Family ASPIDOSTOMATIDAE Jullien, 1888

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Family ASPIDOSTOMATIDAE Jullien, 1888

Compiler and date details

July 2001 - Dr Philip Bock

Introduction

Jullien (1888) created this family for Aspidostoma Hincks (1881), then monotypical for A. giganteum (Busk, 1854). Levinsen (1909) assigned his new genera Labioporella and Crateropora to the Aspidostomatidae and presented a detailed morphological account of the type species of all three genera. Harmer (1926) removed Labioporella to its own family, but described a new aspidostomatid genus, Lagarozoum.

Colonies may be encrusting, unilaminar; or erect, either as bilaminate plates, which may be fenestrate, or with a regular vinculariiform structure. All species are heavily calcified; gymnocystal calcification is minimal or absent, the vertical walls and complete frontal shield consisting of granular cryptocyst. The distal edge of the frontal shield dips basally and is then reflected, defining a vertical tube through which the polypide passes on eversion. Depressor muscles pass through pronounced opesiular slots on each side of the tube.

Aspidostoma species occur in Tertiary deposits of New Zealand, Australia and Patagonia. Four living species are known: A. giganteum (Busk) in the southwest Atlantic, south to the South Shetland Isles; A. lividum Hayward & Cook (1983), from eastern South Africa; A. cylindricum Harmer (1926), from Indonesia, and the endemic Antarctic species A. coronatum (Thornely 1924). Crateropora includes three Recent shallow-water species known only from Indonesia, whereas Lagarozoum is monotypical for L. profundum Harmer (1926), also from Indonesia. Larvapora Moyano (1970) is presently monotypical for the Antarctic endemic L. mawsoni (Livingstone 1928; Hayward 1995).

Autozooids are large, often exceeding 1 mm in length, convex and thickly calcified. The membranous frontal wall is completely underlain by cryptocyst, the operculum more or less coinciding with the short, arched opesia. There is typically a longitudinal median ridge on the cryptocyst, terminating at the proximal edge of the opesia, where the edge of the cryptocyst narrows, dips basally and is reflected to form a smooth, flat or concave lip. The distal edge of the opesia is considerably raised and with its proximal lip defines a vertical tube, termed the 'polypide tube' by earlier authors. Frontal membrane depressor muscles pass on each side of the polypide tube, through defined opesiular slits. Characteristically, the lateral and distal margins of the opesia may be produced into stout, columnar or palmate processes in later ontogeny. The frontal shield has indistinct marginal pores, and scattered frontal pores; vertical walls are perforated by uniporous or multiporous pore plates. The ovicell is recumbent on the distally succeeding autozooid, partially immersed within it in Crateropora and completely immersed in Larvapora. In Aspidostoma it is globular and prominent, with a membranous ectooecium, and a coarsely granular, imperforate entooecium. It has an independent aperture, not closed by the autozooid operculum. Adventitious avicularia are present in Crateropora and Aspidostoma, though only sporadically in the latter; larger vicarious avicularia occur in Lagarozoum and Larvapora.

Both Aspidostoma and Crateropora have a fossil record in the Victorian Tertiary (Maplestone 1902, 1911; MacGillivray 1895).

Diagnosis

Colony encrusting or erect, bilaminar, fenestrate or vinculariiform. Zooids very large. often heavily calcified. Cryptocyst extensive, opesia small. Paired opesial grooves flanking the proximal cryptocyst above a 'polypide tube'. Avicularia interzooidal or vicarious. Ovicells immersed or prominent and hyperstomial with a separate orifice.