Acropora elegans, 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 : 741-743

ACROPORA ELEGANS ( MILNE EDWARDS, 1860)

FIGURE 33

Madrepora elegans Milne Edwards, 1860: 163 , pl. E1 fig. 3

Acropora magnifica Nemenzo, 1971: 148 , pl. 3 fig. 1

Diagnosis

Colonies with determinate growth, plate-shaped, composed of flat branches up to a second degree of ramification, sometimes anastomosed, large appressed tubular to tubular corallites arranged mainly laterally on the branches. Coenosteum of dense elaborated spinules throughout ( Wallace & Wolstenholme, 1998; Wallace, 1999).

Material studied

East Kalimantan: NHMUK PI AZ 6503 , 5 specimens ; NHMUK PI AZ 6674 , 1 specimen ; NHMUK PI AZ 7097 , 1 specimen ; NHMUK PI AZ 7099 , 3 specimens ; NHMUK PI AZ 7106 , 4 specimens ; NHMUK PI AZ 8840 , 9 specimens ; NHMUK PI AZ 8842 , 13 specimens ; NHMUK PI AZ 8982 , 5 specimens ; NHMUK PI AZ 9077 , 28 specimens plus 5 clumps of specimens embedded into a clay-rich matrix.

Modern comparative material: Holotype, MNHN 285 B, locality unknown; MTQ G60789, West Papua, Indonesia, 30 m depth; MTQ G63166, Karang Utara Ronde , Teluk Buli, Halmahera .

Skeletal characteristics

Corallum . Colonies extend horizontally, from field observations they cover an area of at least 40 cm long and 25 cm wide ( Fig. 33A ), flattened branches, main axis diameter 12.34–16.44– 20.43 mm, 6–9 mm thick. Primary branches terete, low degree of anastomosis, flattened irregular in transverse section, extending laterally from main axes, branch length 41.81–64.76– 136.0 mm, angle 29.93–46.88–66.66°, mid branch diameter 6.61–9.87– 16.24 mm, 3–4 mm thick, branch tip diameter 3.60–4.02– 4.70 mm, distance between branches 17.46–32.01– 44.75 mm; growth determinate; terete.

Corallites. Axial corallite, small oval to round calices, 3.01–3.13– 3.20 mm exsert, outer diameter 2.47–2.68– 2.83 mm, inner diameter approx. 1.2–1.5 mm, primary and secondary septa visible but worn, arranged as S1>S2; radial corallites scattered, mostly not touching, laterally arranged on the branches, a few borne on the upper side of the mid section of branches, tubular or appressed tubular, small round to oval calice, profile length 2.01–2.91– 3.38 mm, angle 21.92–29.28–37.48°, outer diameter 0.98–1.32– 1.70 mm, inner diameter 0.57– 0.77– 0.93 mm, wall thickness 0.26–0.37– 0.47 mm, distance between centres 4.48–5.19– 6.50 mm, septa S1>>S2. Corallite arrangement sequence 1–2–2–[2–3]–?.

Coenosteum. Elaborated spinules evenly and densely distributed both on and between radial corallites. Coenosteum amount 1.92–2.69– 3.45 mm.

Occurrence

Early Miocene to Recent. The earliest occurrence of the species is from outcrops TF59 and TF153, near Bontang, of Early Langhian to Late Burdigalian age, 15.3–17.9 Ma. This species also occurs in the outcrop TF79, Samarinda, of Langhian age, 14.8–15.3 Ma. In modern reefs, A. elegans is known from Sulawesi, Halmahera , Flores and West Papua in Indonesia ( Table 4), with a geographical range that extends to north-east Australia, Micronesia ( Wallace et al., 2012), Brunei ( Turak & DeVantier, 2011) and Japan ( Nishihira & Veron, 1995).

Palaeoenvironment

This species occurred in coral settings mainly dominated by large platy coral colonies of agariciids such as Pachyseris and Leptoseris typical of low-light conditions. On modern reefs, low-light environments are mainly depth related and the term ‘mesophotic reef’ has been coined for those reefs below 30–40 m up to 150 m depth ( Kahng et al., 2010). However, studies on the fossil record of Indonesia that combine evidence from larger benthic foraminifera, coralline algae, corals and sedimentology have suggested that low-light conditions in ancient reefs could be related to high influence of siliciclastic input in the delta-front of rivers; palaeoenvironments have been interpreted as shallowwater turbid patch reefs ( Novak et al., 2013; Santodomingo et al., 2015). Other co-occurring branching corals in the fossil settings are Stylophora and Dictyaraea . The colony NHMUK PI AZ 9077 was recovered in addition to a large platy colony of ‘ Echinopora ’ pelarangensis with a diameter of 1 m.

Remarks

Preservation is relatively poor and characterized by internal recrystallization and probably replacement of aragonite in most specimens. Although the holotype MNHN 285B ( Milne Edwards, 1860) was not examined, comparison of images published by Wallace (1999) and the specimens MTQ G60789 and MTQ G63166 allowed interpretation of the fossil material as A. elegans based on similar morphological traits. Fossil specimens of A. elegans can be distinguished from A. tenella and A. pichoni by their more robust colonies bearing larger corallites. Acropora elegans can be distinguished from Acropora darellae sp. nov. by shorter branches that extend from thicker main axes.

Figures

Figure 33. Acropora elegans. A, NHMUK PI AZ 9077, TF153, Bontang, East Kalimantan, Early to Middle Miocene (15.3– 17.9 Ma). Aspect of a colony in the field showing horizontally extended branches. B–D, NHMUK PI AZ 7099, TF153, Bontang, East Kalimantan, Early to Middle Miocene (15.3–17.9 Ma): B and C, AZ 7099a-b, specimens interpreted as the main axes and primary branches of a colony. D, AZ 7099b, flattened irregular branch in transverse section. E, AZ 6503, whole specimen. F and G, NHMUK PI AZ 7097, TF153, Bontang, East Kalimantan, Early to Middle Miocene (15.3– 17.9 Ma): F, whole specimen; G, electron micrograph of tubular appressed radial corallites. H–J, NHMUK PI AZ 6503, TF153, Bontang, East Kalimantan, Early to Middle Miocene (15.3–17.9 Ma): H, electron micrograph of tubular appressed radial corallites; I, incipient branch, triangle indicates radial corallite; J, detail of coenosteum with elaborate spinules.