Size and Weight:
Coelurus was one of the innumerable genera of small, lithe theropods that scurried across the plains and woodlands of late Jurassic North America. The remains of this tiny predator were discovered and named in 1879 by the famous paleontologist Othniel C. Marsh, but they were later lumped in (incorrectly) with Ornitholestes, and even today paleontologists are unsure exactly what position Coelurus (and its other close relatives, like Compsognathus) occupies on the dinosaur family tree.
By the way, the name Coelurus--Greek for "hollow tail"--refers to the lightweight vertebrae in this dinosaur's tailbone. Since the 50-pound Coelurus didn't exactly need to conserve its weight (hollow bones make more sense in huge sauropods), this evolutionary adaptation may well count as additional evidence for the theropod heritage of modern birds.