A review of cosmine: its structure, development, and relationship to other forms of the dermal skeleton in osteichthyans

Cosmine is a unique association of skeletal tissues and soft, presumably sensory, structures. The evolutionary and biological significance of cosmine in the lobe-finned fishes has been a continuing source of controversy in the systematics of this group. The wide variation in cosmine morphology and the inferred developmental pathways producing this variation have not been comprehensively studied by those interested in the systematics of the lobe-finned fishes. In this study, I examine the different arrangements of tissues that cosmine exhibits in dipnoans and “rhipidistians.” I discuss the ontogenetic pathways by which such arrangements could have been achieved and show that a continuum of developmental processes existed between cosmine sensu stricto and other configurations of the dermal skeleton, such as those in coelacanths and actinopterygians. Particular attention is paid to the proposed role of the pore-canal network in the development and maintenance of cosmine. Although the phylogenetic significance of cosmine remains unclear, I conclude that understanding the developmental constraints on the dermal skeletal system provides a framework for reconstructing pathways of evolutionary change.