Acropora halmaherae, WALLACE & WOLSTENHOLME, 1998

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 : 733

ACROPORA HALMAHERAE WALLACE & WOLSTENHOLME, 1998

FIGURE 28

Acropora halmaherae Wallace & Wolstenholme, 1998: 258 , fig. 54

Diagnosis

Colonies arborescent to hispidose with indeterminate growth, composed of slender branches. Radial and axial corallites contribute equally to branch structure. Radial corallites tubular exsert, regularly distributed. Coenosteum reticulate, elaborated spinules ( Wallace & Wolstenholme, 1998; Wallace, 1999).

Material studied

Sumatra: RGM 125830 , 4 specimens ; RGM 791828 , 1 specimen ; RGM 125828 , 4 specimens ; RGM 791813 , 1 specimen .

Modern comparative material: Holotype, MTQ G51513, Halmahera , Indonesia.

Skeletal characteristics

Corallum . Specimens show evidence of arborescent to hispidose colony form, branch length 13.48–19.01– 36.15 mm, branch angle 54.21–59.01–64.97°, mid branch diameter 3.25–5.00– 6.24 mm, round on transverse section, minimum distance between branches 7.80– 8.84– 9.88 mm; growth probably indeterminate.

Corallites. Axial corallite visible in incipient branches and transverse sections, outer diameter 1.55–1.90– 2.26 mm, inner diameter 0.80–0.99– 1.42 mm, wall thickness 0.31–0.39– 0.46 mm, primary septa up to two-thirds R, secondary septa up to one-quarter R; radial corallites evenly sized, irregularly dispersed on the branches, not touching, mostly tubular exsert, round to nariform calices, projected almost perpendicular from the branch in angles of 37.09–53.68–71.62°, profile length 0.59–1.57– 2.39 mm, angle, outer diameter 0.93– 1.25– 1.52 mm, inner diameter 0.45–0.60– 0.77 mm, wall thickness 0.19–0.25– 0.35 mm, distance between centres 2.04–3.47– 4.97 mm, primary septa up to two-thirds R, straight and smooth edges, secondary septa absent or visible as points. Corallite arrangement sequence 1–2– 4–4 up to 6.

Coenosteum. Reticulate with evenly arranged elaborated spinules rarely arranged into costae ( Fig. 28C ). Coenosteum amount 0.95–1.63– 2.21 mm.

Occurrence

Late Miocene (?) to Recent. The earliest occurrence is one poorly preserved specimen from Ci Bining in Java of Tortonian age, 8.5–11.6 Ma, and although it is similar to modern specimens of A. halmaherae , the identification remains uncertain. The other fossil material comes from Gawo and Mola Valley, Nias, Sumatra, of Calabrian to Gelasian age, 0.781 –2.588 Ma. These occurrences extend the distribution of the species to the eastern Indian Ocean in the Pleistocene. On modern reefs, A. halmaherae has a narrow distribution as it is only known from its type locality, Halmahera Island, West Papua, and some sparse records in Papua New Guinea, Micronesia and Marshall Islands ( Table 4, Wallace et al., 2012).

Palaeoenvironment

Fossil corals in Ci Bining from the Ci Lanang beds include a rich fauna of several colony forms and genera such as Galaxea , Stylophora , Hydnophora , Fungia and Pocillopora that were preserved into clay-rich sediments and interpreted as rolled down from upper parts of the reef ( Gerth, 1921). Specimens from Gawo and Mola Valley were recovered with contrasting faunas of azooxanthellate and shallow-water corals ( Gerth, 1925). Acropora halmaherae inhabits protected sandy slopes on modern reefs ( Wallace & Wolstenholme, 1998; Wallace, 1999).

Remarks

The specimen RGM 791828, previously identified as A. duncani ( Gerth, 1925) , was re-interpreted in this study. In addition, the specimens left in open nomenclature including RGM 791813 ( Gerth, 1921), RGM 125828 and RGM 125830 ( Gerth, 1925) were allocat- ed to this species. With their apparently hispidose colony form, slender branches and the typical tubular exsert corallites almost perpendicular from the main axis, these fossil specimens were assigned to A. halmaherae , and they compare well to the holotype MTQ G51513, Halmahera , Indonesia. Its protruding tubular corallites resemble those of A. abrohlosensis , but the latter species has thicker branches and more openly reticulate coenosteum. The fossil specimens of A. halmaherae from Nias represent the westernmost report of the species and extend its distribution to the Indian Ocean in the Pleistocene.

Figures

Figure 28. Acropora halmaherae, RGM125830, Gawo Valley, Nias, Sumatra, Pleistocene (0.8–2.6 Ma). A and B, RGM 125830a: A, whole specimen showing degraded tubular radial corallites; B, upper view showing the axial corallite. C and D, RGM125830b: C, whole specimen showing broken branches, probably hispidose; D, detail showing the axial corallite. E and F, RGM125830c: E, whole specimen showing tubular exsert and nariform radial corallites; F, detail of radial corallite and reticulate coenosteum.