Eric Gorscak is a postdoctoral researcher at The Field Museum focusing on African dinosaurs and paleobiogeography of the southern continents.
There is little doubt that Tyrannosaurus rex is the indisputable "king" of dinosaur popularity and iconic prehistoric predator. Proudly on display in Stanley Field Hall at The Field Museum, SUE is one of the best specimens of this fearsome Cretaceous predator. However, Tyrannosaurus rex is just one of many closely-related theropod dinosaurs within the aptly-named superfamily Tyrannosauroidea. This group of predators had humbler beginnings with ancestors that were much smaller and perhaps not quite as intimidating as Tyrannosaurus rex. Nevertheless, Tyrannosauroidea was a diverse group that originated in the middle Jurassic, around 170–175 million years ago. One hundred million years later, this group had undergone a radiation of large-bodied predators after waiting for the perfect evolutionary opportunity during the end of the Cretaceous Period.
Alioramus altai / remotus
(Kurzanov, 1976; Brusatte et al., 2009)
Alioramus is a tyrannosaur from the Maastrichtian Nemegt Formation in Mongolia. Discovered in the late 1970s, Alioramus remotus is known from a partial skull and several bones of the foot. Interpretation of the strange Alioramus skull was difficult due to several confounding factors, including preservational distortion, potential juvenile features, and several primitive tyrannosaurid traits. In 2009, Dr. Stephen Brusatte and co-authors described a second Alioramus species, Alioramus altai, that helped clear up the mystery of this tyrannosaur. A. altai is nearly complete and displayed some odd features including cranial ornamentation along the snout and around the eyes. Just like A. remotus, A. altai was a small and gracile tyrannosaur with a long and short snout, in contrast to the large and deep snout of Tyrannosaurus rex.
(Fiorillo and Tykoski, 2014)
Tyrannosaurs are known from northern hemisphere continents, mainly Laramidia (western North America) and Asia, and crossed the Bering Strait when sea levels were low enough during the Late Cretaceous. Nanuqsaurus hoglundi was discovered in the North Slope Borough of Alaska, adding further evidence of this trans-continental distribution. Nanuqsaurus was not only the northernmost tyrannosaur, it is also the smallest, being a fraction of the size of Tyrannosaurus rex. At such high latitudes, Nanqusaurus must have adapted to seasonal extremes in ancient Alaska. Though Nanuqsaurus is known from a partial skull, there are enough anatomical similarities to bridge a close relationship with Tyrannosaurus from North America and the Asian tyrannosaur, Tarbosaurus.
(Loewen et al., 2013)
You can think of Tyrannosauroidea as your extended family that you only see during family reunions, whereas Tyrannosauridae is more like your immediate family. Lythronax argestes is the geologically oldest member of Tyrannosauridae, discovered in Grand Staircase-Escalante National Monument in southern Utah. Lythronax is known from a nearly complete skull, most of the vertebral column, and part of the leg. The presence of Lythronax in the middle Campanian (~80 million years ago) Wahweap Formation of Utah suggests that Tyrannosauridae originated in North America. Loewen and co-authors (2013) argued that the rise of the Western Interior Seaway helped divide western North America into distinct northern (Montana, Wyoming, Canada) and southern (Utah, New Mexico) regions, the latter of which Lythronax ruled.
(Carr et al., 2011)
Another discovery from Grand Staircase-Escalante National Monument in Utah (but from the slightly younger Kaiparowits Formation), Teratophoneus curriei is another unusual tyrannosaur that deviates from the classic deep and robust skull seen in Tyrannosaurus rex. The shortened skull and reduced number of teeth in both the upper and lower jaw of Teratophoneous demonstrated that tyrannosaurs of North America evolved a wide range of skull shapes and underwent a radiation. The morphological diversity in tyrannosaur skulls may reflect a change in functional differences in feeding, and perhaps differentiation in ecological roles, for North American tyrannosaurs.
(Li et al., 2009)
Until the discovery of Xiongguanlong baimoensis in 2009 by Li and co-authors (including Dr. Pete Makovicky from The Field Museum), there was a noticeable gap in the evolutionary history of tyrannosaurs. In the Late Cretaceous (84-66 million years ago), there were several known large-bodied species, yet earlier and small-bodied relatives were known in the Early Cretaceous (126 million years ago). Xiongguanlong was the tyrannosaur that strengthened the link between these disparate parts of tyrannosaur evolutionary history.
Xiongguanlong fit perfectly into that gap temporally (~125–100 million years ago), phylogenetically (more closely related to true tyrannosaurs but not quite one itself), and in body mass, with adult estimates that fell between the smaller tyrannosaurs from the Early Cretaceous and the larger tyrannosaurs from the Late Cretaceous. However, Xiongguanlong had a much slenderer snout when compared to the more robust skulls we are familiar with in tyrannosaurs.
Brusatte, S. L., T. D. Carr, G. M. Erickson, G. S. Bever, and M. A. Norell. 2009. A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia. PNAS 106:17261–17266.
Carr, T. D., T. E. Williamson, B. B. Britt, and K. Stadtman. 2011. Evidence for high taxonomic and morphologic tyrannosauroid diversity in the Late Cretaceous (late Campanian) of the American Southwest and a new short-skulled tyrannosaurid from the Kaiparowits Formation of Utah. Naturwissenschaften 98:241–246.
Fiorillo, A. R., and R. S. Tykoski. 2014. A diminutive new tyrannosaur from the top of the world. PLoS ONE 9:e91287.
Kurzanov, S. M. 1976. A new Late Cretaceous carnosaur from Nogon-Tsav Mongolia. Sovmestnaâ Sovetsko-Mongolskaâ Paleontologiceskaâ Ekspeditcia, Trudy 3:93–104 (in Russian).
Li, D., M. A. Norell, K.-Q. Gao, N. D. Smith, and P. J. Makovicky. 2010. A longirostrine tyrannosauroid from the Early Cretaceous of China. Proceedings of the Royal Society B: Biological Sciences 277:183–190.
Loewen, M. A., R. B. Irmis, J. J. W. Sertich, P. J. Currie, and S. D. Sampson. 2013. Tyrant dinosaur evolution tracks the rise and fall of Late Cretaceous oceans. PLoS ONE 8:e79420.