Coralline algae indicate Pleistocene evolution from deep, open platform to outer barrier reef environments in the northern Great Barrier Reef margin

Abstract

Coralline algae from a drill core in Ribbon Reef 5 have been used to interpret changes in the depositional palaeoenvironments in the northern Australian Great Barrier Reef over the last 790 ka. Three main coralline algal assemblages, each dominated by members of a particular subfamily, have been distinguished by quantitative analysis: (1) mastophoroid assemblages, usually occurring as crusts on corals, are typical of the shallowest reef settings; (2) lithophylloid assemblages within algal nodules may represent shallow-water, cooler environments or deeper reef subenvironments; and (3) melobesioid assemblages are characteristic of deeper-water platform areas. The algal assemblages between 96 and 210 m b.s.f. (metre below sea floor) record a fluctuating but progressive shallowing-upwards from deep, outer-platform to shallower, non-reefal depositional environments. Two intervening episodes dominated by mastophoroids represent two phases of reef growth. The section above 96 m b.s.f. comprises several stacked reefs in which mastophoroid crusts similar to the present-day shallow-water assemblages predominate.

Appendix

Percentages of taxa found, thickness range of individual thalli, and total thickness of coralline crusts in the studied samples

Relative proportions have been obtained by estimating the cross-sectional areas occupied by each taxon in the thin sections, according to Perrin et al. (1995). Samples with unidentifiable or very low algal content have not been included. Following Woelkerling et al. (1993a), 0.2 mm is the thickness boundary separating thin and thick thalli.

• Abbreviations in Latin: onk., onkodes; mun., munitum; reinb., reinboldii; Aethes. probl., Aethesolithon problematicum; fosl., fosliei; con., conicum; Spong., Spongites; Lithopor. melob., Lithoporella melobesioides; kots., kotschyanum; pust., pustulatum; incra., incrassatum; Mes., Mesophyllum; Lith., Lithothamnion; Spor., Sporolithon.

• Other abbreviations: TLT unidentifiable thin laminar thalli, assem. assemblages, M mastophoroid, L lithophylloid, B melobesioid. Coralline growth forms: C crusts covering coral skeletons and other bioclasts, B loose branches, F for-algaliths, R rhodoliths, Fr only fragments recorded.

Taxonomy

We include in the Lithophyllum pustulatum group species comprising plants with thallus dimerous with primigenous filaments usually composed of palisade cells, and postigenous filaments composed of only epithallial cells (rarely preserved in fossil plants) or epithallial and a few subtending cells. These characters are shared by several living Lithophyllum species traditionally ascribed to Titanoderma. The diagnostic features to distinguish the different species among the group are not preserved in ancient plants (such as the cells occluding the conceptacle pore canals in L. chamberlainianum Woelkerling and Campbell and L. irvineanum Woelkerling and Campbell) or are difficult to discern in fossil examples. The latter is the case of the terraced surface considered diagnostic to separate L. prototypum (Foslie) Foslie (and L. tessellatum Lemoine considered a younger heterotypic synonym) from L. pustulatum and its heterotypic synonyms by Woelkerling and Campbell (1992).

In the Lithophyllum kotschyanum Unger species group, we include plants which show a certain variability in growth form and thallus zonation. They have tetra/bisporangial conceptacles with columella and conical pore canal, and the cells of adjacent filaments are laterally aligned. Changes in growth patterns and thallus zonation range from those of the thalli illustrated as L. bermudense Foslie & Howe by Woelkerling and Campbell (1992), subsequently included in L. frondosum (Dufour) Furnari, Cormaci & Alongi by Woelkerling (1996), to the ones included in L. tamiense (Heydrich) Foslie and L. moluccense Foslie (for example, the plant illustrated by Johnson 1957, p. 230, pl. 58, Fig. 5).

The species Neogoniolithon conicum (Dawson) Gordon, Masaki & Akioka has recently been attributed to the genus Pneophyllum by Keats et al. (1997) on the basis of tetrasporangial developmental features. While awaiting further testing of the systematic consistency of the character used by Keats et al. (1997) inside the Mastophoroideae, which groups N. conicum with dimerous, mostly epiphytic plants, we have followed the taxonomic criteria proposed by Braga et al. (1993) for fossil corallines, and have included N. conicum in Neogoniolithon.

We consider Neogoniolithon fosliei (Fig. 3B) as a separate entity from N. brassica-floridum (Harvey) Setchell & Mason sensu Woelkerling et al. (1993b). Under the latter name, Woelkerling et al. (1993b) included Melobesia brassica-florida Harvey, Melobesia notarisii Dufour and Lithothamnion fosliei Heydrich, accommodating the morphologies of their types into the variability observed in the specimens referred to as N. fosliei by Penrose (1992) in southern Australia. According to Athanasiadis (1997), however, this broad species concept, although simplifying the species-level taxonomy, conceals a morphological diversity which may correspond to reproductively isolated units. Including N. fosliei in N. brassica-floridum sensu Woelkerling et al. (1993b) would ignore the fact that in plants comparable to the lectotype of N. fosliei, the coaxial arrangement of core filaments is always present at least in some portions of the plants, while such a coaxial arrangement has never been observed in the Melobesia brassica-florida Harvey type or in the isolectotype of Melobesia notarisii Dufour (Athanasidis 1997).

We have attributed to Aethesolithon problematicum Johnson plants common in the intervals dominated by mastophoroids in the Ribbon 5 core section, which cannot been assigned to any modern coralline species. The very typical vegetative anatomy of the peripheral region of the examples from the Quaternary of Ribbon 5 (Fig. 3C, D) is coincident with the one of the holotype and other specimens of A. problematicum from the Miocene of Guam illustrated by Johnson (1964, pl. 9, Figs. 1, 2 and 3). In addition, cores of coaxial filaments occur on the ventral side of the plants (Fig. 3C). A. problematicum is the type species of Aethesolithon, a genus established by Johnson (1964) with Miocene and Pliocene specimens, which requires a reassessment from a modern taxonomical perspective. Such a reassessment, however, is beyond the scope of this paper. Despite the abundance of examples of this species, plants with conceptacles are very scarce and poorly preserved. The conceptacles are uniporate and seem to show the features typical of Hydrolithon according to Penrose and Chamberlain (1993), but better-preserved examples with conceptacles are needed to assess the circumscription of this species and the reports of Aethesolithon to Hydrolithon and other mastophoroid genera. In any case, the uniporate, presumably sporangial conceptacles and the conspicuous fusions of the vegetative cells allow this species to be included in the subfamily Mastophoroideae.