Acropora borneoensis, (FELIX, 1921)

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 : 714-716

ACROPORA BORNEOENSIS ( FELIX, 1921)

FIGURE 18

Madrepora borneoensis Felix, 1921: 54 , pl. 142 fig. 11–11a

Madrepora duncani Reuss, 1866 : pl. 2, fig. 2c (only)

Diagnosis

Colonies arborescent, probably with indeterminate growth, composed of cylindrical, tapering branches. Branch structure axial dominated. Mixed radial corallites, densely packed towards the branch tips, mainly labellate with rounded lips or subimmersed, medium size. Coenosteum composed of large simple spinules, costate on corallite walls and open reticulate in between corallites.

Material studied

East Kalimantan: NHMUK PI AZ 9062 , 1 specimen . TF505 , Bontang , East Kalimantan, 0°8′26.16″N, 117°25′33.60″E, Tortonian age, 9.4–9.8 Ma. Collector N. Santodomingo, 26 June 2011; NHMUK PI AZ 9061 , 27 fragments from the same sample as NHMUK PI AZ 9062 .

Other material studied. East Kalimantan: NHMUK PI AZ 6352 , 61 specimens ; NHMUK PI AZ 7087 , 53 specimens ; NHMUK PI AZ 7088 , 5 specimens . NHMUK PI AZ 7095 , 1 specimen (radials upside down) . Java: NHMW 1959 /334/13, 1 specimen ; RGM 791812 , 1 specimen ; RGM 791816 , 1 specimen ; RGM 791820 , 9 specimens . Sumatra: RGM 791825 , 1 specimen .

Additional material. East Kalimantan: NHMUK PI AZ 5409 , 1 specimen ; NHMUK PI AZ 5999 , 2 specimens ; NHMUK PI AZ 6006 , 23 specimens ; NHMUK PI AZ 7443 , 4 specimens ; NHMUK PI AZ 6232 , 11 specimens ; NHMUK PI AZ 6250 , 56 specimens ; NHMUK PI AZ 6274 , 53 specimens ; NHMUK PI AZ 6276 , 6 specimens . NHMUK PI AZ 6305 , 23 specimens ; NHMUK PI AZ 8962 , 3 specimens ; NHMUK PI AZ 8973 , 4 specimens ; NHMUK PI AZ 9020 , 9 specimens .

Description of NHMUK PI AZ 9062 and AZ 9061 Corallum . Arborescent; NHMUK PI AZ 9062 overall length 23.7 mm, mid branch diameter 8.7 mm, round in transverse section, slightly tapering, branch tip diameter 5.20–5.76– 6.20 mm; growth probably indeterminate.

Corallites. Axial corallite dominates branch structure, visible on one branch tip; exsert 0.54–0.60– 0.64 mm, outer diameter 1.35–1.60– 1.85 mm, inner diameter 0.99–1.0– 1.01 mm, wall porous 0.37–0.42– 0.46 mm thick, primary septa up to three-quarters R, secondary septa up to one-third R; radial corallites in evenly sized, evenly distributed, sometimes giving the appearance of aligned longitudinal rows, mostly subimmersed or with small lips and round calices, lip length 0.08–0.23– 0.44 mm, angle 50.68–57.13–62.95°, outer diameter 0.77–1.31– 1.89 mm, inner diameter 0.65– 0.86– 1.18 mm, wall thickness 0.08–0.17– 0.26 mm, distance between centres 1.06–2.11– 3.84 mm of each other, primary septa up to two-thirds R, distal margin smooth and straight, secondary septa up to one-quarter R or visible as points. Corallite arrangement sequence 1–6– 6–8–10.

Coenosteum. Simple spinules, arranged into irregular costae on radial corallite walls, reticulate with less densely arranged spinules in intercorallite areas. Coenosteum amount 0.37–0.81– 1.21 mm.

Description of other material studied from

East Kalimantan

Corallum . Arborescent; overall length of specimens 8.27– 20.07– 28.25 mm, branching angle 32.09–64.39– 93.86°, basal branch diameter 7.20–9.72– 12.40 mm, mid branch diameter 4.06–7.10– 9.11 mm, branch tip diameter 2.93–3.57– 5.02 mm, round in transverse section; growth probably indeterminate.

