Acropora tortuosa (Dana, 1846)

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 : 731-733

ACROPORA TORTUOSA ( DANA, 1846)

FIGURE 27

Madrepora tortuosa Dana, 1846: 467 , pl. 37 fig. 3 Madrepora implicata Dana, 1846: 466 , pl. 37 fig. 2

Diagnosis

Colonies irregular arborescent with indeterminate growth, composed of long and slender branches. Branch structure 50:50 axial/radial. Radial corallites, rounded tubular to subimmersed, evenly distributed. Coenosteum composed of short spinules with slightly elaborated tips, usually costate on corallite walls ( Wallace & Wolstenholme, 1998; Wallace, 1999).

Material studied

East Kalimantan: NHMUK PI AZ8847 , 2 specimens ; NHMUK PI AZ8810 , 10 specimens .

Modern comparative material: Lectotype, USNM 284 , Fiji; MTQ G34133 Middleton Reef, Australia; MTQ G34425 Middleton Reef, Australia; MTQ G57005, Pulau Talatakoh , Togian Islands , Central Sulawesi, Indonesia.

Skeletal characteristics

Corallum . Long, straight and slender branches with a few short or incipient secondary branches, round in transverse section; branch length 37.57–43.77– 50.06 mm, mid branch diameter 4.94–5.30– 5.91 mm; branching mainly at one side of the main axis, incipient branches along ( Fig. 27C ), a short branch protruding towards the apex, length 5 mm, mid branch diameter 2.30 mm ( Fig. 27B ), distance between branches 10.47– 12.84– 14.56 mm, angle 48.78–51.59–54.39°; growth probably indeterminate.

Corallites. Axial corallite visible in incipient axials and transverse section, outer diameter 1.05–1.37– 1.70 mm, inner diameter 0.73–0.85– 0.97 mm, wall thickness 0.32 mm, primary septa present up to two-thirds R, secondary septa up to one-quarter R; radial corallites evenly sized and distributed, mostly rounded tubular to subimmersed at the base of the branch, not touching, round calices, profile length 1.00– 1.46–1.99 mm, protruding in acute angles 24.48–30.03–43.74°, outer diameter 0.94–1.12– 1.34 mm, inner diameter 0.61–0.70– 0.75 mm, wall thickness 0.11–0.19– 0.22 mm, distance between centres 2.67–3.44– 4.05 mm, primary septa up to one-quarter R, secondary septa visible as points. Corallite arrangement sequence 1–[2–4]–6–6– up to 8.

Coenosteum. Slightly elaborated spinules throughout usually forming costae on corallite walls; coenostum amount longitudinally 1.13–1.84– 2.92 mm, laterally 0.68–0.92– 1.23 mm.

Occurrence

Late Miocene to Recent. Specimens from the East Kalimantan outcrops TF504 and TF508 are the earliest occurrence of the species in the Indo-Pacific, of Tortonian age, 9.4–9.8 Ma. These records also represent the westernmost locality in which the species has ever been recorded. Until the recent record of A. tortuosa described from Sulawesi ( Wallace et al., 2012), this species had a disjunct distribution at the western coast of Australia and several islands of the Central Pacific ( Wallace, 1999).

Palaeoenvironment

Acropora tortuosa was found at TF504 and TF 508 in units characterized by orange fine-grained sand and silt sediments that have been interpreted as lowenergy habitats with moderate input of siliciclastic sediments. In modern reefs, this species inhabits protected habitats such as contained lagoons ( Wallace, 1999; Veron, 2000).

Remarks

The fossil specimens compare well with the modern specimens MTQ G34425 from Middleton Reef, Australia, and MTQ G57005 from Togian Islands, Sulawesi, as both the fossil and the modern specimens have relatively straight branches, rounded tubular corallites of similar dimensions and elaborated spinules with the same morphology. Acropora tortuosa is similar to A. horrida and A. vaughani , but can be distinguished from the former in having radial corallites more uniformly arranged and from the latter in having thinner corallite walls.

Figures

Figure 27. Acropora tortuosa. NHM UK Pl AZ 8847, TF504, Bontang, East Kalimantan, Late Miocene (9.4–9.8 Ma). A, two straight branches interpreted as middle part of a colony. B and C, AZ 8847a: B, detail showing secondary branch; C, electron micrograph showing incipient axial and radial corallites with costate walls, triangle indicates a small radial. D–F, AZ 8847b: D, rounded tubular and subimmersed corallites at the base of a branch; E, transverse section showing axial corallite; F, detail of coenosteum formed of slightly elaborated spinules.