Family METRARABDOTOSIDAE Vigneaux, 1949

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• Metrarabdotosidae Vigneaux, M. 1949. Révision des Bryozoaires néogènes du Bassin d'Aquitaine et essai de classification. Mémoires de la Société Géologique de France, n.s 28: 1-153.

Introduction

First established by Vigneaux (1949), with an emended diagnosis and taxonomic review provided by Cheetham (1968), the Metrarabdotosidae is recognized as a northern hemisphere family. It has an extensive fossil history in the Tertiary deposits of western Europe, North Africa, eastern U.S.A. and the Caribbean. Living faunas occur off West Africa and in the Caribbean and tropical West Atlantic. Polirhabdotos inclusum (Waters), an endemic Antarctic species, was assigned to the family by Hayward & Thorpe (1987).

Species of Metrarabdotosidae form encrusting or erect, branching colonies. The frontal wall of the zooid is an umbonuloid shield, bordered by conspicuous marginal pores; the primary orifice is obscured by a complex peristome bearing denticles and/or ridges on its inner surface. Adventitious avicularia occur. Embryos are brooded in gonozooids, differing from autozooids in having an immersed, globular brood chamber, partly separated from the main cavity of the zooid by a low shelf.

The Antarctic species Polirhabdotos inclusum forms slender, cylindrical, dichotomously branching colonies. The rather elongate zooids are arranged in whorls of three to five. The frontal shield is coarsely nodular, with large and conspicuous marginal pores. The primary orifice is hidden by a deep peristome, with a distinctive ontogeny: initially D-shaped, the orifice develops a broad proximal lip with sharp processes defining a circular median foramen. The lip deepens, developing paired ridges which form a frontally lengthening gutter, and finally a lateral suboral avicularium develops on the outer rim of the peristome. Gonozooids are broadened distally, with a projecting convex hood marking the position of the brood chamber.

Polirhabdotos inclusum appears to be a slow-growing, long-lived species. Colonies may exceed 40 mm high, with gonozooids in successive independent whorls, suggesting multiple reproductive seasons.

Diagnosis

Colony encrusting or erect, branched, heavily calcified, often deeply pigmented. Zooids large, with umbonuloid frontal shield, surrounded by prominent areolae. Secondary orifices with distal oral shelf and proximal pseudosinus. Avicularia oral, adventitious. Brooding zooids dimorphic, with enlarged distal chamber and wide orifice.

General References

Cheetham, A.H. 1968. Morphology and systematics of the bryozoan genus Metrarabdotos. Smithsonian Miscellaneous Collections 153: 1-121

Hayward, P.J. & Thorpe, J.P. 1987. The systematic position of Smittina inclusa Waters, an endemic Antarctic Bryozoan. Journal of Natural History 21: 1469-1476

Vigneaux, M. 1949. Révision des Bryozoaires néogènes du Bassin d'Aquitaine et essai de classification. Mémoires de la Société Géologique de France, n.s 28: 1-153

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