Cold blooded animals obtain their body heat from the environment, as their energy output is low and they do not need to sustain activities over prolonged periods. Thermoregulatory mechanism in reptiles depends on low metabolic rate, with little insulation of skin, resulting in rapid exchange of heat with environment. Any insulation on the body will be a handicap for ectotherms because it will prevent body from warming up by external heat. To maintain warmth, feathers evolved in birds and hairs in mammals, both descendants of ancient reptiles and both warmblooded animals.
Discovery of thecodont fossils such as Sinosauropteryx from Sihetun in China prove this theory, because their bodies were covered with two inches long dense filamentous feathers Use of vaned feathers for flight must have been a later development in some small-sized species which grew them in asymmetrical aerodynamic form on the arms and tail and which could then be used for flight.
FOSSIL BIRDS 
FEATHERED, BIRD-LIKE DINOSAURS
In the last few years, many fossils of feathered dinosaurs have been found near Yianxin, in Liaoning Province, China. Two new Chinese feathered dinosaurs dating from between 145 and 125 million years ago (during the late Jurassic and early Cretaceous periods) have been found, Protarchaeopteryx robusta and Caudipteryx zoui. Feathered dinosaur Sinosauropteryx, found a few years ago, also in the same region of China, and the bird-like Unenlagia found in Argentina.
THE OLDEST-KNOWN BIRDS
Archaeopteryx is one of the most famous and oldest-known fossil birds, and dates from the late Jurassic period (about 150 million years ago). Compsognathus was a bird-sized and bird-like dinosaur. Huxley argued that birds and reptiles were descended from common ancestors. In 1986, J. A. Gauthier looked at over 100 characteristics of birds and dinosaurs and showed that birds belonged to the clade of coelurosaurian dinosaurs. [Gauthier, J.A., 1986. Saurischian monophyly and the origin of birds, in The Origin of Birds and the Evolution of Flight, California Academy of Sciences Memoir No. 8].
BIRD-LIKE ANIMALS
In the chain of creatures leading from dromaeosaurid dinosaurs (advanced theropods) to birds, Sinosauropteryx is the earliest bird-like dinosaur. For now, the bird-like animals include the following:
- Protoavis – (meaning “first bird”) genera. Fossils have been found in Texas, USA.
- Archaeopteryx – The oldest known bird had asymmetrical feathers.
- Sinosauropteryx – Sinosauropteryx had a coat of downy, feather-like fibers and it was a bipedal thecodont reptile. Fossils from Sibetun in China.
- Protarchaeopteryx had long, symmetrical feathers on arms and tail. Fossil from China.
- Caudipteryx – a small, very fast runner covered with primitive, symmetrical feathers and therefore it was flightless but feathers were perhaps meant for display.
- Iberomesornis (meaning “Iberian=Spanish intermediate bird”) was a small, early, toothed bird.
- Unenlagia – a much larger ground-dwelling theropod that had feathers but could not fly.
- Beipiaosaurus was 7 feet long bipedal dinosaur, with 2 inches long feathers covering the body and longer feathers on arms and head that did not assist in any way in flight. Beak had thecodont teeth.
- Oviraptosaurus fossil was found from Mongolia, had feathers but could not fly.
- Patagonykus (meaning “Patagonia claw”) was a lightly-built meat-eater with a single, clawed finger on each hand.
- Velociraptor – a larger, ground-dwelling carnivore with a swiveling wrist bone.
- Mononykus (meaning “single claw”) was a small, insect-eater from the Late Cretaceous period.
- Hesperornis (meaning “western bird”) was an early, flightless bird that lived during the late Cretaceous period.
- Ichthyornis (meaning “fish-bird”) was 8 inch (20 cm) long, toothed, tern-like, extinct bird.
- Eoalulavis (from Spain) – the earliest bird that had good maneuverability while flying.
- Diatryma was a giant terrestrial bird whose fossils were found in USA, was carnivore.
- Phororhacos height was about 6 feet, hooked beak and was a feroceous predator.
- Neocathartes was a terrestrial vulture that could not fly.
- Osteodontornis was a giant oceanic bird much like extant albatroses.
- Aepyornis & Mullerornis were elephant birds of Madagascar.
- Dromornis stirtoni fossils were found by Peter F. Murray from Australia.
- Moas (Dinornis) was 10-12 feet in height, became extinct recently in 13th century.
- Shuvuuia was a bird-like bipedal dinosaur with feathers covering the whole body and forelimbs and tail having long feather. But this animal was not able to fly.