Acropora darrellae, Santodomingo & Wallace & Johnson, 2015

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 : 743-745

ACROPORA DARRELLAE SP. NOV.

FIGURE 34

Diagnosis

Colonies with determinate growth, probably horizontally extended, composed of flat main branches and short secondary branches, none anastomosed, large appressed tubular to tubular corallites sparsely arranged on the lateral sides, rarely on top and undersides of branches. Coenosteum with dense elaborated spinules throughout.

Material studied

Holotype. NHMUK PI AZ 7105 , 12 fragments of the same colony. Type locality: TF56 , Badak , East Kalimantan, 0°19′19.2″S, 117°17′49.2″E, Burdigalian to Langhian age, 14.8–17.9 Ma. Collector N. Santodomingo, 25 June 2011.

Paratype. NHMUK PI AZ 8880 , 13 specimens. Locality : TF56 , Badak , East Kalimantan, 0°19′19.2″S, 117°17′49.2″E, Burdigalian to Langhian age, 14.8– 17.9 Ma. Collector N. Santodomingo, 25 June 2011 .

Additional material. East Kalimantan: NHMUK PI AZ 8873 , 1 specimen ; NHMUK PI AZ 8883 , 2 specimens ; NHMUK PI AZ 8895 , 1 specimen .

Description of the holotype

Corallum . One colony composed of 12 fragments from two main branches, found together embedded in a grey silty matrix, extending horizontally over an area of at least 30 cm long and 10 cm wide ( Fig. 34A ); flattened branches with evidence of primary ramification, oval at the base and flattening towards the tips, laterally alternate from main axes, branch length from fragments puzzled together 143.68–144.65– 145.63 mm, mid branch diameter 7.06–7.92– 9.36 mm, branch thickness 3–5 mm, branch tip diameter 3.03–3.58– 4.13 mm, distance between branches 9.26–11.85– 16.87 mm; growth determinate; terete.

Corallites. Axial corallite, oval to round calice, 3.51– 4.38–6.0 mm exsert, outer diameter 2.30–2.75– 3.20 mm, inner diameter 0.80–1.48– 1.88 mm, wall thickness 0.80–1.48– 1.88 mm, primary septa present, secondary septa absent or visible as points, arranged as S1>>S2; radial corallites widely scattered, mostly not touching, occurring on the flanks of the flat branches, tubular or tubular appressed, round to oval calice, profile length 2.39–3.72– 4.94 mm, angle 37.27– 46.40–53.62°, outer diameter 1.48–1.80– 2.44 mm, inner diameter 0.55–0.73– 0.85 mm, wall thickness 0.66– 0.72– 0.85 mm, distance between centres 7.83–8.66– 9.40 mm, septa S1>>S2. Corallite arrangement sequence 1–1–[1–2]–[2–3]–?.

Coenosteum. Elaborated spinules evenly and densely distributed both on and between radial corallites, relatively aligned in regular rows, reticular appearance. Coenosteum amount 1.64–2.67– 3.70 mm.

Description of the paratype

Thirteen fragments of a colony, one of them horizontally encrusting on an oyster shell, taken as evidence of a flattened form. Fragment length 25–57 mm, mid branch diameter 6–10 mm, primary branches mostly broken at the point of junction with the main axes, length up to 18 mm, mid branch diameter 4–5 mm. Coenosteum worn, evidence of elaborated spinules, aligned and intermingled giving the appearance of a reticulum.

Occurrence

Early to Middle Miocene. This species is only known from its type locality TF56, Badak, of Langhian to Burdigalian age, 14.8–17.9 Ma.

Palaeoenvironment

The specimens were found in two different fossiliferous beds of outcrop TF56: first in the middle of the section embedded in grey silt-rich sediments, and co-occurring with oysters and platy corals such as Leptoseris and Pachyseris , branching corals such as Dictyaraea and Goniopora , and second at the top of the section cooccurring with a very rich coral assemblage. These assemblages have been preliminarily interpreted as deltafront patch reefs typical of low-light conditions ( Renema et al., 2015).

Remarks

The most similar species among the extant fauna is A. elegans . Acropora darrellae sp. nov. can be distinguished from A. elegans based on its simpler branching pattern, consisting of slightly sparser primary branches and radial corallites. Both species have large radial corallites, but in A. darrellae sp. nov. radials are more uniformly arranged as alternating on either sides of the branches. Acropora darrellae sp. nov. has generally thinner and longer branches, probably a more tri-dimensional structure, and neither thick main axes nor anastomoses typical of A. elegans were observed. Additionally, the thin branches and radial corallite arrangement of A. darrellae sp. nov. are similar to secondary branches of Acropora rongelapensis Richards & Wallace, 2004 . However, in the fossil specimens only thin branches were recovered (max 9.36 mm) and there is no evidence that the new fossil species developed thicker main branches with abundant radial corallites (up to 10 radials) as seen in A. rongelapensis holotype MTQ G57574 and paratype MTQ G57575 from the Marshall Islands.

Etymology

This species is named after our friend and colleague Jill Darrell, curator of fossil corals at NHMUK, as a tribute to her dedication for caring for palaeontological collections and palaeontologists over more than 30 years.

Figures

Figure 34. Acropora darrellae sp. nov. Holotype, NHMUK PI AZ 7105, TF56, Badak, East Kalimantan, Early to Middle Miocene (14.8–17.9 Ma). A, aspect of a colony in the field showing two main branches horizontally preserved. B, composite reconstruction of the same branches. C, AZ 7105a, detail of main branch showing the arrangement of primary branches, their broken bases and axial corallite in transverse section. D, AZ 7105b, detail of a branch showing the arrangement of appressed tubular radial corallites. E, AZ 7105c, electron micrograph of a tubular axial corallite with incipient radial. F, AZ 7105d, electron micrograph of a primary branch showing tubular appressed radial corallites. G, AZ 7105d, detail of coenosteum with aligned and reticular elaborate spinules.