One hundred and twenty eight million years ago, a killer stalked eastern Utah. Known as Utahraptor, this distant relative of Velociraptor of Jurassic Park fame was also equipped with a sickle-claw on its hind feet. The name means “Utah’s raptor” with “raptor” being the informal name commonly (but incorrectly) used for the sickle-clawed dromaeosaurid theropods. Utahraptor was named in 1993 by Dr James Kirkland for bones from the Gaston Quarry (also known as Yellow Cat Quarry), north of Arches National Park. The Gaston Quarry occurs in the lower part of the Yellow Cat Member of the Cedar Mountain Formation. The presence of an ankylosaur related to Polacanthus at the Gaston Quarry suggests that the Yellow Cat Member is the same age as the Wealden Formation on the Isle of Wight, in other words, it lived 125 to 130mya. A radiometric date of 126 +2.5mya was obtained from the Yellow Cat Member which supports the dinosaur evidence for the age. Utahraptor has been found at several other sites in the Yellow Cat Member, so must have been relatively widespread in the region. Nevertheless, most of this material remains undescribed.
Until recently, what Utahraptor looked like relied a great deal on imagination. Several recent scientific studies have shown that Utahraptor is related to Achillobator, a dromaeosaurid from the middle of the Late Cretaceous of Mongolia. This theropod is represented by parts of the skeleton that are yet unknown in Utahraptor, but can be used to approximate what the missing bones might have looked like. Using this information, as well as information from the best known dromaeosaurids – Deinonychus and Velociraptor – a new skeletal reconstruction was recently completed. Casts of the original Utahraptor were used, as well as sculpted parts modelled on other Utahraptor and dromaeosaurid specimens scaled to a large-sized individual. The reconstructed skeleton stands one metre at the hips and is 4.6m long. The resulting skeleton had several surprises: the snout is slightly upturned as it is in Velociraptor, rather than straight and tapering as is usual among theropods. The orbits or eye sockets are very large compared to the skull size, suggesting the eyeball was very large. Large eyes in owls allow them to hunt in low light, so perhaps Utahraptor primarily hunted at dawn or dusk, when light levels were low, or in the low light of the forest understory.
We know from Deinonychus and Velociraptor that the arms and hands were long and slender. Structurally, they are well suited for grasping and holding prey as the sickle-claw was used to kill its prey. At one time, it was thought that dromaeosaurids leaped at their prey to disembowel with the sickle claw. However, the discovery of a Velociraptor skeleton locked in tight embrace with the skeleton of its prey (a Protoceratops) shows that the claws were used as puncturing or stabbing weapons. The pose selected for the Utahraptor skeleton is based on a small bronze Marquette (see Fig. 3), so is an example of science imitating art. A few changes were necessary, including the upward angle of the tail, which is based on the Velociraptor locked in combat with the Protoceratops. In life, having the tail angled upwards would keep the centre of gravity close while lashing out with sickle-clawed foot. Another change is the arms are not as far forwards because of the limitations imposed by the shoulder.
Once the skeleton was completed, it was possible to make a life restoration. We know from the discovery of Chinese feathered dinosaurs that dromaeosaurids (such as Sinornithosaurus) had a covering of simple, hair-like feathers. For the restoration, these were modelled from the body feathers of an emu and the wing feathers of a vulture. The eyes are those of an alligator. Although the restoration looks believable based on what we currently know, its accuracy can only be determined with a time machine. Any takers?