The holotype and only known specimen, BMNH R3718, consists of a single left hand bone, discovered around 1905 near Wollaston, on the Lightning Ridge.[3] The fossil has been opalised.[3] The bone has a length of seven centimetres.[2] This manual element shows a prominent dorsomedial process, a feature shared with the much smaller Ornitholestes which occasioned the specific name.[3] The process with Ornitholestes is much less distinctive though.[3] On its upper end there is only one cotyle, from which von Huene deduced it must have been a metacarpal.[2] However, several coelurosaurian groups lack a second cotyle on the first phalanx also. If Rapator had a build like Australovenator, it would have attained a considerable size: a body length of 9 metres (30 ft) has been estimated.