Acropora kirstyae, VERON & WALLACE, 1984

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 : 734-736

ACROPORA KIRSTYAE VERON & WALLACE, 1984

FIGURE 29

Acropora kirstyae Veron & Wallace, 1984: 247 , figs 593, 596–597; Wallace & Wolstenholme, 1998: 254, fig. 50; Wallace, 1999: 292, pl. 79, map 79; Wallace et al., 2012: 104, fig. 50.

Diagnosis

Colonies arborescent, probably with indeterminate growth, composed of thin, straight, tapering branches. Radial corallites tubular to appressed tubular, round calices, evenly sized and distributed. Coenosteum composed of short elaborated spinules throughout ( Wallace & Wolstenholme, 1998; Wallace, 1999).

Material studied

East Kalimantan : NHMUK PI AZ 8845 , 20 specimens ; NHM UK PI AZ 8861 , 12 specimens .

Modern comparative material: Holotype, MTQ G55077, Palm Island , Great Barrier Reef , 6 m depth; MTQ G51298, Great Barrier Reef ; MTQ G48957, Pulau Talatakoh, Togian Islands, Central Sulawesi, Indonesia.

Skeletal characterstics

Corallum . Broken branches that may have formed a small arborescent colony; specimens representative of base, main axis and tips; axis thick from which primary branches arise in open angles 56.46–61.84–70.29° ( Fig. 29A ); a few secondary and incipient branches present; all branches tapering, slender, oval to round in transverse section ( Fig. 29D ), branch length 37.53– 48.09– 64.82 mm, basal branch diameter 8.92–10.82– 12.95 mm, mid branch diameter 4.16–5.46– 6.87 mm, branch tip diameter 2.56–3.61– 5.43 mm; distance to the nearest branches 10.87–16.30– 24.13 mm; growth indeterminate.

Corallites. Axial corallite visible in transverse section and at one branch tip, 0.95 mm exsert, outer diameter 2.12–2.19– 2.32 mm, inner diameter 0.93–1.02– 1.13 mm, wall thickness 0.45–0.46– 0.46 mm, primary septa two-thirds R, secondary septa up to onequarter R or visible as points, S1>S2; radial corallites evenly sized and distributed, long tubular appressed with round calices, profile length 1.92–2.32– 2.71 mm, acute angles 17.26–19.78–22.49°, outer diameter 0.83– 1.05– 1.38 mm, inner diameter 0.40–0.54– 0.69 mm, wall thickness 0.17–0.20– 0.22 mm, distance between centers 3.88–4.75– 7.10 mm, primary septa up to one-third R, secondary septa absent or reduced to spines in some corallites. Corallite arrangement sequence 1–2–[2–3]– [3–4]-up to 6.

Coenosteum. Simple spinules with probably slightly elaborated tips arranged into dense regular pattern throughout. 2.03–2.21– 2.51 mm.

Occurrence

Early Miocene to Recent. The earliest occurrence of the species is from the outcrop TF56, Badak, East Kalimantan, of Langhian to Burdigalian age, 14.8– 17.9 Ma. This fossil material is the westernmost record of the species, previously known only from modern reefs of Japan ( Nishihira & Veron, 1995) and Sulawesi, Halmahera , Papua, Solomon Islands, Micronesia and New Caledonia ( Wallace et al., 2012).

Palaeoenvironment

Fossil specimens of A. kirstyae were recovered from the top bed of the outcrop TF56, Badak, characterized by orange silty sediments, and high abundance and richness of reef corals. This environment has been interpreted as a shallow-water patch reef similar to the Stadion patch reef in Samarinda ( Santodomingo et al., 2015) because it overlies a bed of carbonaceous shales (N.S., field observations). In modern reefs, A. kirstyae occurs only in protected subtidal lagoons ( Wallace, 1999).

Remarks

Even though the fossil specimens have some degree of recrystallization, morphological features such as small arborescent colony, long appressed tubular radials and corallite arrangement compare well with the holotype MTQ G55077 and the modern specimens MTQ G48957 from Sulawesi and MTQ G51298 from the Great Barrier Reef. These fossil specimens also closely resemble the extant species Acropora proximalis Veron, 2000 , described from the Indonesian– Philippines Archipelago, supporting that the latter is a junior synonym of A. kirstyae as suggested by Wallace et al. (2012).

Figures

Figure 29. Acropora kirstyae. NHMUK PI AZ 8845, TF56, Badak, East Kalimantan, Early to Middle Miocene (14.8– 17.9 Ma). A, colony fragments showing evidence of arborescent form. B and C, AZ 8845a: B, electron micrograph composition of a branch tip showing axial corallite and appressed tubular radial corallites; C, electron micrograph of an appressed tubular corallite. D and E, AZ 8845b: D, transverse section of a branch showing axial corallite; E, electron micrograph showing variation of radial corallites. F, AZ 8845a, electron micrograph of reticulate elaborated spinules on coenosteum.