Extant corals symbiotic with sipunculans, i.e., the caryophylliid Heterocyathus and the dendrophylliid Heteropsammia, develop corallum modifications (in comparison with 'ordinary' representatives of these families) that seem to meet the needs of the coral's worm partner. We distinguish two types of corallum modifications, designated the monoporous and the polyporous types. In the adult monoporous type, the shell inhabited by the sipunculan is usually overgrown only in part by the coral base. There are two orifices: the main one and a smaller pore in the upper part of the corallum. In the polyporous type the shell inhabited by the sipunculan is entirely overgrown and the coral produces a spiralled sipunculan housing. In addition to the main orifice there are several pores in the lower part of the corallum. Heterocyathus priscus sp. n. from the Early Cretaceous (Albian) of France is the oldest example of symbiosis, in which the monoporous-type corallum was modified in the same way as in extant monoporous Heterocyathus. We speculate that the monoporous type was ancestral, as only this type is known to occur among Cretaceous corals. Morphological similiarities between Heteropsammia and certain species of Heterocyathus, such as the Pourtalés plan of septal arrangement and skeleton porosity, may point to a close phylogenetic relationship.

Key words: Scleractinia, Sipuncula, Caryophylliina, symbiosis.

Jarosław Stolarski [stolacy@twarda.pan.pl], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-818 Warszawa, Poland; Helmut Zibrowius [hzibrowi@com.univ-mrs.fr], Centre dOcéanologie de Marseille, Rue Batterie des Lions, 13007 Marseille, France; Hannes Löser, Estación Regional del Noroeste, Instituto de Geología, UNAM, Apartado Postal 1039, Hermosillo, Sonora, México 83000.