Caudipteryx zoui was another one who was definitely a bird and definitely a dinosaur, both at the same time. A true transitional species, even though its lineage never led to modern aves.
Like so many of the other feathered dinosaurs before it, the wings of Caudipteryx were useless for flying. They served instead as as insulation for the unhatched brood and as displays to attract mates. Doubtless, this was the function of the tail as well.
A "half-winged", warm-blooded feathered reptile that is a transitional species of 100% dinosaur AND 100% bird is exactly the thing my 100% creationist family demanded all my young life. "No such creature will ever be found", they insisted along with so many others, "because if it were, it would prove evolution".
They stopped saying that in 1997. Now the excuses are:
1. that all these feathered fossils were faked.
2. Its just a bird with a long tail and clawed hands.
3. Its just a dinosaur with long feathery scales, (like longisquama).
4. Birds and dinosaurs have impossibly different respiration.
5. It can't be a transitional, because it isn't half of one thing
and half of something else that is completely different.
Archaeopteryx, (five of them) were the only feathered dinosaur ever found after more than a century of constant searching. So creationists felt confident that no others would ever be. Of course, when their bluff is called, and the criteria for their own ultimatum finally met, they "move the goal posts", so to speak. Suddenly what they once demanded as solid proof is no longer acceptible once it exists.
And just for the record.....
1. One of these dozen or so species of feathered dromaeosaurs found so far actually was a fake. Achaeorapter. It was purchased from some inventive Chinese farmers by a amateur fossil collecting hobbyist and sold to a national magazine as opposed to ever undergoing serious scientific analysis. Once it finally was examined, (by "evolutionists") it was immediately revealed to be a composite of two different fossils, the tail of a dromaeosaur, and the body of an ancient Cretaceous bird. Ironically, the dromaeosaur was later found to be feathered and the bird had some distinctly dinosaur traits in some of its skeletal subtleties.
2. A bird with clawed hands only needs a tail with at least partially unfused vertibrae to be considered a dinosaur. And Caudipteryx has one.
3. The fossil impressions left by Caudipteryx feathers (as well as by more obvious birds like Confuciousornis) all leave a shadow trace, which is all true plumage can really do when fossilized. Longisqama's "feathers" don't do that. Even the very edges leave an impression when the spine is pressed to the ground. Scales can do that. Feathers can't.
4. Birds and their coelosaurian ancestors first appeared in the mid-Jurassic, about 150 million years ago. And no one knows exactly what the respiratory system of a dinosaur is like. Or rather, they didn't until the discovery of Scipioniyx, a diminuative Italian maniraptor with traceable elements of soft tissue fossilized intact. And its internal organs are exactly like a bird's.
5. A "transitional" species is one that appears to belong completely to two different, but closely-related taxonomic groups at the same time. Which is what Caudipteryx (and at least a hundred other animals) are. Half dog / half bird is impossible. That's why creationists have started to demand one. Everything else they've ever challenged biology to produce in the past has been found in spades, sometimes even before they asked for it!