Question

For assistance with my studies, what is the homologies of sclaes types between chondrichyans (placoid sclaes), actinopterygians (ganoid sclaes, teleost scales), aquatic sarcopterygians (cosmoid scales), and reptile (epidermal scales)?

Answer 1

Placloid scales - these are the most primitive type of scales which are found in the cartilaginous fishes such as sharks .

Ganoid scales - These are derived from cosmoid scales , and are found in sturgeons and paddle fishes .

Teleost scales - They are silver like scales found in the Actnopterygii . They are seen in fishes that dwells deep into the sea.

Cosmoid scales - These scales are seen in the lung fishes and the Crossopterygii . These are formed probably by the fusion of placoid scales .

Epidermal scales - These are derived from the dermis of the the skin and seen in reptiles and in animals living in the dry lands to prevent loss of excess water from the body .

All of the above mentioned scales , except the reptile scales are homologous , because all of them are derived from dermis , except reptile scales which are derived from epidermis .

Another point of homology is the structure . Placoid , ganoid , cosmoid and teleost have a very similar structure as they are made up of enamel like material , which is then covered with dentine or dentine like material .

Reptiles scales on the other hand is made of keratin .

So , all the fish scales are homologous to each other , but the reptile scales are not homologous to them .