Question

Scales of fish, reptile, bird, and mammal. Which scales are homologous (derived from same ancestral/embryonic structure) and which are not?

What embryonic layer they are from (dermal/mesoderm; epidermal/ectoderm)?

Answer 1

Scales of Fish: a) Placoid scales: It is found on cartilaginous fish, modern and early shark. Outer layer made up of vitrodentine(enamel like substance) and inner layer made up of dentine and pulp cavity included nerves and blood vessels. It have a toothlike structure. b)  Cosmoid scales:It is found on Sarcopterygians. Hard Outer layer made up of enamel and inner layer made up of cosmine and vascular layer isopendine.c) Ganoid scales : It is found on gars, bichirs, and reedfishes .Outer layer made up of ganoin (enamel like substance) and inner layer made up of cosmine and bony vascular layer. d)Cycloid scales : Thin and translucen bony scale have outer smooth edge found on  salmon and carp e) Clenoid Scale: like a cycloid scales have a tooth outer edge found on bass and crappie.

Scales of Reptile:Lizard Scales:Tubular,platelike and Overlapping.Outer sarface is horny and keratinized and underline with bony plates called as osteoderms.

Snake scales: entire body of snake is covered with scales and scutes which lack the osteoderms and scutes also found on turtle and crocodiles unlike the overlapping structure of snakes scales.Same dermal scutes are found in the feet of birds and tails of some mammals and are primitive form of dermal armour in reptiles.

Scales of Birds(Avian): The scales and scutes of birds were similar and homologous to those of reptiles,

Scales Of Mammals: Scales are consist of keratin and found on pangolin. Foot pad epidermal tissues in most mammal species homologous to avian reticulae are formed from stunted feathers.

Scales ofsome mammals and avian are homologues to each other but non homologous to fish and reptiles. Reptiles and mammals have same genes for scale but scales are not homologous.

During embyonic develompment scales are derived from the epidermal layer of mesoderm of embryo .