Family BIFAXARIIDAE Busk, 1884

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Introduction

The Bifaxariidae was introduced by Busk (1884) for Bifaxaria (Harmer 1957; Hastings 1966). The family is characterised by rooted erect colonies, branching in one plane, in which the zooids are arranged back to back and face in opposite directions. Uniquely among living ascophorines, the zooids have an outer, porous umbonuloid shield over an inner costal shield, although the costae can be reduced or lacking. Generally, the umbonuloid wall is in two halves with a median longitudinal suture between, each half wrapping around the sides of the parent zooid and sometimes extending over neighbouring zooids. The secondary orifice depends on the extent of peristomial development. As in all umbonuloid species, the primary orifice is uncalcified and the operculum is contiguous with the membranous frontal wall and often concealed by the peristome. There are no oral or other spines. Characteristically, a pair of lateral oral avicularia, arising from small marginal septular pores, occur one on either side of the orifice. In some genera, additional avicularia occur, their subrostral chambers contributing to branch thickening. The ovicell is recumbent, immersed in secondary calcification, and opens into the peristome (Gordon 1988).

Currently, only two species belonging to two different genera have been recorded in the Australian region. Raxifabia Gordon (1988) is represented by Raxifabia tunicata, found at 1573 m. depth on the western slope of the Lord Howe Rise. Raxifabia has only a single suboral pair of costal spines, unlike Bifaxaria, which has a concealed costal shield composed of several flattened spines. The second genus, Diplonotos Canu & Bassler (1930) is represented by Diplonotos novus, from 1280 m. on the Macquarie Ridge south of Macquarie Island (Gordon 1988). Diplonotos has a primary costate frontal shield like that of Bifaxaria, with four pairs of costae, but differs in the extent of the spines. Whereas the colonies of Bifaxaria and Raxifabia are jointed and candelabriform, those of Diplonotos and the other three genera introduced by Gordon (1988) (Aberrodomus, Domosclerus, Smithsonius) are unjointed and generally have pinnate or semireticulate branching.

The family is apparently an ancient one - it is known with certainly from at least the Lower Eocene (Taylor 1993).

In the possession of a double-shielded morphology, this remarkable, mostly deepsea, family shares a characteristic of some Upper Cretaceous Cribrilinidae, from which it is apparently derived (Gordon & Voigt 1996).

Diagnosis

Colony erect, with proximally jointed branches, lensoid in cross section, the zooids laterally compressed and carinate. Orifice broad, transverse, with a raised proximal rim. Primary spinocyst of several pairs of separated spines. Umbonuloid wall with a median suture and rows of pores. Ovicell immersed. Uniporous mural septula present (Gordon, 1988).

General References

Busk, G. 1884. Polyzoa. Pt. I. Cheilostomata. Report on the Scientific Results of the Voyage of H.M.S. Challenger 1873–1876, Zoology 10: xiv, 216

Canu, F., & Bassler, R.S. 1930. The Bryozoa of the Galapagos Islands. Proceedings of the United States National Museum 76: 1-78

Gordon, D.P. 1988. The bryozoan families Sclerodomidae, Bifaxariidae, and Urceoliporidae and a novel type of frontal wall. New Zealand Journal of Zoology 15: 249-290

Gordon, D.P. & Voigt, E. 1996. The kenozooidal origin of the ascophorine hypostegal coelom and associated frontal shield. pp. 89-107 in Gordon, D.P., Smith, A.M., & Grant-Mackie, J.A. (eds). Bryozoans in Space and Time. Wellington : NIWA.

Harmer, S.F. 1957. The Polyzoa of the Siboga Expedition. Part 4. Cheilostomata Ascophora II. Siboga-Expéditie Report 28D: 641-1147

Hastings, A.B. 1966. Observations on the type material of some genera and species of Polyzoa. Bulletin of the British Museum (Natural History) Zoology 14: 57-78

Taylor, P.D. 1993. Bryozoa. pp. 465-489 in Benton, M.J. (ed.). The Fossil Record 2. London & New York : Chapman & Hall 845 pp.

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