Acropora pichoni, WALLACE, 1999

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 : 745-747

ACROPORA PICHONI WALLACE, 1999

FIGURE 35

Acropora pichoni Wallace, 1999: 306 , pl. 84

Diagnosis

Colonies horizontally extended with determinate growth, composed of flat branches and short secondary branches, with frequent anastomosis; coenosteum sometimes filling the spaces between branches. Radial corallites appressed tubular to tubular, not touching but closely arranged laterally, rarely on top and undersides of branches. Coenosteum with dense elaborated spinules throughout ( Wallace, 1999).

Material studied

East Kalimantan: NHMUK PI AZ 5294–5296 , 3 specimens ; NHMUK PI AZ 6491 , 1 specimen ; NHMUK PI AZ 6579 , 3 specimens ; NHMUK PI AZ 7226 , 2 specimens ; NHMUK PI AZ 7279 , 4 specimens ; NHMUK PI AZ 7302 , 3 specimens ; NHMUK PI AZ 8838 , 1 specimen ; NHMUK PI AZ 8843 , 4 specimens .

Modern comparative material: Holotype, MTQ G53270, Kimbe Bay , Papua New Guinea, 16 m depth; NHMUK 1893.4 .7.168, Manila, Philippines; NHMUK 1975.8 .5.3,

locality unknown; MTQ G64450, Papua New Guinea,

25 m depth; MTQ G56512, Togian Islands , Central Sulawesi, Indonesia .

Skeletal characteristics

Corallum . Colonies show evidence of horizontal extension ( Fig. 35A ), flattened branches, length up to 7 cm, main axis diameter 8.09–10.61– 12.55 mm, thickness 2.5–3.5 mm, larger and oval at the base, basal branch diameter 3–16 mm, thickness 6–12.5, flattening towards the apices. Primary branches, mid branch diameter 5.43–6.26– 7.61 mm, thickness 1.5–2.5 mm, length up to 1.2 cm, anastomosis observed as coenosteum fill the space in between primary branches or lateral radial corallites increasing the flattened appearance ( Fig. 35A, E ), branches laterally alternate from main axes, extending from the same junction or separated 8–13 mm between each other ( Fig. 35A–C ); growth determinate; terete.

Corallites. Axial corallites only visible in transverse section, oval to round calices, outer diameter 2.5 mm, inner diameter 0.8–1.2 mm, primary septa developed, secondary septa observed as points and arranged so that S1>>S2; radial corallites, mostly not touching, closely arranged on both sides of the flat branches in angles 58.92–70.09–80.33° ( Fig. 35B–E ), exceptionally borne on the centre of the branch, separated by a distance of 2.14–2.96– 3.41 mm between each other, tubular or tubular appressed ( Fig. 35C, D ), length 1.25–1.50– 1.62 mm, outer diameter 1.8–2 mm, inner diameter 0.5–0.8 mm, calice round to oval, septa S1>>S2. Corallite arrangement sequence 1–[1–2]–[1–2]–[1–2].

Coenosteum. Elaborated spinules evenly and densely distributed both on corallite walls and between radial corallites, relatively more aligned on the lateral corallites ( Fig. 35F ). Coenosteum amount 0.70–1.14– 1.60 mm, laterally extends as branch diameter.

Occurrence

Early Miocene to Recent. The earliest occurrence of A. pichoni is from outcrop TF59. Additional specimens from the Stadion sections TF51 and TF57, at the Tortonian–Serravallian boundary, 11.6 Ma. These fossil specimens extend the distribution of the species to East Kalimantan during the Miocene. On modern reefs, this species has a narrow distribution from Sulawesi, Halmahera and Irian Jaya ( Table 4) to the Solomon islands ( Wallace et al., 2012).

Palaeoenvironment

This species was rare in clay or silt-rich sediments and most of the specimens were recovered from the Stadion outcrops, TF51 and TF57, that have been interpret- ed as shallow delta-front patch reefs that developed in low-light conditions, under the influence of high siliciclastic inputs ( Santodomingo et al., 2015). It co-

occurs with A. darrellae sp. nov. in outcrop TF59 and

with A. tenella in TF51 and TF57.

Remarks

Comparisons with modern specimens NHMUK 1893.4.7.168 (Manila, Philippines), NHMUK 1975.8.5.3 (locality unknown) and MTQ G64450 ( Papua New Guinea, 25 m depth) showed that the fossil material has the same distinctive flattened branches, closely arranged corallites and branches almost restricted to the lateral sides of the colony, as well as coenosteum filling the spaces in between corallites and branches. Some of the specimens are similar to A. tenella , but A. tenella is more slender and with radial corallites distributed more sparsely and in a zigzag pattern. Before its description by Wallace (1999), specimens of A. pichoni were mostly identified as A. tenella ( Nishihira & Veron, 1995) and A. elegans ( Wallace, 1994) , but examination of a large range of material allowed the separation of the three species ( Wallace, 1999). Further examination of additional fossil material from other localities in the Indo-Pacific would provide more clues for the distinction of the three species.

Figures

Figure 35. Acropora pichoni. A, NHMUK PI AZ 8843, TF57, Stadion, Samarinda, East Kalimantan, Middle to Late Miocene (11.6 Ma), fragments of a colony showing horizontally extended branches and anastomosis as coenosteum fills the space between primary branches and corallites. B and C, NHMUK PI AZ 6579: B, fragments of a colony showing radial corallites closely arranged on the flanks of flattened branches; C, electron micrograph of radial corallites. D, NHMUK PI AZ 8838, TF59, Bontang, East Kalimantan, Early to Middle Miocene (15.3–17.9 Ma), whole specimen interpreted as the main branch of a colony showing flattened shape, lateral arrangement of corallites. E–G, NHMUK PI AZ 5294, TF13, Bontang, East Kalimantan, Early to Middle Miocene (15.3–17.9 Ma): E, whole specimen showing arrangement of radial corallites; F, electron micrographs of appressed tubular radial corallite; G, detail of coenosteum in between corallites formed of dense elaborate spinules.