Raptorex' 15 minutes of fame

When first unveiled Raptorex caused quite a stir. Here was an early Tyrannosauroid that had evolved all the features of T. rex... at 1/100th of it's size.
But it wouldn't be long before these claims would be seriously challenged, ultimately reducing Raptorex to a nomen dubium.

Raptorex kriegsteini compared to a human (image by Conty, from Wikipedia)

Raptorex kriegsteini was described by Paul Sereno as a new Tyrannosauroid from the early Cretaceous of China. The specimen was described as a young adult of 3 meters in length and an estimated weight of 65kg. In terms of size and weight that would be comparable to other Late Jurassic/Early Cretaceous Tyrannosauroids like Dilong and Guanlong. But it's bodyplan was radically different, Dilong and Guanlong were much closer to their Coelurosaurian ancestors, with long arms, three-fingered hands and a small head. Raptorex on the other hand was remarkably similar to Late Cretaceous Tyrannosaurids like Tyrannosaurus and Tarbosaurus.
This seemed at at the time to shed a new light on Tyrannosaur evolution, Raptorex was according to Sereno an intermediate stage between the small long-armed Tyrannosaurs and the large short-armed Tyrannosaurids.

A chequered past
The fossil that was described already had something of a history, it had already changed hands several times before being sold to Henry Kriegstein as a juvenile Tarbosaurus. Kriegstein had originally planned to use the fossil as decoration for his living room (money can't buy hapiness, but it can buy a genuine Tyrannosaur fossil!).

Raptorex skull and life reconstruction (with manes!)

During the preparation palaeontologist Paul Sereno was contacted  for advice, he identified it not as a juvenile but as a subadult of a completely new species. He also convinced Kriegstein to donate the fossil to science.
To date this suddenly new find Sereno used the fossil of a Lycoptera fish embedded in the same rock, which meant that it originated from the early Cretaceous. Sereno also speculated that the fossil had originally come from the Yixian formation in China, but had been smuggled to Mongolia for sale on the black market. 
That was not the first U-turn in the identification of the fossil, before it was sold as a juvenile Tarbosaurus from Mongolia it had tentatively been considered an Alectrosaurus (a medium-sized Tyrannosauroid from the Late Cretaceous). Such differences is identification are perhaps unsurprising, identifying juvenile specimens is complicated. Like most Dinosaurs Tyrannosaurs underwent major changes to their body during growth, so a juvenile specimen might look very different from the adult form. Knowing the origin of the fossil would of course narrow things down, but that information is generally lost when the fossil is traded on the black market.



The gloves come off

Raptorex skull (Conty)

There had already been some doubts cast on the description of Raptorex as a new species when in 2011 a reanalysis of the specimen  appeared in PLoS One. This paper reads like a scientific hatchet job, all of Sereno's original claims were disputed (if not outright dismissed). The specimen was not a 6 year old subadult, but a 3 year old juvenile. The fish found in the rock alongside was not Lycoptera but an undetermined species of a family that was common in China throughout the Cretaceous (and beyond) Tracing back the origin of the fossil showed no evidence that it came from China as Paul Sereno had speculated. The skull was also found to be extremely similar to a juvenile Tarbosaurus skull of the same size.
This led to a devastating conlusion: 

Unless stronger evidence is presented, Raptorex should be considered a nomen dubium. LH PV18 [the Raptorex fossil] more likely represents the juvenile of a larger tyrannosaurid from the Late Cretaceous of Mongolia, such as Tarbosaurus. 

And with that "Raptorex" had come almost full-circle. Originally sold to Kriegstein as a juvenile Tarbosaurus it had enjoyed a brief period of fame as a totally new species of Tyrannosaur, but was once again provisionally identified as a juvenile Tarbosaurus.
Of course this is still a very tentative identification, there is no positive evidence linking the specimen to a geological age much less a specific location. Analysis of the rock matrix in which the fossil was embedded might reveal more about the origins of this specimen, but until someone actually goes out and does that any identification will remain speculative.