The possession of scales, laying of eggs with shells and bony structure of the head…

Explanation

Its long been recognized that birds and reptiles share many characteristics:

Both have skulls that abut the first neck vertebra with a single ball-and-socket joint.

Both birds and reptiles excrete nitrogenous wastes as uric acid; mammals excrete urea, fish ammonia.

Both lay similar highly yolked eggs
Bird feather is homologous to the reptile scale. Both develop from an epidermal elevation over a nourishing dermal core. Scales flatten, feather buds roll into a cylinder shaped tube

There is an explanation video available .