… or at least its reputation is. It is becoming more clear that whatever morphological distinctiveness lies at the heart of the animal we’ve come to know and love/hate as Raptorex kriegsteini (Sereno et al., 2009), its identity is going to have to shift with new data. That identity has always depended on two things:
1. That the specimen is a relatively mature, near-adult animal showing unique morphological characteristics and skeletal maturity indicated by nearly-complete neurocentral fusion along the spine.
2. That the specimen hails from the Early Cretaceous due mostly to the presence of teleost vertebrae in the entombing matrix that were identified as the fish Lepidotes (or something quite like it) but which aren’t known from the Late Cretaceous.
The first point was raised to show the specimen purports a distinctive morphological quality, a diagnosis that would assert itself regardless of where it was found; the second point, however, would seek to ensure that Raptorex kriegsteini would not be confused with what are essentially similar tyrannosaurids from the Nemegt Formation. This is because the specimen that forms the holotype of Raptorex kriegsteini, LH PV18, was not recovered from a known locality but was collected privately and then sold and acquired by the Long Hao Institute of Geology and Paleontology in Hohhot, Inner Mongolia, China. In their initial paper, Sereno et al. (2009) purported the specimen was collected from the border between Inner Mongolia and Liaoning Province, an area rife with Early Cretaceous, Late Jurassic and Middle Jurassic formations, and to support this they used a pelycopod and a fish vertebra to assert the material was certainly Early Cretaceous and derived from one of the Yixian Formation localities, specifically suggested as deriving from the Lujiatun beds, known from their three-dimensional preservation of skeletons in an otherwise lagerstatt-dominated Formation.
However, doubts arose, as I summarize here. Denver Fowler and colleagues had raised not only concerns on Sereno et al. (2009) interpreting too many LAGs, but also questioned the identity of the vertebra. The specimen, they argued, looked amazingly like a juvenile Tarbosaurus bataar, a taxon well-known from the latest Late Cretaceous. Interpreting many of the skeletal features (including vertebral fusion) as a product of an unconstrained understanding of how fusion of the vertebral column precedes skeletal maturity — even adults can have unfused vertebrae, and juveniles can fuse early — Fowler et al. indicated the specimen probably belongs to Tarbosaurus or some very similar species. Having tackled claim 1 above, claim 2 was handled more carefully: Fowler et al. raised doubts to the affinities of the vertebra and sought to study teleost vertebrae from the Cretaceous by which the material preserved with LH PV18 could be compared. If the bone was lycopteran, it would raise a morphological gap of over half of the Cretaceous; indeed, Raptorex kriegsteini might be supported as a distinct taxon on stratigraphy alone. But if the vertebra weren’t Lycoptera…
Newbrey et al. (2010) had recently published work on ellimmichtyiform fishes from the early Maastrichtian, and these fishes seemed similar. The hunt was on. This study culminated today with Newberry et al. (2013), who determine that the vertebra illustrated by Sereno et al. (2009) belongs to Hiodontidae, a group of teleost fish comprising the extant mooneye and goldeye fishes (Hiodon spp.) and some extinct, fossil taxa.
Hiodontidae is known from the Nemegt Formation, and Newberry et al. (2013) set out to describe these fishes and then provide context for asserting that the vertebra associated with LH PV18 is hiodontid in nature … not ellimmichtyiform, but not Lycoptera either. Thus, it seems almost certain now that the material probably derives from a Tarbosaurus-like tyrannosaurid theropod, not a convergent, smaller-bodied tyrant.
So, is the Raptorex kriegsteini debate over? No, not likely. Tsuihiji et al. (2011) suggested that while LH PV18 has all the earmarks of a juvenile tyrant, and not a non-tyrannosaurid tyrannosauroid (a “despot,” if you will) as Sereno et al. had suggested, there are morphological characteristics that distinguish the two, and that means Raptorex kriegsteini might represent another Nemegt tyrannosaurid … albeit a juvenile of one. Though based on less-than-ideal material, Hone et al. (2011) described another Nemegt-aged Chinese tyrant, Zhuchengtyrannus magnus, which while broadly comparable to Tarbosaurus bataar (or Tyrannosaurus bataar, if you follow Thomas Carr’s philosophy [n1]) is distinct enough that it suggests the likelihood of not one, but two coeval tyrants stomping around the floodplains/wetlands of northeast central Asia. Raptorex kreigsteini might even be Zhuchentyrannus magnus, which would make it the senior synonym, though much, much more data is needed to even consider this reasonably.
[n1] That, in case it wasn’t apparent, is a plug for Dr. Thomas Carr’s new blog, Tyrannosauroidea Central, which seeks to set out the thoughts and teaching experience of the world’s foremost expert/fanboy/enthusiast of all things tyrannosauroid.
Hone, D. W. E., Wang K.-B., Sullivan, C., Zhao, X.-j., Chen S.-x., Li D.-j., Ji S.-’a., Ji W. & Xu X. (2011). A new, large tyrannosaurine theropod from the Upper Cretaceous of China. Cretaceous Research 32 (4): 495-503.
Newbrey, M. G., Murray, A. M., Brinkman, D. B., Wilson, M. V. H. & Neuman, A. G. 2010. A new articulated freshwater fish (Clupeomorpha, Ellimmichthyiformes) from the Horseshoe Canyon Formation, Maastrichtian, of Alberta, Canada. Canadian Journal of Earth Sciences — Revue canadienne de sciences de la Terre 47 (9): 1183-1196.
Newbrey, M. G., Brinkman, D. B., Winkler, D. A., Freedman, E. A., Neuman, A. G., Fowler, D. W. & Woodward, H. N. 2013. Teleost centrum and jaw elements from the Upper Cretaceous Nemegt Formation (Campanian–Maastrictian) of Mongolia and a re-identification of the fish centrum found with the theropod Raptorex kriegsteini. pp.291-303 in Arratia, G., Schultze, H.-P. & Wilson, V. H. (eds.) Mesozoic Fishes 5 – Global Diversity and Evolution. Proceedings of the International Meeting (Saltillo, 2010).
Sereno, P. C., Tan L., Brusatte, S. L., Kriegstein, H. J., Zhao X.-j. & Cloward, K. 2009. Tyrannosaurid skeletal design first evolved at small body size. Science 326: 418-422.
Tsuihiji T., Watabe M., Tsogtbaatar K., Tsubamoto T., Barsbold R., Suzuki S., Lee, A. H., Ridgely, R. C., Kawahara Y. & Witmer, L. M. 2011. Cranial osteology of a juvenile specimen of Tarbosaurus bataar (Theropoda, Tyrannosauridae) from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia. Journal of Vertebrate Paleontology 31 (3): 497-517.