These new specimens clinch the argument. Archaeopteryx is no longer on its own, a single species that attests to the reality of an evolutionary transition from reptiles to birds. Below it, on the evolutionary tree, stretch countless theropod dinosaurs that become ever more birdlike through time, and above it stretch numerous bird species that bridge every step of the way from Archaeopteryx to fully-fledged birds. A long series of fossils through the Jurassic and Cretaceous periods, a span of 140 million years, document the evolutionary transition from reptile to bird. Mammals can be traced back to reptilian origins.
Jaws tell the story of reptile to mammal transition.
The evolutionary route from reptile to mammal is known in just as much detail. Between the Permian and Triassic periods, mammal-like reptiles evolved from basal forms that were fully reptilian. Through dozens of intermediate steps they evolved into mammals by the Late Triassic, some 225 million years ago. In reptiles, both today and in the past, the jaw joint lies between the articular bone at the back of the lower jaw, and the quadrate bone in the skull. In mammals, on the other hand, the jaw joint is between the dentary and the squamosal element of the skull.