Callovosaurus is based on BMNH R1993, a nearly complete left thigh bone. This specimen was collected from the middle Callovian–age (Middle Jurassic) Peterborough Member (former Lower Oxford Clay) of the Oxford Clay Formation of Fletton, near Peterborough in Cambridgeshire, England. The bone is 28 centimetres (0.92 ft) long, and is estimated to have belonged to an animal approximately 2.5 metres (8.2 ft) in length. A partial shin bone from the same site or nearby, SMC J.46889, may also belong to Callovosaurus.[1]