SHALLOW-WATER SPOROLITHON RHODOLITHS FROM NORTH ISLAND (NEW ZEALAND)

Shallow-water Sporolithon rhodoliths from New Zealand are described here on the basis of shape, size, composition, internal structure, and major taphonomic attributes, with the aim of discussing their significance in the framework of current ecological and paleoecological models of rhodolith formation and accumulation. The very shallow water environment (<2 m) of the Whangaparaoa Peninsula undergoes daily tidal currents and seasonal storm conditions. These factors, along with the availability of cobbles and pebbles as suitable substrate, lead to the accumulation of spherical, fruticose, monospecific, nucleated, and internally compact rhodoliths. The observed taphonomic features include apical abrasion, intercalary growth of protuberances after mechanical breakage, and multiple growth stages with distinctive bioerosion (by bivalves, annelids, and cyanobacteria), which are identified by internal abrasion surfaces and dark layers. A review of the pattern of global distribution of the coralline genera comprising very shallow-water rhodoliths (<2 m) identifies five major categories: (A) unattached protuberances, commonly monospecific, made up of one or more genera (Phymatolithon, Lithothamnion, Lithophyllum) in cool to cold waters at middle to high latitudes; (B) rhodoliths composed of dominant Hydrolithon and other mastophoroids (Neogoniolithon or Spongites) with subordinate Lithophyllum in the tropics; (C) unattached protuberances of Neogoniolithon associated with seagrass meadows, from middle latitudes, in warm to warm-temperate waters; (D) mainly fruticose, monospecific, rhodoliths composed of Mesophyllum, Lithothamnion, Hydrolithon, and Neogoniolithon under tidal currents in cool waters in the Southern Hemisphere; and (E) Sporolithon rhodoliths of cool waters in the Southern Hemisphere, which suggest an austral polar emergence of the genus.