Article: Hexaphyllia: a spiny heterocoral from Lower Carboniferous reef limestones in Derbyshire, England

Exceptionally well preserved and abundant corallites of Hexaphyllia are recorded from limestones of the Lower Carboniferous Castleton Reef Belt, north Derbyshire, UK. Details of corallite morphogenesis are presented and the growth attitude of corallites is determined. Tabulae curve down at their margins and fuse together to form the tabulotheca. Conversely, spines curve upward and point in the direction of corallite growth. Soft tissue reconstructions infer the presence of polyps sitting exposed upon and totally enclosing the distal tips of the corallites, with polyp lobes extending down their sides. Much of the corallite is therefore regarded as endoskeletal in origin. Rows of spines projecting from between the polyp lobes gave some degree of protection to the exposed polyps. Assemblages of corallites from different positions in the reef show notable differences in morphology. Variations in shape, wall thickness and tabulae spacing are attributed to contrasting growth rates at different positions within the reef. Examination of approximately 1300 corallites from two localities in the reef reveals the presence of a single species, Hexaphyllia marginata (Fleming), which shows considerable intraspecific variation. Systematic studies indicate that criteria used to distinguish Hexaphyllia species in the past are invalid and that the majority of previously described taxa are junior synonyms of H. marginata. Heterocoral mode of life is discussed in the light of observations made on this species.