Interesting story by Darren Naish over at Scientific American. He unveils details of his paper in a recent issue of Science (August 1st, 2014). As, he states theropod dinosaurs (the "meat eaters") were shrinking in size and becoming more and more "birdy" as time passed. What about Tyrannosaurus rex, you say? T-rex (and his/her large friends) were the exception rather than the rule.
Some time round about 165 million years ago, the group of small, feathered dinosaurs that we call birds evolved from within the theropod radiation (theropods are the so-called ‘predatory dinosaurs’: the great group that includes animals like Tyrannosaurus and Velociraptor as well as the birds). As anyone reasonably familiar with recent palaeontological discoveries will know, we’re now aware of a large number of fossils that blur and smudge whatever distinction there might have been between ‘dinosaur’ and ‘bird’. Archaeopteryx may or may not be one of the oldest members of the bird lineage, but it’s evident that – when it was alive – it belonged to just one of several lineages of small, long-legged, fully feathered, omnivorous or predatory theropods, all of which possessed a mosaic of ‘bird-like’ and ‘unbird-like’ characters.
Indeed, if we look at theropod history across the whole of the Triassic, Jurassic and Cretaceous, we see a gradual, cumulative acquisition of bird-like features, ranging from wishbones and a pneumatised skeleton to complex feathers, a reduced, three-fingered hand, an enlarged sternum (breastbone) and tiny size.
This trend of increasing birdiness is well known and has often been commented on. Allosaurus is more bird-like than Coelophysis,Tyrannosaurus is more bird-like than Allosaurus, Ornithomimus is more bird-like than Tyrannosaurus, Velociraptor is more bird-like than Ornithomimus, and Archaeopteryx is more bird-like than Velociraptor. Incidentally, while this general trend might imply that theropods are a ‘bird factory’ (Holtz 2000) and not much else, remember that evolution is about divergence and diversification, with the umpteen lineages that split off along the way all having complex, intricate and independent histories of their own.