[1][2] While initially considered to have come from the Yixian Formation of China, dated to approximately 125 million years ago during the early Cretaceous period, later studies showed that such an early date for the fossil are unlikely, and given its extremely close similarity to juvenile tyrannosaurids of the late Cretaceous, it probably came from the Nemegt or similar formation.

Kriegstein notified American paleontologist Paul Sereno of the specimen, who proposed it represented a subadult of a new species from the Yixian Formation of China.

Sereno initially stated to the press that numerous index fossils present in the rock slab surrounding the specimen showed it belonged to Early Cretaceous.

[1][3] Sereno and colleagues identified the fish vertebra as being similar to the genus Lycoptera, a key index fossil of the early Cretaceous Yixian formation.

Rather than evolving their distinct anatomy of a large head, long legs and tiny, two-fingered arms only after large body size, a small, early Cretaceous species with similar proportions to adults of true tyrannosaurids would indicate that the characteristic tyrannosaur shape appeared prior to the advent of giant size in the group.

[1] In October 2010 an online Nature News report[6] (not peer-reviewed) challenged the provenance and classification of Raptorex as a basal tyrannosauroid.

Because the specimen was donated by a collector without detailed provenance information, Larson doubted the assigned age, which was based only on the Lycoptera vertebra and mollusk shell found alongside the dinosaur fossil.

[8] In 2022, the results of a restudy of the Raptorex specimen were included in an article focusing on the tyrannosauroids of the Iren Dabasu Formation by researcher Thomas Carr.

[4] The only known Raptorex specimen shows the same basic proportions as juvenile tyrannosaurids: a comparatively large and solidly-constructed skull, long legs with adaptations for running, and tiny, two-fingered forelimbs.

[9] Proceratosaurus bradleyi Kileskus aristotocus Guanlong wucaii Sinotyrannus kazuoensis Juratyrant langhami Stokesosaurus clevelandi Dilong paradoxus Eotyrannus lengi Bagaraatan ostromi Raptorex kriegsteini Dryptosaurus aquilunguis Alectrosaurus olseni Xiongguanlong baimoensis Appalachiosaurus montgomeriensis Alioramus altai Alioramus remotus Tyrannosauridae Carr (2022) considered Raptorex to be closely related to derived tyrannosaurines, because it and a juvenile tyrannosaurid from the Iren Dabasu Formation shared unique characteristics with juvenile Tyrannosaurus and adult Tarbosaurus.