Fossils show a great record of the adaptation from the fully aquatic morphology of fish to the semi-aquatic amphibian morphology. There are many fossils of fish that had increased thickness and strength of the shoulder girdle, and more limb-like front fins. If you look at the fossil of Tiktaalik you will notice that it has eyes and nostrils on the top of its head, much like an alligator or crocodile. Its front limbs and pectoral and shoulder muscles are also well developed. There are other lungfish that clearly show above water respiratory capabilities as well.

The bee hummingbird (Mellisuga helenae) is "the smallest dinosaur that ever lived". Of course your question only applies to non-avian dinosaurs.

For over a century the smallest known dinosaur was the 3 foot Compsognathus, otfen compared to chickens in size. The discoveries in China for the last 20 years have revealed many smaller dinosaurs. Currently the smallest known dinosaur is Epidexipteyx, at only 1 foot long.

The smallest dinosaur that ever lived on earth was "westlothiana," it is almost the same size as a fully grown human's toe!

The first reptiles evolved from amphibians about 310-320 million years ago. It is known that amphibians live life in both water and land, but have to return to the water to lay their soft eggs.

As amphibians differentiated one group known as reptiliomorph labyrinthodonts became more accustomed to the dry land. They spent more time on land than in water. Eventually they adapted by growing hard, dry skin rather than the soft, wet skin of amphibians. They also adapted by laying hard shelled eggs on land rather than laying soft eggs in the water.

The first reptiles were largely overshadowed by the larger amphibians, which were more diverse and dominant prior to the Permian mass extinction. Hylonomus is the oldest-known reptile, and was about 8 to 12 inches (20 to 30 cm) long. Westlothiana has been suggested as the oldest reptile, but is for the moment considered to be more related to amphibians than amniotes.