FIGURE 32

Acropora filiformis Veron, 2000, (1): 418; 2002: 66– 68, figs 132–135. Wallace et al., 2012: 68.

Diagnosis

Colonies upright or prostrate; long, slender, straight branches. Radial corallites, tubular to appressed tubular, sparsely distributed. Coenosteum composed of dense reticulate elaborated spinules throughout (Veron, 2002).

Material studied

East Kalimantan: NHMUK PI AZ 7441, 4 fragments from the same branch.

Modern comparative material: Topotype, MTQ G60255, Calamian Islands, Palawan, Philippines, 12 m depth.

Skeletal characteristics

Corallum. Long, sparse, straight branches were observed in situ embedded into a clay-rich sediment matrix (Fig. 32A), branch length measured from field images 49.53–89.51– 120.26 mm, basal branch diameter 5.90– 6.84– 7.44 mm, mid branch diameter 3.84–4.97– 6.02 mm, no branch tips recovered; primary branches mostly broken, angle 65.63–67.02–68.09°; four specimens in the collection are fragments the same branch (Fig. 32 B-C): length 9.94–13.60– 19.55 mm; growth probably indeterminate.

Corallites. Axial corallite only visible in transverse section, outer diameter 1.32–1.37– 1.42 mm, inner diameter 0.79–0.83– 0.88 mm, wall thickness 0.22–0.28– 0.34 mm, primary septa up to three-quarters R, secondary septa up to one-quarter R or visible as points; radial corallites evenly sized, sparsely distributed, not touching, mostly appressed tubular, some subimmersed, small round calices, profile length 2.19–2.40– 2.69 mm, exert in acute angle 17.23–22.11–33.50°, outer diameter 1.12–1.26– 1.54 mm, inner diameter 0.82–0.97– 1.15 mm, wall thickness 0.21–0.29– 0.34 mm, distance between centres 3.39–5.20– 5.95 mm, primary septa present up to three-quarters R, secondary septa up to one-quarter R or visible as points, S1>S2. Corallite arrangement sequence 1–?–2–2– up to 4.

Coenosteum. Reticulate, elaborate spinules sometimes laterally flattened (Fig. 32H), coenostum amount 2.76–3.34–4.00 mm longitudinal, 3.72–4.75– 6.13 mm lateral.

Occurrence

Late Miocene to Recent. These fossil specimens are from the outcrop TF500, Bontang, of Tortonian age (9.4– 9.8 Ma), and are the earliest record of the species. On modern reefs, A. filiformis is only known from its type locality, the Calamian Islands, Philippines (Table 4).

Palaeoenvironment

Stratigraphic correlations have placed the outcrop TF500 at the base of the Bontang Gardens sequence (Renema et al., 2015). The long branches of A. filiformis were found embedded into yellowish clay-rich sediments, and the branches were easily broken during sampling resulting in small fragmented specimens. This species occurred together with branching corals of the genus Dictyaraea and Seriatopora, and it overlays a bed with abundant platy corals and Acropora russelli. In modern settings A. filiformis has been observed in shallow reef environments protected from wave action, as isolated colonies or forming large stands extending across several metres (Veron, 2000, 2002).

Remarks

The nomenclatural status of A. filiformis as an available species was confirmed by Wallace et al. (2012), following the ICZN (2011). Comparisons with the figured topotype MTQ G60255 (Veron, 2002) allowed the interpretation of the fossil material as A. filiformis. The long, slender, straw-like branches, and sparse, small, appressed tubular corallites are distinctive characteristics of this species. It is similar to A. derawanensis, but costae are more developed in A. filiformis.