| Abstract: | Members of the bryozoan family Petraliellidae share the capacity to develop basal rhizoids, which anchor the unilaminar, semi-repent parts of the colonies above the substratum, and enable them to overgrow other, competing sessile forms. Little is known of the larval behaviour and settlement, or the early astogeny of species. Ancestrulate colonies of the Australian Tertiary lunulitiform species Smittia biincisa are referred to the genus Riscodopa, and together with Riscodopa paucipora sp. nov. are described and compared with the Recent species R. cotyla and R. parva from New Zealand, and with R. hyalina sp. nov. from New South Wales, Australia. All the Recent species are known to develop basal rhizoids, and an early astogeny similar to that of many other small, rooted bryozoans, comprising the post-metamorphosis development of a binary complex, including rhizoid and feeding elements, is inferred for Riscodopa. Observations on living Hippopetraliella magna from Queensland suggest that both the ancestrular morphology and early astogeny show a capacity for semi-repent growth, even though they do not include rhizoid development. Larvae metamorphose without direct attachment, and the ancestrula develops elongated, partially calcified supporting processes, which raise the early stages of growth above the substratum. A similar kind of ancestrula has been found in preserved specimens of Mucropetraliella ellerii. |
