Acropora monticulosa

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

ACROPORA MONTICULOSA ( BRÜGGEMANN, 1879)

FIGURE 7

Madrepora monticulosa Brüggemann, 1879: 576 .

Diagnosis

Colonies with determinate growth, digitate to corymbose shape, thick conical branches. Branch structure axial dominated. Tubular to nariform radial corallites. Coenosteum with slightly flattened spinules, costate on corallite walls and reticulo-costate between corallites ( Wallace & Wolstenholme, 1998; Wallace, 1999).

Material studied

East Kalimantan: NHMUK PI AZ7098 , TF522 , One colony plus three small fragments of the same colony. Modern comparative material: Lectotype, NHMUK 1876.5 .5.93, Rodriguez Island, Indian Ocean ; MTQ G27404, Tijou Reef , Queensland Australia, 1–2 m depth; MTQ G47311, Ruang Island , Sangihe Islands , North Sulawesi, Indonesia, 3 m depth; MTQ G47309, Ruang Island , Sangihe Islands , North Sulawesi, Indonesia.

Skeletal characteristics

Corallum . One large digitate to corymbose colony, length 33 cm, width 31 cm, height 16 cm, broad base approx. 10 cm diameter, highly recrystallized with most of the basal part compacted into the same coenosteum and distal part of branches free; branches strongly tapered, conical shape, round to polygonal in transverse section, length 29.2–37.8– 60.5 mm, basal branch diameter 14.50– 20.73– 25.50 mm, mid branch diameter 15–18.64– 23.5 mm, branch tip diameter 9.5–10.8– 13 mm. Branching from a focused origin in angles 21.72– 37.48–45.65°, distance to the nearest branches 20.2–30.2– 42 mm.

Corallites. Axial corallite dominate branch structure, worn in most cases, 0.89–1.15– 1.55 mm exsert, outer diameter 1.8–2.48– 3.2 mm, inner diameter 0.9– 1.2– 1.4 mm, wall thickness 0.7–0.88– 1 mm, primary septa up to three-quarters R, secondary septa up to one-quarter R; radial corallites evenly sized and distributed, short tubular to nariform with round and slightly dimidiate calices, profile length 1.2–1.62– 2.1 mm, angle 20.95–30.63–40.41°, outer diameter 1.1– 1.42– 1.8 mm, inner diameter 0.8–0.96– 1.2 mm, wall thickness 0.3–0.46– 0.6 mm, closer towards the branch tips, distance between centres 1.6–2.06– 2.8 mm, slightly farther at the basal part of branches 2.2–2.8– 3.5 mm, primary septa up to one-half R, secondary septa visible as points. Corallite arrangement sequence 1-[4– 6]-8-[14–16]-up to 28.

Coenosteum. Slightly flattened simple spinules, arranged into dense irregular costae on radial corallite walls, reticulate with less densely arranged spinules in intercorallite areas. Coenosteum amount 0.58–0.72– 0.92 mm.

Occurrence

Middle Miocene to Recent. This record represents the earliest occurrence of the species with one colony found in the locality TF522, Kari Orang, Bengalon, East Kalimantan of Serravallian age, 11.6–13 Ma. In the Recent, A. monticulosa has been found in various localities of Sulawesi ( Table 4), and it is generally widely distributed in the Indo-Pacific ( Wallace et al., 2012).

Palaeoenvironment

The hauling road locality TF522 has been interpret- ed as a regressive sequence, with A. monticulosa found at the top bed of the section interpreted as deposited in the shallowest habitat recorded. The ochre-coloured sand–silty sediments typical of the bed in which A. monticulosa was found are indicative of relatively high water energy. It is most likely that A. monticulosa occupied similar environments in the Miocene and Recent. In modern reefs, they are typically found in shallow exposed or semi-exposed waters of less than 4 m depth ( Wallace, 1999; Wolstenholme et al., 2003).

Remarks

The robust habit of the species and high recrystallization allowed the removal of the fossil colony of A. monticulosa in one piece. Although internal structures are poorly preserved, external features of some branches and corallites were clearly visible and comparable to the modern specimens of A. monticulosa MTQ G 47309 and G47311 from North Sulawesi and G27404 from Australia. Measurements are within the range of variation of modern specimens ( Wallace, 1999; Wolstenholme et al., 2003), including the lectotype NHMUK 1876.5.5.93, Rodriguez Island. This species can be distinguished from other species within the humilis group based on the typical conical shape of the branches.

Figures

Figure 7. Acropora monticulosa, NHMUK PI AZ 7098, TF522, Kari Orang, East Kalimantan, Middle Miocene (11.3–13 Ma). A, upper view of the digitate to corymbose colony. B, lateral view, showing the broad base of the colony. C, AZ7098a, apical fragment of a branch, showing a relatively conical shape and the radial corallites arrangement. D, AZ7098b, transverse break through a polygonal branch with axial corallite. E–H, AZ7098c: E, basal part of a branch showing tubular to nariform radial corallites, evenly sized and distributed; F, lateral view of a radial corallite showing spinules aligned in costae; G, detail showing thickened outer corallite wall; H, electron micrograph detail of coenosteum in between corallites formed of flattened spinules in reticulate arrangement.