“Sue” specimen, Field Museum of Natural History, Chicago
Tyrannosaurus rex is the most iconic dinosaur of all time. It was discovered in the Hell Creek Formation by Barnum Brown in 1902, and later described by Henry Fairfield Osborn in 1905. Osborn actually named two large Hell Creek tyrannosaurids, T. rex and Dynamosaurus imperiosus. He later realized that Dynamosaurus imperiosus and Tyrannosaurus rex were synonymous, but Tyrannosaurus has priority, as it preceded Dynamosaurus in the description (Osborn, 1906).
T. rex and its closest relatives, known as tyrannosaurids, comprise the clade Tyrannosauroidea, a relatively derived group of theropod dinosaurs, more closely related to birds than to other large theropods such as allosauroids and spinosaurids, that originated in the Middle Jurassic, approximately 165 million years ago. All tyrannosaurs were bipedal predators characterized by premaxillary teeth with a D-shaped cross section, fused nasals, extreme pneumaticity in the skull roof and lower jaws, a pronounced muscle attachment ridge on the ilium, and an elevated femoral head (Brusatte et al., 2010). For most of their evolutionary history, tyrannosauroids were mostly small-bodied animals and only reached gigantic size during the final 20 million years of the Cretaceous.
Skulls of the basal tyrannosauroids Guanlong (A), Dilong (B); Skulls of juvenile (C) and adult (D)Tyrannosaurus. (Adapted from Brusatte et. al., 2010)
During the past 15 years, new discoveries from Russia, Mongolia and China helped to build the Tyranosaurs family tree. The oldest and most basal tyrannosaurs comprise a subclade, Proceratosauridae, which includes Kilesksus, Gualong, and Proceratosaurus. They were small-bodied animals no larger than a human, with elaborate cranial crests, extremely elongated external naris, a short ventral margin of the premaxilla, and depth of the antorbital fossa ventral to the antorbital fenestra is greater than the depth of the maxilla below the ventral margin of the antorbital fossa (Rauhut et al., 2010; Averianov et al., 2010).
Kileskus artistotocus, from the Middle Jurassic (167 mya), was discovered in 2010 in Western Siberia, by Alexander Averianov on the basis of an associated maxilla and premaxilla, a mandible fragment, and some possible associated postcranial elements. The cranial crest is currently unknown for Kileskus.
Pedal ungual phalanx of Kileskus aristotocus. Abbreviations: ft – flexor tubercle; lgr – lateral groove. Scale bar = 1 cm (From Averianov et. al.; 2010)
Guanlong wucaii, from the Late Jurassic of China, was first described in 2006. The generic name is derived from the Chinese Guan (crown) and long (dragon). The specific epithet wucaii (five colours) referred to the colours of rock of the Wucaiwan area where the fossil was found. The most striking trait of Guanlong is the complex nasal crest consisting of a highly pneumatic median crest that is about 1.5 mm thick for most of its length, and four supporting lateral laminae (Xu et al., 2006).
Proceratosaurus bradleyi, discovered in Gloucestershire, England in 1910 and described by Arthur Smith Woodward, it was originally thought to be an ancestor of Ceratosaurus. Some of the characters uniting Proceratosaurus with Guanlong are the strongly enlarged nares, and a midline cranial crest or horn on the nasals (Rauhut et al., 2010).
Guanlong wucaii. (Image adapted from Xu et al., 2006)
The giant, feathered tyrannosaur Yutyrannus huali, lived during the early Cretaceous period in what is now Northeastern China. It was discovered in 2012 by Chinese palaeontologist Xing Xu. Yutyrannus weighed about 1,400 kilograms and was at least 8 metres in length, and shares some features, particularly of the cranium, with derived tyrannosauroids, but is similar to other basal tyrannosauroids in possessing a three-fingered manus and a typical theropod pes.
Dilong paradoxus, also described by Xu, was discovered in 2004. This small tyrannosauroid shows a mosaic of characters, including a derived cranial structure resembling that of derived tyrannosauroids and a primitive postcranial skeleton similar to basal coelurosaurians. And at least one specimen was preserved with remnants of protofeathers.
Yutyrannus skeleton (From Wikimedia Commons)
Eotyrannus lengi, from the Early Cretaceous of the Isle of Wight, United Kingdom, was described in 2001. The holotype of Eotyrannus are estimated to have measured about 4 m (13 ft) long. However, as it is believed to have been juvenile, an adult specimen might have been somewhat larger.
Qianzhousaurus sinensis, was discovered in 2014 in China. Nicknamed “Pinocchio rex”, this long-snouted tyrannosaurids along with Alioramus, shows that these type of tyrannosaurids were widely distributed in Asia.
Nanuqsaurus hoglund, was a small dinosaur discovered in Alaska in 2014. The name is the combination of ‘nanuq’ the Iñupiaq word for polar bear and the Greek ‘sauros’ (lizard). The specific name, hoglundi, honors the Texas philanthropist Forrest Hoglund.
Skull of Qianzhousaurus sinensis (Image credit: Junchang Lü et al.)
Until recently, all tyrannosaurs fossils were limited to Asia and North America, but the latest discoveries suggest a more cosmopolitan distribution during their early evolution. Tyrannosaurs more derived than Eotyrannus, exhibit a purely Asian or North American distribution, which indicates an increasing Laurasian-Gondwanan provincialism during the final stages of the Age of Dinosaurs (Brusatte et al., 2010).
Averianov, A., Krasnolutskii, S., Ivantsov, S. 2010. A new basal coelurosaur (Dinosauria: Theropoda) from the Middle Jurassic of Siberia. Proceedings of the Zoological Institute RAS 314, 1: 42–57.
Brusatte SL, Norell MA, Carr TD, Erickson GM, Hutchinson JR, et al. (2010) Tyrannosaur paleobiology: new research on ancient exemplar organisms. Science 329: 1481–1485. doi: 10.1126/science.1193304
Fiorillo AR, Tykoski RS (2014) A Diminutive New Tyrannosaur from the Top of the World. PLoS ONE 9(3): e91287. doi:10.1371/journal.pone.0091287
Loewen MA, Irmis RB, Sertich JJW, Currie PJ, Sampson SD (2013) Tyrant Dinosaur Evolution Tracks the Rise and Fall of Late Cretaceous Oceans. PLoS ONE 8(11): e79420. doi:10.1371/journal.pone.0079420
RAUHUT, O. W. M., MILNER, A. C. and MOORE-FAY, S. (2010), Cranial osteology and phylogenetic position of the theropod dinosaur Proceratosaurus bradleyi (Woodward, 1910) from the Middle Jurassic of England. Zoological Journal of the Linnean Society, 158: 155–195. doi: 10.1111/j.1096-3642.2009.00591.x
Xu X., Clark, J.M., Forster, C. A., Norell, M.A., Erickson, G.M., Eberth, D.A., Jia, C., and Zhao, Q. (2006). “A basal tyrannosauroid dinosaur from the Late Jurassic of China”, Nature 439 (7077): 715–718. doi:10.1038/nature04511