As I discussed in the previous post, the fossil record tells a story that at first seems implausible: birds are descendants of dinosaurs. Part of what’s surprising about this idea is that dinosaurs typically appeared very reptilian, whereas birds do not.
Without providing a formal definition of “reptile”, you probably have a general image in your head. This is because reptiles possess a suite of characteristics that intuitively unite them into a group. For instance, reptiles are covered in scales, walk around on all fours, have tails, typically have simple conical teeth and warm up by basking in the sun.
Birds, by contrast, are covered in feathers, walk around on just their hind legs and/or fly, lack tails, have no teeth whatsoever, and are able to generate their own heat, similar to mammals.
Anybody that is at least vaguely aware of animal diversity probably would never mistake a bird for a reptile. So besides a pattern in the fossil record and remnants of tooth genes in their genomes, is there any other evidence that birds are descended from dinosaurs and, ultimately, other reptiles?
One line of evidence comes from comparisons of DNA. When researchers have compared the genes of birds, reptiles, and other animals, they find something that perfectly fits the conclusion of the fossil record: birds are genetically nested within reptiles. In fact, crocodilians are more genetically similar to birds than they are are to turtles or lizards. As just one example of a study that demonstrates this, Chiari et al.  compared 248 genes, with a total of 187,026 letters of DNA, among multiple species of reptiles, birds and other vertebrates and found this very pattern:
The green branches on this phylogenetic tree indicate lizards, red are turtles, blue are crocodilians and purple are birds.
The link between crocs and birds isn’t entirely surprising to anatomists, who have long remarked that modern and ancient crocodilians share a number of traits with dinosaurs, including teeth set in sockets (thecodonty), holes in the skull in front of the eyes and in the lower jaw (antorbital and mandibular fenestrae), and an extra ridge (trochanter) on the femur. However, birds no longer have most of these traits, and the one trait that they do have (antorbital fenestrae) is not found in modern crocodilians. As such, this conclusion was not always intuitively obvious.
Nonetheless, here we have an excellent example of where DNA and fossils tell the same story. Fossils appear to document a transition from large reptilian progenitors to modern birds and DNA suggests that birds are not only relatives of reptiles, but are descendants of reptilian ancestors shared with crocodilians, turtles and lizards..
Questions for Creationists
Why do birds have DNA more similar to crocodilians than crocodilians do to turtles and lizards? Is it just a coincidence that bird DNA and the fossil record seem to be telling the same story, that birds are descended from reptiles? What kinds of evidence might overturn this hypothesis?