Birds and dinosaurs, dinosaurs and birds, did one evolve from the other or did they evolve in similar environments under similar conditions? Man may never know, but there are certain similarities between the two that are interesting to note.
First and foremost, as every schoolchild knows, both birds and many early dinosaurs were oviparous, egg laying. The fossil evidence indicates that they built nests, laid eggs and then cared for their offspring similar to the ways birds do today. Of course dinosaurs had bigger nests, bigger eggs and probably more voracious offspring than most birds, but much of the reproduction was similar!
The bone structures of many types of dinosaurs was very similar to birds. Dinosaurs can be separated into two basic categories based upon their pelvic bones: ostrich hips and lizard hips. The ostrich hips generally refers to those dinosaurs that ran around on two legs, T. Rex, Iguanodon, and such. Three-toed feet and even the shape and function of the knees show similarities between dinosaurs and many birds. This does not mean that the two were related, merely that this type of bone structure was best for going about on two legs!
The bones of many dinosaurs also appear to have been hollow, which is also seen in birds. This allows for sufficient strength and yet light weight. It also means the dinosaurs were likely breathing using air sacs and lungs similar to those found amongst avian species. Avian lungs are fixed and the muscles pull in oxygen as the animal moves. This is very different from the diaphragms found in mammals.
Skin is another shared similarity between dinosaurs and birds. Under all those feathers, the epidermis of birds is pebbly and rough, just like the dinosaur skin imprints seen in fossils. Both creatures have scales (Look at the legs of birds, they are scaly and not feathered!). Recently, fossilized skin has been found that shows some dinosaurs actually had small feathers.
Similarities between dinosaurs and birds continue. The shape of the skull, the size and position of the eyes and even the nasal cavities are all very close in shape and proportion between the two species. This indicates either a common ancestor, very similar environment, or that one possibly evolved from the other. Did the dinosaurs become birds?
Based upon the similarities, it is possible. What little is known about dinosaur DNA indicates that it is very similar to birds, but then DNA from existing mammals, reptiles and birds appear closely related. That can indicate a similar beginning, but not that one necessarily became the other. All the similarities does is stimulate the imagination and cause one to wonder why the similarities are there.