Colonies are multiserial and erect, forming large bifoliate expansions that may be folded. Early astogeny has not been described in material from the Crags.
Autozooids average about 0.50 mm long by 0.24 mm wide, and in outline shape are typically rectangular to very slightly rhomboidal (i.e. attaining a maximum width about mid-length). They are arranged in well-defined rows separated from those of adjacent rows by a narrow groove along which the colony may split. The opesia is longitudinally elliptical and occupies most of the frontal surface with the exception of the cryptocyst which forms a narrow, inwardly sloping shelf at the proximal end of the zooid. Cryptocystal calcification is granular, the granules becoming smaller and fewer towards the opesia. Short spinules are sometimes present on the proximal edge of the opesia, often accentuated by overgrowths of epitaxial cement, but there is no obvious plectriform apparatus. Gymnocyst and gymnocystal tubercles are not developed. Ovicells are lacking.
Avicularia are absent.
The erect bifoliate colonies comprising zooids with large, longitudinally elliptical openings (opesia) mark this species out from all others found in the Crags of East Anglia. Material from the Pliocene of Belgium considered by Lagaaij (1952, p. 19) to be conspecific differs in having raised ridges delineating the zooidal boundaries, as well as gymnocystal tubercles at the corners of at least some zooids (see Bishop 1987, figs 1-3).
Despite being one of the commonest erect bryozoans in the Coralline Crag, the correct generic and specific names for this species are both unclear. Acanthodesia is regarded by many as a synonym of Biflustra (see Bishop 1987, p. 6), an issue yet to be settled. Lagaaij (1952, p. 19) considered the Coralline Crag species to be a synonym of Biflustra savartii (Audouin, 1826), originally described from the Recent of Egypt. The paucity of skeletal morphological characters, together with the absence of type material of Audouin’s species, makes it difficult to test this synonymy. Therefore, the species epithet delicatula, which was introduced principally for material from the Coralline Crag, is here employed in preference to savartii.
Interestingly, Busk (1859, pl. 1, fig. 1) also included some Recent material from Australia when introducing Acanthodesia delicatula. While this Recent material bears a striking overall resemblance to specimens from the Coralline Crag, the boundaries between zooids in linear series are much more pronounced and a plectriform apparatus is present, strongly suggesting that the two species are distinct.
Pliocene, Late Zanclean–Early Piacenzian, Coralline Crag Formation, Suffolk, UK.