The birds are the phylum chordate, from the class Aves. They are the avian dinosaurs and birds are technically considered as reptiles because they descended from certain diapsids dinosaurs. The earliest species of the class Aves to be known is Archaeopteryx. They are characterized by the bony beak without teeth, the laying of hard – shelled eggs, high metabolic rate and a light strong skeleton. The most important characteristic of the bird is flight or that they fly.
The dinosaurs were the subphylum of chordates, specifically, those with backbones or spinal columns that conquered the terrestrial ecosystem over 160 million years. They are a group of diapsids reptiles that firstly evolved from archosauriform ancestor during the olenekian, like modern crocodilians. They first appeared about 230 million years ago, and became extinct 65 million years back at the time of cretaceous, and they were no longer dominant on land. The clade maniraptoran is the group of the theropod dinosaurs that many paleontogists believe birds were derived from some 150 or so million years ago, in the Jurassic period on the other side of the phylogenetic taxonomy birds are by definition maniraptoran and other maniraptoran are their closest relatives.
The birds are the direct descendants of meat eating dinosaurs (eoraptor). However, the dinosaurs are not birds or reptiles at all. They are a separate special group, having straighter legs. They had unique movement, walked with their legs underneath them. The little dinosaur meat-eaters were probably the ancestors of birds. The dinosaurs also laid eggs, even though their eggs differ somewhat from those of birds, and at least some dinosaurs nested in colonies and cared for their young in the nest as the birds do, they take care of their young one until they are able to fly on their own.
The birds and the dinosaurs were comparable, because the birds have hollow bones, but so did pterosaurs. Dinosaurs walked upright as birds do and meat-eating dinosaurs have very similar skulls to birds. Some of the smaller dinosaur meat-eaters have bodies and arms similar to birds' as well. The first bird, archaeopteryx, looks nearly identical to the small meat-eating dinosaur of the same time, compsognathus, except it has feathers.
The birds share many characteristics with their extinct relatives and those characteristics enable them to fly. ‘The birds have evolved remarkable specializations for flight, they resembled the flight from the Ornithomimids and other small theropods.: a unique "one-way" breathing system, light yet strong hollow bones, a skeleton in which many bones are fused or lost, powerful flight muscles, and -- most importantly – feathers’.
1. [http://teacher.scholastic.com/researchtools/articlearchives/dinos/evolut.htm, accessed on 10/05/2006, 09:00].
2. [http://learn.amnh.org/courses/dinosaurs.php, accessed on 10/05/2006, 09:40].
3. [http://en.wikipedia.org/wiki/Dinosaurs, accessed on 10/05/2006, 12:00].
4. [http://en.wikipedia.org/wiki/Phylogenetics, accessed on 10/05/2006].