: 468 ; 1893: 143, pl. 31 fig. A, pl. 6 fig. C.

Acropora wallaceae Veron, 1990: 99 , figs 4–6.

Diagnosis

Colonies with determinate growth, corymbose to caespito-corymbose, branches terete or slightly tapering, up to 15 mm in diameter, short, evenly sized and regularly spaced. Branch structure axial-dominated. Short tubular radial corallites, mostly not touching. Coenosteum composed of short laterally flattened spinules, costate on corallite walls and reticulate between corallites ( Wallace & Wolstenholme, 1998; Wallace, 1999).

Material studied

East Kalimantan: NHMUK PI AZ7103, TF505, 3 fragments of a colony.

Modern comparative material: Lectotype, NHMUK 1875.10 .2.8, Samoa; MTQ G49128, Kakaban Island , East Kalimantan, Indonesia.

Skeletal characteristics

Corallum . Corymbose or caespito-corymbose colony as observed in field pictures. Recovered specimens are three broken branches, terete or moderate tapering, with signs of primary branching, round in cross section, overall length of fragments 24.0– 25.1–26.4 mm, basal branch diameter 11.5–12.67– 14 mm, mid branch diameter 10.5–11.5– 12 mm, branch tip diameter 4.01–4.22– 4.43 mm; growth determinate branching in angle 49.26– 55.10–64.87°, distance to the nearest branches 12– 16.45– 25 mm.

Corallites. Axial corallite dominates branch structure, visible on a secondary branch at one of the specimens and on transverse section of broken branches, exsert 0.49 mm, outer diameter 1.87–1.96– 2.01 mm, inner diameter 0.9–0.96–1.0 mm, wall thickness 0.69– 0.82–1.0 mm, primary septa up to three-quarters R, secondary septa up to one-quarter R; radial corallites evenly sized and distributed, short tubular with round calices, profile length 1.1–1.42– 1.8 mm, angle 35.45– 46.33–55.10°, outer diameter 1.0– 1.1–1.2 mm, inner diameter 0.7–0.88–1.0 mm, wall thickness 0.3–0.36– 0.5 mm, closer towards the tips, distance between centres 1.4–1.98– 2.5 mm, slightly farther at the basal part of branches 1.9–2.3– 2.6 mm, primary septa present up to three-quarters R, secondary septa up to onequarter R or visible as points. Corallite arrangement sequence 1–6–8–[12–14]–?.

Coenosteum. Slightly flattened simple spinules, arranged into irregular costae on radial corallite walls, reticulate with less densely arranged spinules in intercorallite areas. Coenosteum amount 0.28–0.83– 1.95 mm.

Occurrence

Late Miocene to Recent. The fossil specimens found at the locality TF505, Bontang, are the earliest occurrence of the species as they are assigned an age of 9.4–9.8 Ma (early Tortonian). The previously known fossil record includes the specimen MTQ G55171-2 from Niue (Pacific Ocean) of Pleistocene age ( Wallace, 1999). In the Recent, A. samoensis is widely distributed across the Indo-Pacific and the Red Sea along a broader depth range than other species of the humilis group ( Wallace et al., 2012). This species is encountered on most of the modern reefs throughout Indonesia as shown in Table 4 ( Wallace & Wolstenholme, 1998).

Palaeoenvironment

Locality TF505 has been interpreted as a shallow environment with calm waters and high influence of siliciclastic sediments probably associated with a protected lagoon or sheltered environment. Acropora samoensis co-occurred with Seriatopora spp. and robust colonies of branching Porites into a compact grey silt-rich sediment layer. Stratigraphic units interbedding contain typical seagrass fauna rich in wellpreserved molluscs and specimens of the aspera species group.

Remarks

The three specimens from the same colony have wellpreserved morphological features such as short tubular radial corallites as well as evidence of caespitocorymbose branching pattern. They compare well with the lectotype NHMUK 1875.10.2.8 and the specimen MTQ G49128 from East Kalimantan. The branches are the thinnest among all of the fossil specimens identified within the humilis species group. These traits could be measured and compared with modern specimens, confirming their identification as A. samoensis . This species can be distinguished from A. monticulosa and A. digitifera based on its relatively smaller dimensions of branches and corallites, and its characteristic short tubular radial corallites as compared with the more exsert tubular-nariform corallites characteristic of A. monticulosa . Two of the specimens have evidence of boring, with smooth oval cavities interpreted as Lithophaga sp. boreholes that occupy a large proportion of the branch. The borings are 7.1 mm long × 4 mm wide × 9.1 mm deep and 7.8 mm long × 6.4 mm wide × 7.1 mm deep ( Fig. 8D ). This suggests that skeletons of Acropora served as a substrate for this type of bivalve–coral ecological association since at least the Miocene in the Coral Triangle.

Figures

Figure 8. Acropora samoensis, NHMUK PI AZ 7103, TF505, Bontang gardens sections, East Kalimantan, Late Miocene (9.4–9.8 Ma). A, aspect of a colony in the field found within grey silt-rich sediments, showing a probably caespitocorymbose growth form. B, AZ 7103a, upper view of a branch fragment. C, three branches showing short tubular radial corallites, anticlockwise: AZ 7103a,b,c. D, AZ 7103b (up) and AZ 7103c (down), boring holes interpreted as being made by Lithophaga sp. E, AZ 7103b, electron micrograph showing the upper view of an axial corallite.