In Depth
Raptorex is a very dubious genus of tyrannosaur that many consider to be a juvenile Tarbosaurus. When Raptorex was first revealed to the world the holotype had already been dug out of the ground and shipped to Japan where it was purchased by an American businessman. Back then the fossil was being identified as a juvenile Tarbosaurus and coming from the Yixian Formation of China. Alarm bells should already be ringing at this for two reasons. First, there is a law in China that makes it illegal to export genuine fossils from beyond China’s borders, and any legitimate original fossils would have to be on the black market, a far from reliable scientific source as black market fossils dealers will say anything about fossils just to sell them.
Proof of this deception comes from the second point to be made. The original dealer said that this specimen was a juvenile Tarbosaurus from the Yixian Formation. However, Tarbosaurus is known to have lived during the Campanian and Maastrichtian periods of the Late Cretaceous period roughly around 70-66 million years ago. The Yixian Formation however is dated to the Barremian/Aptian of the Early Cretaceous around 125-121 million years ago. This leaves two possibilities, one is that Tarbosaurus has the longest temporal span in the fossil record of any tyrannosaur, and also indicates that advanced form tyrannosaurs were living at a time when previously only primitive forms were known, or, alternatively, the fossil dealer lied about the specimen coming from the Yixian Formation.
When the specimen was shipped to the United States it was sold on again at the Arizona Gem, Mineral and Fossil Show in Tucson. The buyer was a Dr. Henry Kreigstein, himself a fossil collector. Kreigstein contacted one of the more noted American palaeontologists, Paul Sereno, to examine the specimen, stating that it was bought as a juvenile Tarbosaurus from the Yixian Formation. The first thoughts of Sereno and his team about the specimen were that coming from the Yixian Formation the specimen was not a juvenile Tarbosaurus, but a whole new genus of previously unknown tyrannosaur. Naming the specimen Raptorex in 2009, they concluded that the specimen was six years old at the time of death and almost fully grown, and concluded that the body form of later tyrannosaurs had already become established before they grew to the large sizes that they are known for from the late Cretaceous.
Sereno’s team also tried to locate the area and time that that the fossil deposit was laid down. To do this they looked at the occurrences of other smaller fossils of creatures that were embedded around the Raptorex holotype. They identified vertebrae belonging to fish as coming from the genus Lycoptera, a genus known to have lived in China up until the same time in the early Cretaceous as well as be present in the Yixian Formation. For Sereno and his team this was proof enough that the specimen did indeed come from the Yixian Formation, and that Raptorex was a ground breaking discovery in our understanding of tyrannosaur evolution. The first publicised doubts about Raptorex came in 2010 when after examining the holotype fossil, Peter Larson of the Black Hills Institute of Geological Research, Inc. (a private fossil excavation company), could only identify the fossil remains as a juvenile Tarbosaurus. Larson also suggested that a much more detailed examination of the matrix, the rock that the Raptorex fossils were attached too should be undertaken, ideally with an objective to try and locate pollen to identify known plant types of the Cretaceous eras. This criticism appeared in the publication Nature, but in a response at the time, Sereno stated that no peer reviewed paper had produced evidence to refute his analysis.
However, in 2011, and less than one year after Larson, had first made doubts, a scientific paper refuting the interpretation was published, focusing upon the Raptorex fossils and the matrix. In 2009 Sereno had stated that the vertebrae of Raptorex were nearly fused, denoting a subadult and almost fully grown specimen. However in the 2011 paper Sereno’s team were criticised for misinterpreting growth pattern data, and that not only were the vertebrae of Raptorex unfused, but the age of the animal would have only been three years old. They also examined the fish vertebrae that Sereno’s team assigned to the Lycoptera genus, and stated that not only were they not of Lycoptera, they were not even of the same type of fish as Lycoptera.
In 2013 things were narrowed down even further with a paper by Newbrey et al, which confidently identified the fish vertebrae as belonging to a hiodontid fish. In addition to this, the species of hiodontid fish that the vertebrae on the matrix match up to are only known from the Nemegt Formation which is in Mongolia. This is a key development as not only is the Nemegt Formation Late Cretaceous in age, Tarbosaurus fossils are also known from here, in fact they are surprisingly common with large numbers already being recovered. It is for these reasons that Raptorex is largely regarded as a dubious genus, and probable juvenile of Tarbosaurus by most.
Further Reading
- Tyrannosaurid skeletal design first evolved at small body size. - Science 326 (5951): 418–422. - P. Sereno, L. Tan, S. L. Brusatte, H. J. Kriegstein, M. Zhao & K. Cloward - 2009. - Reanalysis of Raptorex kriegsteini: A Juvenile Tyrannosaurid Dinosaur from Mongolia. - PLoS ONE 6(6): e21376. - D. W. Fowler, H. N. Woodward, E. A. Freedman, P. L. Larson & J. R. Horner - 2011. - Teleost centrum and jaw elements from the Upper Cretaceous Nemegt Formation (Campanian-Maastrichtian) of Mongolia and a re-identification of the fish centrum found with the theropod Raptorex kreigsteini. - Michael G. Newbrey, Donald B. Brinkman, Dale A. Winkler, Elizabeth A. Freedman, Andrew G. Neuman, Denver W. Fowler & Holly N. Woodward - In, Mesozoic Fishes 5 – Global Diversity and Evolution. Verlag Dr. Friedrich Pfeil. pp. 291–303 - Gloria Arratia, Hans-Peter Schultze & Mark V. H. Wilson (eds) - 2013.
