Coelurus

Coelurus is a genus of small theropod dinosaurs from the Late Jurassic Morrison Formation of North America. The type species, Coelurus fragilis, was named by Othniel Charles Marsh in 1879. The name means “hollow tail,” reflecting the relatively delicate, lightweight nature of the known fossil material.

Description and size estimation can only be approximate due to fragmentary remains. Coelurus was a small, bipedal predator with a gracile skeleton and long hind limbs, suggesting relatively high speed. Estimates for body length generally fall in the range of about 1 to 1.5 meters (roughly 3 to 5 feet), with a body mass likely around 10 to 20 kilograms. The forelimbs were relatively short compared to the hind limbs, and the skull is poorly known from the available fossils, limiting detailed reconstructions of its feeding habits.

Fossil record and paleobiology: Fossils attributed to Coelurus come from the Morrison Formation, a sedimentary sequence dating to roughly 155 to 150 million years ago. The remains are incomplete, comprising portions of the vertebral column and limb bones, which constrains precise anatomical interpretations and paleoecological inferences. As a small carnivore, Coelurus would have occupied a role as a quick, agile predator feeding on small vertebrates or other suitable prey available in its environment.

Classification: Historically, Coelurus has been placed in the family Coeluridae, a group of small theropods. However, the exact phylogenetic position of Coelurus within Theropoda is uncertain, and various analyses have placed it as a basal coelurosaur or as incertae sedis within Theropoda. The limited and fragmentary nature of its remains contributes to ongoing debates about its relationships and broader groupings among small Jurassic theropods.