Type species: Riscodopa parva Gordon, 1989.
Description
Colony discoid, free living, budded radially, anchored by rhizoids originating from basal septular pores. Ancestrula either tatiform or similar to succeeding zooids. Zooid oriŽces with oral spines, paired lateral denticles and a proximal lyrula. Avicularia lateral and oral, sometimes proximal and associated with a mucro. Ovicell hyperstomial, prominent, with numerous small pores or tubercles, known or inferred not to be closed by the operculum.
Remarks
Species here assigned to Riscodopa fall into two groups; the Recent species, which have a tatiform ancestrula, and the fossil species, which do not. Genera which include species with diVerent kinds of ancestrula have been documented (Cook, 1985: 51), and the genus Mucropetraliella, which is usually characterized by the presence of a proximal mucro and avicularium, does include some species in which these do not occur (Cook and Chimonides, 1981a: 117). Gordon (1989: 57) stipulated that there was ‘no suboral aviculiferous mucro’ in Riscodopa. A mucro occurs in R. biincisa and R. paucipora, but is absent from R. parva, R. cotyla and R. hyalina. However, the similarities in colony form and basal septular pores suggest that all the species described here may be contained within the genus Riscodopa. In all species, the ancestrula is surrounded by a circlet of Žve or six primary autozooids. These appear to be budded as a distal pair or triad, followed by a proximal triad, which may be derived equally from the lateral zooids as well as from the ancestrula. Riscodopa parva and R. cotyla live, anchored by basal rhizoids, on particulate substrata in deep water (477–4059 and 320–660 m, respectively), from the New Zealand shelf; R. hyalina occurs oV the coast of New South Wales from 429 to 503 m. The numerous other fossil bryozoan species associated with R. biincisa and R. paucipora in the Tertiary samples from Victoria suggest, in contrast, that although similar kinds of particulate sea-bottom occurred in Australian Tertiary seas, it is very unlikely that any of these fossil forms lived in deep water (Wass et al., 1970). Similar apparent discrepancies between the known depth range of Recent species, and the depths inferred for their fossil congeners, occur among several groups of species in the Australian Tertiary, and have been noted for Selenariopsis (Bock and Cook, 1996), Siphonicytara (Bock and Cook, 2001) and for Chelidozoum and other species (Bock and Cook, in press). One possible explanation is that the hydrodynami c conditions of the shelf environments of the mid-Tertiary were much less agitated by wave activity than the modern shelf of southern Australia. This in turn, may be related to the narrower zone of open ocean that existed between Antarctica and Australia, or that the severe storms in this zone were less frequent or less intense during this interval of milder climate. The lack of strong bottom current or wave activity is also re ected in the presence of clay-rich sediments over large intervals and areas in the Tertiary, whereas clay-rich sediments are generally little-represented on the modern continental shelf.