Corallites. Axial corallite exsert 0.40–0.73– 1.01 mm, outer diameter 1.32–1.77– 2.45 mm, inner diameter 0.85– 1.08– 1.37 mm, wall porous 0.08–0.23– 0.68 mm thick, primary septa up to three-quarters R, secondary septa up to l/3 R; radial corallites towards the branch tips as described for the NHMUK PI AZ 9061–9062, profile length 0.05–0.27– 0.46 mm, angle 33.51–48.78–61.97°, outer diameter 0.85–1.21– 1.80 mm, inner diameter 0.62– 0.89– 1.43 mm, wall thickness 0.08–0.19– 0.28 mm, radial corallites towards de base of the branches smaller and mostly subimmersed, outer diameter 0.58–0.63– 0.66 mm, inner diameter 0.38–0.43– 0.48 mm, distance between centres 0.9–1.9– 3.8 mm, primary septa up to two-thirds R, secondary septa up to onequarter R or visible as points.

Coenosteum. Simple spinules, arranged into irregular costae on radial corallites, reticulate with less densely arranged spinules in intercorallite areas.

Occurrence

Middle to Late Miocene. The earliest occurrence is the re-identified material NHMW 1959 / 334/13 from Gunung Sela, Ci-Lanangthale, Rongga District, of Serravallian– Tortonian age (7.246–13.7 Ma). New material collect- ed in outcrops belonging to the same formation of the type locality in East Kalimantan of Tortonian age, 9.4– 9.8 Ma, from localities TF102 , TF154 , TF505 , TF 508 and TF15 = TF151 .

Palaeoenvironment

Co-occurs in the same environments as those described for A. herklotsi and A. duncani .

Remarks

The location of the type specimen of Acropora borneoensis Felix (1921) is unknown. Johannes Felix was a professor at Leipzig University by the time he published his monographs on fossils of Indonesia ( Felix, 1913, 1920, 1921). Part of this material is available at the collections of Leipzig University according to the catalogue of Löser, Bach & Müller (2002), but none corresponds to this fossil species. In a note on his manuscript from Trinil, Felix mentioned that he received material from Prof. Branca in Bonn ( Felix, 1913: 40) for his studies. However, this material has yet not been located either in the Alexander Koenig Research Museum in Bonn (Zoologisches Forschungsmuseum Alexander Koenig) or in the Humboldt Museum in Berlin (Museum für Naturkunde, Berlin). Other museums included in the unsuccesful search were the State Museum of Natural History Stuttgart, the Natural History Museum of Basel, the Museum of Natural History Vienna and the Naturalis Biodiversity Center in Leiden. Recent developments at Leipzig University have revealed uncatalogued specimens studied by Felix, so we cannot discard yet finding them in the near future (H. Löser, pers. comm., 2014). Here, we describe separately the specimen lot NHMUK PI AZ 9062 collected in Bontang, because they are the best-preserved fossil specimens that better match the morphological characteristics described and illustrated by Felix (1921). In the case that the type material cannot be located, these specimens would be the best neotype candidates for A. borneoensis . On the other hand, we also describe other studied material of East Kalimantan to underline the full range of variation of this species in the region.

Acropora borneoensis is common and locally abundant in several localities near Bontang. The examined material shows high variability in the development of the lip-like corallite wall that could be interpreted as a response to different environmental conditions. A similar trend was seen in extant species of the aspera species group ( Veron & Wallace, 1984). Acropora borneoensis can be distinguished from other species in the group by having larger corallites, almost twice the size in comparison with A. duncani and 1.5 times in comparison with A. herklotsi . Corallites are also more crowded because radials are arranged in hexameral symmetry around the axial. Remarkable similarities between A. borneoensis and the extant species Acropora pulchra ( Brook, 1891) and Acropora aspera ( Dana, 1846) are also observed, yet more delicate branches and predominance of shorter corallite lips or mostly subimmersed radial corallites are characteristic of this species.

Figures

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.