On cosmine: its origins, biology and implications for sarcopterygian interrelationships
Cosmine is a combination of tissues (enamel, dentine, and bone) and a structure (pore-canal system) not found in living vertebrates. Long considered a synapomorphy of sarcopterygians (lobe-finned fishes), new discoveries and phylogenetic interpretations have shown that the assembly of cosmine is a deeper evolutionary event than previously thought and that cosmine constitutes a primitive feature of osteichthyans (bony fishes). A precursor of both cosmine and ganoine might have been present in the last common ancestor of sarcopterygians and actinopterygians (ray-finned fishes). However, despite numerous and thorough studies on cosmine since the late 19th century, its role and biological properties are still a mystery. Currently, the most consensual hypothesis considers cosmine as a metabolic regulator of mineral ions through intricated processes of deposition and resorption of dermal tissues. A new ontogenetic and phylogenetic scenario is emerging from microtomographic data revealing that a pore-canal system initially associated with multiple superimposed odontodes in stem osteichthyans and early sarcopterygians was reorganized and simplified in rhipidistians into a uniform cosmine sheet with a single layer of enamel overlying the dentine (eucosmine). In this review, I will summarize the current knowledge of cosmine origins, structure, development, and possible function based on previous research. I will also present new histological data on poorly known early sarcopterygians to highlight the importance of histological studies for upcoming analyses of sarcopterygian interrelationships. Future efforts to unravel the evolution of cosmine will need new virtual and three-dimensional approaches to paleohistology that will change our current framework of studying early vertebrate evolution and certainly revolutionize our understanding of this distinctive feature.