The Scleral Ossicles of Opisthocomus and Their Phylogenetic Significance

Dried scleral rings of Opisthocomus, Galliformes, Cuculiformes, and other birds were examined to determine the pattern of ossicle overlap in Opisthocomus and its bearing on the phylogenetic relationships of this taxon. Although Opisthocomus shares a derived number of 12 scleral ossicles with cuculid Cuculiformes, the pattern of ossicle overlap differs. Nevertheless, fewer modifications are required to derive the number and pattern of ossicles in Opisthocomus from the conditions in cuculid or musophagid Cuculiformes than from the conditions in any galliform. Our findings also indicate that the scleral rings of birds, unlike those of lizards, often do not conserve overlap relations between adjacent ossicles during phylogeny.