Acropora duncani, (REUSS, 1866)

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 : 711-714

ACROPORA DUNCANI ( REUSS, 1866)

FIGURES 16 , 17

Madrepora duncani Reuss, 1866: 171 , pl. 2 fig. 2a, b

Madrepora duncani Martin, 1880: 146 , pl. 25 fig. 11 (only specimen RGM 3990)

Diagnosis

Colonies probably arborescent with indeterminate growth, thin cylindrical branches. Branch structure axial dominated. Tubular to nariform radial corallites, small in size, relatively random distribution. Coenosteum composed of slightly flattened spinules, costate on corallites and reticular to costate in between corallites.

Material studied

Lectotype. NHMW 1959 /334/12. Type locality: Gunung Selah-Ci-Lanangthale, Java. Tortonian, 8.5–11.6 Ma. Collector F. V. Hochstetter, Novara Expedition 1857– 1859.

Other material studied. East Kalimantan: NHMUK PI AZ 6351 , 930 specimens ; NHMUK PI AZ 7086 , 143 specimens ; RGM 42980, 1 specimen . Java: RGM 3990 , 1 specimen ; RGM 3992 , 1 specimen ; RGM 4000 , 2 specimens ; RGM 791821 , 2 specimens ; Indonesia: RGM 791815 , 2 specimens .

Additional material. East Kalimantan: RGM 791836 , 2 specimens ; NHMUK PI AZ 6007 , 29 specimens ; NHMUK PI AZ 6233 , 23 specimens ; NHMUK PI AZ 6273 , 136 specimens ; NHMUK PI AZ 6304 , 30 specimens . Java: RGM 3994 , 3 specimens ; RGM 3996 , 3 specimens .

Description of lectotype

Corallum . One specimen with scars of two broken branches and three incipient branchlets, and broken branch tip, length 19.38 mm, mid branch diameter 6.61– 7.37– 8.23 mm.

Corallites. Axial corallite dominates branch structure, visible only on transverse section and incipient branchlets, outer diameter 1.72 mm, inner diameter 1.15 mm, wall porous 0.48 mm thick, primary septa up to three-quarters R, joining deep in the fossa, secondary septa up to one-third R; radial corallites evenly sized and randomly distributed, mostly subimmersed or with small lips and round calices, profile length 0.27– 0.35– 0.41 mm, outer diameter 0.94–1.07– 1.37 mm, inner diameter 0.52–0.57– 0.63 mm, wall thickness 0.12– 0.16– 0.19 mm, distance between centres 2.26–2.79– 3.19 mm, primary septa up to two-thirds R, secondary septa up to one-quarter R or visible as points. Corallite arrangement sequence 1–?–6–?.

Coenosteum. Composed of flattened simple spinules, arranged into irregular costae on radial corallite walls, reticulate with less densely arranged spinules in intercorallite areas. Coenosteum amount longitudinal 0.87–1.53– 2.28 mm, coenosteum amount lateral 0.88–1.70– 2.28 mm.

Description of East Kalimantan fossil specimens Corallum . Some of the studied specimens show evidence of small arborescent colonies, branching near the base of the fragments, opposite or alternate, distance between branches at least 3.40–5.65– 8.40 mm, angles 57.56–68.17–75.65°, although mostly broken in small pieces, overall length 5.99–15.05– 27.80 mm, generally with no scars other than the one of attachment; branches tapering, basal diameter 3.93–5.25– 9.60 mm, mid branch diameter 2.10–4.00– 8.66 mm, branch tip diameter 1.75–2.11– 2.41 mm, round in cross section; growth probably indeterminate.

Corallites. Axial corallite dominates branch structure, visible on many branch tips, 0.42–0.57– 0.65 mm exsert, outer diameter 1.16–1.56– 1.90 mm, inner diameter 0.80–0.95– 1.10 mm, wall porous 0.10–0.33– 0.45 mm thick, primary septa up to three-quarters R, secondary septa up to one-third R; radial corallites evenly sized and randomly distributed, mostly subimmersed or with small lips and oval calices, profile length 0.12–0.31– 0.49 mm, angles 22.35–34.91– 54.69°, outer diameter 0.46–0.74– 0.91 mm, inner diameter 0.27–0.51– 0.82 mm, wall thickness 0.10–0.14– 0.22 mm, distance between centres 0.90–1.48– 2.16 mm, primary septa up to two-thirds R, secondary septa up to one-quarter R or visible as points. Corallite arrangement sequence 1–4–4–6–[6–8]–?.

Coenosteum. As described for the lectotype, varying from reticulate to aligned costate arrangement on the intercorallite areas ( Fig. 17C–F ). Coenosteum amount 0.27–0.78– 1.42 mm.

