Feathers are complex structures designed for are found only in birds today. The theory scale to feather evolution postulates that scales were evolved in feather during course of time in birds from reptiles.Scientists put many supportive evidence in favor of evolution. Important ones are as follows:
1. Archaeopteryx : Archaeopteryx is an extinct bird like animal which is regarded as connecting link between reptiles and bird , suggesting that birds were evolved from the reptiles. Archaeopteryx shares characters of both bird and reptiles. It has teeth & tail like of reptiles and wings like birds. So perhaps scales of reptiles stretched out from generation to generations resulting in wing capable of flight.
2. Skeleton similarity between Birds and Theropods dinosaurs: Theropods were animals having short forelimbs and walk on their Hind limbs. It is believed that birds are living descendants of theropods due to similarity in skeleton structure. Sinosauropteryx a theropod was having feathers, thus supporting the evolution of birds from dinosaurs.
3.Developmental evidence: Both feathers of birds and scales of birds develop from Placode. Placode is an epidermal thickening of dermal cells which in later stages of development develops into either scales or feather thus sharing same developmental ancestors.
Evidence against evolution.
1. Archaeopteryx : Archaeopteryx feathers are nearly identical feathers of today birds and even before Archaeopteryx fully functional feathers were there, so in a sense it does not provide any conclusive evidence on origin or evolution of feathers. however it has other common characters of birds and reptiles.
2. Morphological difference between Scale & feathers : A morphological difference exist between both. And to evolve from scales , a number of transition stages are required. Evidence of such transition stage are not yet discovered in fossils.
3. Developmental Stage: Leaving the common origin of feather & Scales , both follow very distinct path in their development. Further feathers have more similarity with hair both morphologically and biochemically.
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