Coelurus

Chapter • Meet the dinosaur

Overview

Coelurus fragilis was a small, lightly built predatory dinosaur from the Late Jurassic of what is now Wyoming, USA. At roughly two to two and a half metres long and weighing only about 13–20 kilograms, it was little bigger than a large turkey with a long tail. Its skeleton is remarkably delicate: the neck vertebrae are extremely elongated, the back vertebrae are deeply excavated by air sacs, and the limb bones are long and slender. The lower leg and foot in particular are stretched out, a proportion usually interpreted as an adaptation for fast, agile movement. Coelurus belongs to Coelurosauria, the great branch of theropods that also contains tyrannosaurids, dromaeosaurids and birds. No feather impressions are known for Coelurus itself, but because simple filamentous feathers are documented in several early coelurosaurs, modern reconstructions usually give it a coat of downy fibres. It stood in a horizontal posture with the tail held out as a counterweight, never dragging on the ground. Very little can be said with confidence about its behaviour, because the material is fragmentary and no trackways can be reliably linked to the species. The long, slim legs and the slender hands with elongated fingers suggest an active animal that chased small, quick prey rather than one that wrestled with large victims. Its modest size means Coelurus occupied a completely different ecological role from the big predators of the same environment, such as Allosaurus and Torvosaurus, and direct competition was probably limited. Its diet most likely consisted of small vertebrates: early mammals, lizards, amphibians, hatchling dinosaurs and perhaps eggs, supplemented by large insects. Teeth that can be assigned to Coelurus with certainty are rare, so dietary reconstructions rest heavily on body size, build and comparison with related small theropods. Coelurus fragilis was named in 1879 by the American palaeontologist Othniel Charles Marsh, based on material from Quarry 13 at Como Bluff, Wyoming, in the Morrison Formation. The name means roughly "hollow tail", referring to the excavated vertebrae, while the species name fragilis reflects how delicate the bones are. In 1884 Marsh also named Coelurus agilis, later shown to belong to the same animal. For decades there was considerable confusion, with Coelurus material mixed up with Ornitholestes and other small theropods. Only when John Ostrom re-examined the specimens in 1980 was Coelurus given a clear definition, and it became evident that all secure remains come from just a handful of individuals.

Chapter • Where it lived

Habitat

Coelurus lived about 155–150 million years ago, in the Late Jurassic (Kimmeridgian–Tithonian), in the Morrison Formation of western North America. The landscape was one of broad floodplains crossed by meandering rivers, seasonally drying lakes and gallery forests of conifers, ginkgophytes and ferns, under a climate with a pronounced dry season. All secure finds come from Como Bluff in Wyoming, above all from Quarry 13, a site that has also yielded sauropods such as Camarasaurus and Diplodocus and predators such as Allosaurus.

Chapter • Wild behavior

Behavior & Ecology

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Chapter • Scientific place

Taxonomy