Occurrence

Early to Late Miocene. The earliest occurrence of A. duncani, RGM 3992, is from Ci Talahab, Java, 18– 13 Ma (Burdigalian to Serravalian). Reidentified material from Naturalis collections are mainly from Java localities including Liotjitjangkang, Ci Burial and Ci Beber of Serravallian to Tortonian age, 8.5–13.5 Ma. New specimens are from the East Kalimantan localities around Bontang, TF102 and TF154, of Tortonian age, 9.4–9.8 Ma.

Palaeoenvironment

Acropora duncani was locally abundant in some localities around Bontang that have been interpreted as shallow-water coral carpets influenced by clay-rich siliciclastic inputs ( Kusworo et al., 2015). These habitats are dominated by the extinct poritid Dictyaraea and it co-occurs with other branching corals such as Seriatopora , Porites and other two species of the aspera group, A. borneoensis and A. herklotsi .

Remarks

Acropora duncani was adopted as a working name to identify most of the Acropora fossil specimens found in Indonesia, because only poor-preserved material was available for most researchers during the early 20th century and collections were composed of a few specimens ( Gerth, 1921, 1923). Reuss (1866) distinguished A. duncani from A. herklotsi as A. duncani has smaller (∼ 0.5 mm calice diameter) and more irregularly arranged corallites than A. herklotsi . From the two specimens included in the syntype series of A. duncani, NHMW 1959 /334/12 ( Fig. 16 ) represents better the diagnostic characteristics described by Reuss (1866). Therefore, this specimen is designated as the lectotype of A. duncani . According to the diagnostic characters re-described here, we confirmed that nine specimen lots in historical collections of the Naturalis museum correspond to A. duncani ( Table S2). Measurements performed on the second specimen described by Reuss (1866), NHMW 1959/334/13 ( Fig. 18C, D ), match better the range of variation of the fossil material interpret- ed as Acropora borneoensis ( Felix, 1921) , for which details are explained below. Fossil specimens were compared with modern representative specimens of the aspera group, including A. pulchra , A. aspera , and A. millepora , but none of them show a similar character combination and the proportions described here for A. duncani , A. herklotsi and A. borneoensis .

Figures

Figure 16. Acropora duncani. Lectotype, NHMW 1959/334/12, Gunung Selah, Ci Lanangthale, Java, Late Miocene (8.5– 11.6 Ma). A, branch fragment and detail figured in Reuss (1866, pl. 2, fig. 2a, b). B, transverse section of the branch showing axial corallite. C, lateral view of lectotype specimen showing small radial corallites and incipient secondary branches.

Figure 17. Acropora duncani. East Kalimantan. A–C, E and F, H, NHMUK PI AZ 6351, TF 154, Bontang, East Kalimantan, Late Miocene (9.4–9.8 Ma). D, G, I, NHMUK PI AZ 7086, TF 154, Bontang, East Kalimantan, Late Miocene (9.4–9.8 Ma). A, thin branches recovered in the same sample, some with signs of primary branching. B and C, AZ 6351a: B, upper view of a branch tip showing axial corallite, relatively thick corallite wall, and septal arrangement S1>S2; C, lateral view of the same specimen showing radial corallites. D–F, electron micrographs of three branches showing morphological variation of the coenosteum and horizontal lips of radial corallites: D, AZ 7086a, free simple spinules and subimmersed corallites; E, AZ 6351b, slightly fused simple spinules and mixed subimmersed and horizontal lipped corallites; F, AZ 6351c, fused spinules into costate coenosteum and mostly lipped corallites, exceptionally radial corallites are observed on alternating longitudinal rows. G, AZ 7086a, detail of free spinules. H, AZ 6351c, detail of fused spinules into costae. I, AZ 7086a, detail of coenosteum in between corallites formed of simple spinules.

Figure 18. Acropora borneoensis. A, fragments in the field of probably the same collapsed colony, NHMUK PI AZ 9061- AZ 9062, TF505, Bontang gardens sections, East Kalimantan, Late Miocene (9.4–9.8 Ma). B, NHMUK PI AZ 9062, one of the fragments showing the best preservation. C, NHMUK PI AZ 9061, fragments collected in the sample with lot AZ 9062, showing signs of primary branching. D and E, NHMW 1959/334/13, Gunung Selah, Ci Lanangthale, Java, Late Miocene (8.5–11.6 Ma), specimen presented by Reuss (1866) re-identified here as A. borneoensis: D, upper view showing axial corallite and round branch;E, same specimen in lateral view, showing cylindrical branch shape and incipient branch. F, NHMUK PI AZ 7087, TF154, Bontang, East Kalimantan, Late Miocene (9.4–9.8 Ma), electron micrograph of branch tip, upper view, showing axial corallite. G, NHMUK PI AZ 6352, TF154, Bontang, East Kalimantan, Late Miocene (9.4– 9.8 Ma), electron micrograph of branch tip, lateral view, showing reticular coenosteum and slightly lipped radial corallites. H and I, NHMUK PI AZ 9062: H, lateral view of small branch, showing axial and radial corallite arrangement; I, detail of free simple spinules.