Acropora tenella (Brook, 1892)

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 : 747-749

ACROPORA TENELLA ( BROOK, 1892)

FIGURE 36

Madrepora tenella Brook, 1892: 464 ; 1893: 193 p1.29 fig. E

Diagnosis

Colonies probably horizontally extended with determinate growth, composed of delicate and slender flattened branches, with some degree of anastomosis among secondary branches, appressed tubular to tubular corallites, not touching, sparsely arranged on the lateral sides, or central part of branches. Coenosteum with dense elaborated spinules ( Wallace, 1999).

Material studied

East Kalimantan: NHMUK PI AZ 7054 , 4 specimens ; NHMUK PI AZ 7109 , 1 specimen ; NHMUK PI AZ 7370 , 1 specimen ; NHMUK PI AZ 7386 , 2 specimens ; NHMUK PI AZ 8841 , 1 specimen ; NHMUK PI AZ 9098 , 2 specimens .

Modern comparative material: Syntype, NHMUK 1892.10 .17.33, Macclesfield Bank , South China Sea, Stn. 18, 68 m ; Syntype, NHMUK 1892.10 .17.34, Macclesfield Bank , South China Sea, Stn. 3, 57 m; MTQ G61254, Pulau Rouw , West Papua , 1–25 m depth; MTQ G56515, Togian Islands , Central Sulawesi, Indonesia .

Skeletal characteristics

Corallum . Slender flattened branches, mostly broken, thicker and oval at the base, flattening towards the tips; branch length 25–26 mm, basal branch diameter up to 8 mm, mid branch diameter 3.39–3.77– 4.14 mm, thickness 2.5–3.5 mm; primary branch length up to 9 mm, angle 38.39–39.50–41.41°, mid branch diameter 3 mm, thickness 1.5 mm; no anastomosis observed in the material studied, branches alternate laterally from main axis, sparse, mostly broken at the base, distance between branches about 10 mm; growth determinate; terete.

Corallites. Axial corallites only visible at tips of primary branches and on transverse sections, oval to round calice, up to 2 mm exsert, outer diameter 1.07– 1.48– 1.81 mm, inner diameter 0.53–0.70– 0.83 mm, wall thickness 0.30–0.34– 0.37 mm, primary septa present, secondary septa observed as points and arranged as S1>>S2 ( Fig. 36H ); radial corallites, mostly not touching, sparsely arranged on the flanks of the flat branches in a zig-zag pattern ( Fig. 36 B-C), tubular or appressed tubular, round to oval calice, profile length 1.49–1.88– 2.15, angle 37.42–47.26–55.06°, outer diameter 0.94– 1.38– 1.68 mm, inner diameter 0.46–0.48–0.50, distance between centres 3.59–4.83– 5.48 mm, septa S1>>S2. Corallite arrangement sequence 1–1–1–1–[up to 2–3].

Coenosteum. Elaborated spinules evenly and densely distributed both on and between radial corallites, relatively more aligned on radial corallite walls. Coenosteum amount longitudinally 1.00– 1.26–1.60 mm, laterally extends as branch diameter.

Occurrence

Early Miocene to Recent. The earliest occurrence of A. tenella is material recovered from outcrop TF59, near Bontang, of Early Langhian to Late Burdigalian age, 15.3–17.9 Ma. Additional fossil specimens are from TF51 and TF57, dated to the Serravallian–Tortonian boundary, 11.6 Ma. Acrpora tenella occurs in modern reefs of Indonesia ( Table 4), Japan, Palau, New Guinea and Micronesia ( Wallace et al., 2012).

Palaeoenvironment

A few specimens were found in clay or silt-rich sediments. They occur together in a rich assemblage of platy corals such as Pavona and Pachyseris and a few other branching corals including Madrepora sp. All samples included fenestrate bryozoans. Some of these assemblages have been interpreted as living on shallow deltafront patch reefs that developed in low-light conditions, under the influence of siliciclastic inputs ( Santodomingo et al., 2015). In the Stadion outcrops (TF51 and TF57) A. pichoni co-occurs with A. tenella . In modern reefs A. pichoni occurs on deep reef walls ( Wallace, 1999).

Remarks

Comparisons with modern specimens, including the syntypes NHMUK 1892.10.17.33-34 ( Macclesfield Bank , South China Sea ) and reference material MTQ G61254 ( Pulau Rouw , West Papua ) and MTQ G56515 ( Togian Islands , Sulawesi), allowed interpretation of the fossil material as A. tenella . This species can be distinguished from its congenerics by its slender branches as are evident on most of the fossil specimens examined in this study .

Figures

Figure 36. Acropora tenella. A–C, NHMUK PI AZ 7109, TF59, Bontang, East Kalimantan, Early to Middle Miocene (15.3– 17.9 Ma): A, fragment of a flattened branch, shown from its underside (left) and upperside (right), with radial corallites arranged in zig-zag pattern on the flanks; B and C, electron micrograph of the same branch. D and E, NHMUK PI AZ 7054, TF51, Stadion, Samarinda, East Kalimantan, Middle to Late Miocene (11.6 Ma): D, four fragments of a colony showing thicker basal branch (left corner) and thinner and flattened primary branches; E, AZ 7054a, electron micrograph of radial corallites and incipient axial on top. F, NHMUK PI AZ 7370, TF 57, Stadion, Samarinda, East Kalimantan, Middle to Late Miocene (11.6 Ma), branch with laterally arranged corallites. G–I, NHMUK PI AZ 8841, TF59, Bontang, East Kalimantan, Early to Middle Miocene (15.3–17.9 Ma): G, specimen showing sparse primary branches; H, electron micrograph of a primary branch with appressed tubular radial corallite with elaborated spinules relatively aligned; I, detail of coenosteum in between corallites formed of dense elaborate spinules.