Acropora bartonensis, WALLACE, 2008

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 : 721-722

ACROPORA BARTONENSIS WALLACE, 2008

FIGURE 21

Acropora bartonensis Wallace, 2008: 326–327 , fig. 10; Wallace & Bosellini, 2014: 14, fig. 10.

Diagnosis

Colony hispidose, slender axis with short branches; smaller radial corallites mainly clustered around the branchlets, or irregularly sparse along main branches, evenly sized. Coenosteum with lateral flattened simple spinules aligned throughout ( Wallace, 2008).

Material studied

East Kalimantan: NHMUK PI AZ 6879 , 1 specimen. Fossil comparative material: NHMUK PI AZ R 14571, Holotype, Barton, Hampshire, Eocene , Barton Beds ; NHMUK PI AZ R 19125, Barton , Hampshire, Eocene , Barton Beds.

Skeletal characteristics

Corallum . Overall length of specimen 50.1 mm, mid branch diameter 7.10–8.23– 10.10 mm, multiple short branchlets extending from main branch indicating hispidose form; branchlets arising regularly from main branch in angles of 49.68–63.53–88.45°, distance between branchlets 8.20–9.42– 10.46 mm, with tips mostly abraded, tapering, branchlet length 3.2–5.3– 7.2 mm, basal branchlet diameter 4.90–5.36– 6.20 mm, middle branch diameter 3.90–4.58– 5.40 mm, surfaces extending between branches with a few sparse radial corallites.

Corallites. Axial corallites visible at some branchlet tips, round calice, outer diameter 1.51–1.93– 2.19 mm, inner diameter 0.96–1.24– 1.43 mm, wall thickness 0.29–0.31– 0.39 mm, primary septa visible but worn, secondary septa may be present; radial corallites mostly worn and crowded in short branchlets, three to six radials per branchlet, mostly not touching, probably short tubular to labellate with lower wall slightly thickened, profile length 0.29–0.42– 0.51 mm, angle 43.04–53.55–62.10°, outer diameter 0.82–0.86– 0.94 mm, inner diameter 0.47–0.60– 0.74 mm, wall thickness 0.12– 0.14– 0.16 mm, septa worn out, evidence for primary septa larger than secondary septa. Corallite arrangement sequence 1–[3–4]-6–10–?.

Coenosteum. Simple flattened spinules evenly distributed and aligned, both on corallite walls and between radial corallites. Coenosteum amount 1.11–1.85– 2.29 mm.

Occurrence

Eocene to Middle Miocene. The earliest occurrence of the species is from its type locality, Barton, Hampshire, UK, of Bartonian age, 38–41 Ma ( Wallace, 2008). Additional records include Miocene specimens from France and Italy of Aquitanian and Burdigalian age, 16–23 Ma ( Wallace & Bosellini, 2014). The specimen studied was recovered from the locality TF79, Samarinda, East Kalimantan, of Langhian age, 14.8– 15.3 Ma. This record is the youngest occurrence of this extinct species.

Palaeoenvironment

The mixed siliciclastic–carbonate limestones of locality TF79 have been interpreted as patch reefs ( Renema et al., 2015).

Remarks

This specimen compares well with the holotype of A. bartonensis by sharing regularly distributed branchlets indicative of hispidose form and with radial corallites arranged throughout the main axis and also clustered around the branchlets. Although the coenosteum of the Indonesian fossil specimen is not as costate as in the holotype of A. bartonensis , simple flattened spinules are aligned and it is probable that the costae have not been preserved in this highly recrystallized specimen. Together with Acropora lavandulina , these are the only Tethyan species also recorded in the fossil record of the Indo-Pacific.

Figures

Figure 21. Acropora bartonensis. NHMUK PI AZ 6879, TF79, Batu Cermen, Samarinda, East Kalimantan, Middle Miocene (14.8–15.3 Ma). A, whole specimen showing short branchlets indicative of hispidose form. B, detail of branchlet showing axial corallite and radial corallites. C, electron micrograph showing incipient branchlet and radial corallites. D, electron micrograph of incipient branchlet with radial corallite. E, electron micrograph of coenosteum formed of aligned simple flattened spinules.