Ecophenotypic asymmetry in the middle Danian brachiopod Obliquorhynchia flustracea caused by adaptation to attachment on the coral Dendrophyllia candelabrum

The rhynchonellid brachiopod Obliquorhynchia flustracea from the middle Danian (Paleocene) cool-water coral mounds of the Faxe Formation, Denmark, exhibits commissural asymmetry, a rare feature in articulate brachiopods. It has been much discussed whether the underlying reasons for this asymmetry were species specific caused by pedicle attachment in the limited space between the branches of the scleractinian Dendrophyllia candelabrum. The two species are always found together, and the brachiopod is essentially missing outside facies characterized by D. candelabrum. Based on morphological analyses and statistical tests, we suggest that ecophenotypic variation is the main reason for the variability in the external shell morphology and development of commissural asymmetry. Conditions such as sexual dimorphism and defects can be dismissed as a facilitating reason of phenotypic variation. This is a rare case where it is possible to directly link commissural asymmetry to mode of attachment on a specific host.