Coelurus

Type Species: Coelurus fragilis 

Classification: Dinosauria – Saurischia – Theropoda – Tetanurae – Coelurosauria – Tyrannoraptora – Maniraptoromorpha  

Time Period: Late Jurassic 

Location: United States 

Diet: Carnivore

Coelurus was the first small theropod from the Morrison Formation to be named, and it was named after the numerous hollow spaces – called pleurocels – oddly distributed among its vertebrae. These pneumatic vertebrae lightened the skeleton, making Coelurus more swift of foot and lightweight. It grew up to eight feet in length, stood two feet tall at the hips, and weighed around forty pounds. Its elongated neck vertebrae gave it a longer neck than is seen in most theropods, and its long, slender hind legs indicate that it was a lightly-built swift runner, certainly faster than its similar but shorter-footed contemporary Ornitholestes. It shared its environment with much larger dinosaurs, and it may have needed to use its speed to avoid being preyed upon by larger theropods (or from being trampled by hapless sauropod herds). It likely hunted lizards and early mammals, though it may have preyed upon smaller ornithopods such as Dryosaurus and Othnielosaurus, particularly if they were weak or juvenile. While there’s no indication that Coelurus hunted in packs, it’s possible that it utilized pack-hunting behavior in taking down smaller-sized ornithopods. 

Coelurus lends its name to the clade Coelurosauria, which in the early days of dinosaur taxonomy included all small theropod dinosaurs. Our knowledge of dinosaur relationships has greatly increased, so now Coelurosauria includes all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria branches off into well-known dinosaur groups such as the dromaeosaurs, the ornithomimosaurs, and the tyrannosaurs. Coelurus belongs to the Tyrannoraptorans,  members are all descendants of the last common ancestor of Tyrannosaurus rex and the common house sparrow; within that clade, Coelurus is a member of the smaller clade Maniraptoromorpha, which is a ‘transition’ clade between the tyrannoraptorans and the ‘New Coelurosaurs’ (Neoceolurosauria), which includes all compsognathids and the maniraptoriformes. Coelurus’ placement indicates that it’s somewhat of an oddball in theropod taxonomy, and its only concrete neighbor was its Morrison Formation contemporary Ornitholestes.