Acropora hasibuani, Santodomingo & Wallace & Johnson, 2015

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 : 751

ACROPORA HASIBUANI SP. NOV.

FIGURE 38

Diagnosis

Colony arborescent to open caespitose, with long axial corallites, radial corallites tubular or tubular appressed, evenly sized, evenly distributed, coenosteum formed of dense arrangement of elaborated spinules with tendency to be aligned on corallites ( Wallace, 1999).

Holotype

NHMUK PI AZ 7107 , 1120 fragments of the same colony, including 152 branch tips. Type locality: TF517 , Sangatta , East Kalimantan, 0°34′4.1″N, 117°38′2.4″E. Tortonian age, 9.3–8.9 Ma. Collector N. Santodomingo, 2 July 2011.

Description

Corallum . One colony probably arborescent or open caespitose extending over a surface 45 cm long, 24.5 mm wide, and 25.5 mm high, formed of slender cylindrical branches, extending mainly from the basal part of the colony and some sparse from distal parts, tapering into long axial corallites; overall length of branches from reconstructed parts of the colony and field pictures ( Fig. 38A, B ) 68.35–97.08– 142.87 mm, length of specimens recovered 11.08–26.25– 41.59 mm, angle 44.19–52.36–72.55°, basal branch diameter 8.59–11.32– 13.82 mm, mid branch diameter 5.44–7.13– 8.19 mm, branch tip diameter below the axial corallite 2.41– 3.78– 4.85 mm, distance between branches 17.08–35.92– 45.90 mm.

Corallites. Axial corallites long tubular, 2.75–5.77– 7.95 mm exsert, round to oval calice, outer diameter 2.16–2.81– 3.30 mm, inner diameter 1.23–1.50– 1.75 mm, wall thickness 0.39–0.48– 0.55 mm, primary septa threequarters R, secondary septa up to two-thirds R or present as spines, arranged as S1>S2 ( Fig. 38D, E ); radial corallites evenly sized, evenly distributed, relatively arranged in longitudinal rows opposed or alternating to accommodate as many as 4 to 5 corallites around the perimeter of the branch, not touching, sparsely distributed, appressed tubular to nariform with round calices, profile length 1.96–2.89– 3.78 mm, angle 26.30–32.98–45.24°, outer diameter 1.04–1.61– 2.44 mm, inner diameter 0.58–0.75– 0.96 mm, wall thickness 0.31– 0.36– 0.43 mm, distance between centres 3.14–4.49– 5.36 mm, primary septa up to three-quarters R, secondary septa up to two-thirds R; smaller corallites around axial corallites, around half the size, profile length 0.67–1.36– 1.78 mm, outer diameter 0.79–0.92– 1.09 mm, inner diameter 0.31–0.45– 0.56 mm, calice round to oval. Corallite arrangement sequence 1–[3– 4]–[3–4]–up to 5.

Coenosteum. Elaborated spinules evenly and densely distributed between radial corallites with tendency to be aligned towards the axial and radial corallite walls, coenosteum amount 1.45–3.20– 4.79 mm.

Occurrence

Late Miocene. This species is only known from its type locality TF517 , Bontang, of Tortonian age, 8.9–9.3 Ma .

Palaeoenvironment

Acropora hasibuani sp. nov. was found within silty sandrich sediments together with large colonies of merulinids of the genera Dipsastraea , Favites and Platygyra , and adjacent to organic-rich sediments with probable fossil seagrass-leaves ( Reich et al., 2015). This environment has been interpreted as shallow, calm water, probably a backreef lagoon.

Remarks

Corallum and corallite dimensions of A. hasibuani sp. nov. are similar to Acropora walindii Wallace, 1999 , in the elegans species group, but it does not have the distinctive flattened branches. Acropora hasibuani sp. nov. also resembles Acropora caroliniana Nemenzo, 1976 as both have similar appressed tubular radial corallites and elaborated spinules on the coenosteum, but A. caroliniana develops abundant secondary branches and coenosteal spinules do not align into costae on corallite walls. Acropora hasibuani sp. nov. is placed in the elegans group based on its similarities to A. walindii . Fossil specimens are well preserved and most morphological features could be described. However, the skeleton is very porous and readily breaks and softens when soaked in water. Original aragonite may be present but in some branches the pores and intracalicular spaces are filled with gypsum crystals.

Etymology

This species is named after Prof. Fauzie Hasibuan who made possible our collaboration with the Indonesian Geological Agency. He has provided guidance in the geography and geology of Indonesia and always cheerful advice.

Figures

Figure 38. Acropora hasibuani sp. nov. Holotype, NHMUK PI AZ 7107, TF517, Sangatta, East Kalimantan, Late Miocene (8.9–9.3 Ma). A, whole colony in the field showing arborescent to open caespitose growth form. B, reconstructed part of the holotype colony showing long branches with basal ramification. C, AZ 7107a, b, detail of two fragments of the holotype showing relatively aligned radial corallites. D and E, AZ 7107c, d, electron micrographs long axial corallites and radial corallites. F and G, AZ 7107c, electron micrographs of tubular and tubular appressed radial corallites. H, AZ 7107c, electron micrograph of dense reticulate coenosteum formed of elaborated spinules.