Article: Sensory spines in the Jurassic brachiopod Acanthothiris
M. J. S. Rudwick
The rhynchonelloid Acanthothiris has tubular external spines similar to the spines of productoids and chonetoids. Their structure, arrangement and mode of growth are described in detail, on the basis of a study of exceptionally well-preserved specimens of A. spinosa. They are compared to the spines of the related genus Acanthorhynchia, and to those of the living oyster Crassostrea echinata. On the basis of the known sensory physiology of living brachiopods, it is inferred that the spines near the valve-edges of Acanthothiris contained extensions of the mantle, and that their tips bore portions of the highly sensitive mantle-edge tissue. These sensitive spine-tips could have provided the brachiopod with effective 'early warning' protection against potentially harmful agents in the environment; and the radiating spines themselves could have formed a protective grille straddling the apertures leading into the mantle cavity. Spines further from the valve-edge are blocked, and are regarded as having been superseded functionally. This interpretation is discussed in relation to the spines of other brachiopods and Crassostrea, and in relation to the ontogeny, ecology and phylogeny of Acanthothiris.