Acropora horrida, 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 : 728-730

ACROPORA HORRIDA ( DANA, 1846)

FIGURE 25

Madrepora horrida Dana, 1846: 472 , pl.39 fig. 2, 2a Madrepora angulata Quelch, 1886: 160 Madrepora inermis Brook, 1891: 462 ; 1893: 194, pl.

29 fig. A, B Acropora sekiseiensis Veron, 1990: 107 , figs 11, 12

Diagnosis

Colonies arborescent to hispidose–arborescent with indeterminate growth, composed of irregular, slender branches. Axial and radial contribute equally to the branch structure. Radial corallites, tubular to subimmersed, irregularly distributed. Coenosteum composed of aligned simple spinules that can become costate throughout ( Wallace, 1999).

Material studied

East Kalimantan : NHMUK PI AZ 7104 , 250 specimens from the same thicket; NHM UK PI AZ 5623 , 1 specimen ; NHM UK PI AZ 5624 , 2 specimens ; NHM UK PI AZ 8822 , 8 specimens ; NHM UK PI AZ 8846 , 1 specimen ; NHM UK PI AZ 8889 , 10 specimens ; NHM UK PI AZ 8890 , 39 specimens ; NHM UK PI AZ8891 , 14 specimens ; RGM 77688, 9 specimens . Java : RGM 77689, 9 specimens . Sumatra : RGM 791830 , 1 specimen .

Modern comparative material: Holotype, USNM 291 , Fiji; NHMUK 1886.12 .9.235, Zamboanga, Philippines, Madrepora angulata holotype; MTQ G56622, Kakaban Island, East Kalimantan, Indonesia, 14 m; MTQ G48964, Karang Tababinga , East Kalimantan, Indonesia.

Skeletal characteristics

Corallum . Specimens in situ in the outcrop TF518 were observed forming a thicket of arborescent colonies up to 30 cm height ( Fig. 25A ); irregular slender branches, length 34.3–46.1–75.0 mm ( Fig. 25B ), branching angle 40.62–45.76–52.26°, terete or slightly tapering, basal diameter 12.96–15.26– 18.55 mm, mid branch diameter 7.74–8.69– 9.80 mm, branch tip diameter 5.31–6.35– 8.54 mm; growth probably indeterminate.

Corallites. Axial corallite visible in a few branch tips, 0.64–0.87– 1.05 mm exsert, outer diameter 1.60–1.73– 1.80 mm, inner diameter 1.00– 1.10–1.18 mm, fragile walls usually worn out, wall thickness 0.16–0.20– 0.24 mm, primary septa present up to two-thirds R, secondary septa up to one-quarter R; radial corallites evenly sized, irregularly distributed, mostly tubular to conic, some subimmersed, not touching, round calices, profile length 0.87–1.34– 1.74 mm, angle 40.98–57.38– 90.97°, outer diameter 1.08–1.31– 1.60 mm, inner diameter 0.77–0.86– 0.99 mm, wall thickness 0.20–0.22– 0.26 mm, distance between centres 2.16–3.60– 4.74 mm, primary septa up to one-half R, secondary septa up to one-quarter R. Corallite arrangement sequence 1–[3– 4]–[4–6]–up to 8.

Coenosteum. Aligned simple spinules, arranged into costae throughout, coenosteum amount 0.85–1.67– 2.63 mm.

Occurrence

Late Miocene to Recent. The earliest occurrence of A. horrida is represented by specimens collected in Kampung Narut outcrops, TF518 and TF171, of Messinian to Tortonian age, 5–8 Ma. Specimens from the Naturalis museum are from Gunung Linggapadang of Messianian age, 5.33–7.25 Ma, and some Pleistocene repositories from Gawo Valley of Calabrian to Gelasian age, 0.781 –2.588 Ma. Previous fossil records include specimens from the Era Beds of 2–3 Ma ( Veron & Kelley, 1988). On modern reefs A. horrida is common and widespread in the Indo-Pacific, from the Seychelles to the Cook Islands ( Wallace, 1999). Records from Indonesian reefs include occurrences in Java, Kalimantan, Nusa Tenggara, Banda Sea and Irian Jaya ( Table 4).

Palaeoenvironment

In the outcrop TF518, Kampung Narut, Acropora horrida produced a monospecific layer 30–50 cm thick, within a grey silt-rich sediment matrix ( Fig. 25A, B ). The A. horrida thicket overlies a bed with platy corals and is overlain by a unit with an indurated matrix with a few Goniopora , Seriatopora and Alveopora branching corals. In modern settings this species has been observed forming almost mono-specific banks in Alor Islands on some of the sandy reef slopes ( Wallace & Wolstenholme, 1998), suggesting that probably this type of dominant development has been possible since the Late Miocene.

Remarks

Comparisons with the modern specimens MTQ G56622 and MTQ G48964 from East Kalimantan and NHMUK 1886.12 .9.235 from Zamboanga, Philippines, allowed the interpretation of the fossil material as A. horrida . This species can be distinguished by its irregular slender branches and irregularly distributed radial corallites .

Figures

Figure 25. Acropora horrida. NHMUK PI AZ 7104, TF518, Kampung Narut, East Kalimantan, Late Miocene to Early Pliocene (5–8 Ma). A, thicket of A. horrida preserved in a silt-rich sediment matrix. B, detail of colony in the field showing arborescent colony form. C–E, AZ 7104a–c, fragments of irregular slender branches. F, AZ 7104c, electron micrograph of a corallite. G, AZ 7104d, electron micrograph of branch tip with appressed tubular corallites. H, AZ 7104d, electron micrograph of axial corallite. I, AZ 7104c, detail of coenosteum in between corallites formed of simple spinules arranged into costae.