Systematics and life history of antoniettella exigua, a new genus and species of cribrimorph bryozoan from the miocene of East Kalimantan (Indonesia)

A new genus and species of cheilostome bryozoan, Antoniettella exigua gen. et sp. nov., is described from East Kalimantan (Indonesia) in rocks around the Burdigalian-Langhian boundary (Lower-Middle Miocene). Colonies encrust the undersides of platy scleractinian corals that formed patch reefs in turbid shallow waters. Included in the cribrimorph ascophorans, the monospecific genus Antoniettella differs from all existing cribrimorph genera and can be distinguished by a combination of its keyhole-shaped orifice and lack of pelmata, intercostal lacunae and ovicells. Colonies are always small in size, subcircular in outline, thick and usually multilayered, with new layers spreading outwards from the centre of the colony. Ecological categorization of Antoniettella as a 'spot colony' is discussed and its life history inferred. A survivorship curve suggests low initial rates of mortality followed by a constant rate of mortality. Colonies appear to have been well adapted to occupy and defend small patches of substrate space, possibly in a low-nutrient microenvironment.