Let me get this out of the way up front: Birds are dinosaurs. Birds are not LIKE dinosaurs. They are not BASICALLY dinosaurs. They are not RELATED to dinosaurs. They are not dinosaurs *nudge*nudge*wink*wink*. They are literally dinosaurs. Period. To this end, scientists use the term “non-avian dinosaurs” when they want to describe the things we called dinosaurs when we were kids but not birds. With a little bit of genetic manipulation, you can actually turn a bird into a more recognizable dinosaur.
Scientists have known this for some time, now, and the evidence is still mounting. Birds have skeletal features that clearly place them within the dinosaur lineage, but there is more exciting evidence. Within the last 20 years, scientists have discovered that many non-avian dinosaurs had feathers and were warm-blooded, just like modern birds. Although birds are the only living lineage of reptiles that are bipedal, they belong to a lineage of dinosaurs that was also bipedal — the “Theropods.” This lineage also includes fan favorites such as the Tyrannosaurus rex and Velociraptor, as well as the Dilophosaurus, Allisaurus, Ornithomimus, and Compsognathus. More specifically, modern birds belong to the “Maniraptora” lineage, which makes them particularly close relatives of the Velociraptor and Deinonychus. As you can see, birds are not classified as dinosaurs by a mere technicality. Birds are buried so deeply within the dinosaur lineage that it is basically impossible to classify anything that we like to call dinosaurs as dinosaurs without including birds. Although birds superficially resemble the pterosaurs, pterosaurs are not the direct ancestors of birds, nor are pterosaurs true dinosaurs. The following cladogram shows the evolutionary relatedness of birds, non-avian dinosaurs, and other reptiles (for help in interpreting cladograms, go here):
The Archaeopteryx is a fantastic transition fossil between birds and their ancestors, possessing traits of both modern birds and the ancient Maniraptors. Like modern birds, Archaeopteryx had large wings with large feathers, and a beak. Like the ancient Maniraptors, the Archaeopteryx had small teeth, and lacked the ridged or “keeled” sternum that serves as an attachment point for the flight muscles of modern birds. The hand bones in the front limbs of Archaeopteryx were not fused together like modern birds, but separated into long fingers just like the Velociraptor.
Modern discussions of the relatedness among organisms typically involve mention of DNA evidence. For example, part of the reason that we know that humans are more closely related to chimpanzees than we are to gorillas is that our DNA is more similar to that of chimpanzees than it is to that of gorillas. The reason that I haven’t mentioned this yet is because we don’t have any. DNA is a fairly stable molecule, but it will break down over time. Under the best conditions, we can sequence DNA that is tens or hundreds of thousands of years old. In warm, humid climates, however, DNA doesn’t last more than a few thousand years — this is why scientists were unable to sequence the genome of the recently discovered “hobbit” fossils (Homo floresiensis), even though they are less than 10,000 years old. It may be possible in the future, but with modern technology no one has ever extracted DNA from a sample 65 million years old.
But DNA is not the only type of molecular analysis. In 2005, a group of paleontologists discovered, wait for it, soft tissue from a T. rex that was still soft and flexible. I’m going to pause for a second to let that sink in. Take all the time you need.
By some fluke of nature, the conditions were perfect for this tissue to remain soft, flexible, moist, and mostly undamaged for the last 68 million years, which is the estimated age of the fossil. My own excitement over this cannot come close to that of the scientists who made this discovery.
Although the scientists were unable to recover DNA from the sample, the proteins were still intact. DNA works by telling what amino acids to assemble and in what order to form a protein. Similarity in this strand of amino acids can be used to determine relatedness among organisms in the same way that is done with strands of DNA. Sure enough, they compared this sequence to that of modern birds and it was a close enough match to place birds within the dinosaur lineage.
Consider this: most Americans eat dinosaurs on Thanksgiving.
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