A New Giant Tyrant, Zhuchengtyrannus

"While 2010 was celebrated as the year of ceratopsians by many," paleontologist Dave Hone wrote at Archosaur Musings yesterday, "it should not be overlooked the huge number of tyrannosaurs that have cropped up in the last year or so." He's right. For a long time Albertosaurus, Gorgosaurus, Daspletosaurus, Tarbosaurus and, of course, Tyrannosaurus made up most of what we knew about the tyrant dinosaurs, and all were large, Late Cretaceous apex predators. Only in the last decade or so have we started to understand the origins and early evolution of these carnivores, and many new species of tyrannosaur have been turned up at field sites and in museum drawers.

Hone, along with a large team of collaborators, has just added another member to the tyrant dinosaur pantheon. Described in an in-press Cretaceous Research paper, the new dinosaur has been named Zhuchengtyrannus magnus. It was an enormous, Late Cretaceous meat-eater.

For the moment, the known remains of Zhuchengtyrannus are limited to part of an upper jaw (the maxilla) and the tooth-bearing portion of the lower jaw (the dentary). They were found in a quarry near the city of Zhucheng in China's Shandong Province dating to a little more than 73.5 million years ago, not far from where the giant hadrosaur Shantungosaurus was discovered. Isolated tyrannosaur teeth and a toe bone had previously been found in the area, but the new skull material is unique enough to know that Zhuchengtyrannus was distinct from other large tyrants, including a second, as-yet-undescribed tyrannosaur found at the same location.

The distinguishing traits of the Zhuchengtyrannus fossils are subtle features that it takes a paleontologist's eye to see. How the various fenestrae, fossa, and other landmarks on the skull are organized make all the difference, especially since Zhuchengtyrannus was comparable in size to Tarbosaurus, another tyrannosaur living in the same area at the same time.

Both Zhuchengtyrannus and Tarbosaurus were tyrannosaurines, which is the group of familiar, large-bodied tyrant dinosaurs that lived in Asia and western North America during the Late Cretaceous. The new tyrant was also just as large as some of the largest Tarbosaurus specimens, which themselves were nearly as big as some of the biggest Tyrannosaurus from North America. Together Zhuchengtyrannus, Tarbosaurus, and the unnamed species from Zhucheng mark a high diversity of tyrant dinosaurs around the 74-million-year mark in Cretaceous China.

In fact, the discovery of the new tyrannosaurs from Zhucheng may make Tyrannosaurus an especially unusual predator. Hone and co-authors point out that many prehistoric ecosystems hosted multiple species of large predatory dinosaurs, from the Jurassic Morrison Formation of North America to the Late Cretaceous deposits of Morocco. Although the effects of time-averaging have to be taken into account, the general trend appears to be that multiple species of enormous, carnivorous dinosaurs lived alongside one another and likely had different feeding habits to allow this sort of partitioning.

Where Tyrannosaurus stands out is that it appears to have been the only large predator in many of the places where it has been found. Either there are some yet-undiscovered giant predators waiting to be found in the latest Cretaceous rocks of North America, or there was something different about the ecosystems where Tyrannosaurus lived. (For example, juvenile Tyrannosaurus may have hunted different prey, taking the role that might otherwise be played by a different species of large predator in other environments.) Familiar as they are, many mysterious still surround the tyrant dinosaurs.

For more on Zhuchengtyrannus, see Dave Hone's posts (I, II, III) on his Archosaur Musings blog.

References:

Hone, D.; Wang, K.; Sullivan, C.; Zhao, X.; Chen, S.; Li, D.; Ji, S.; Ji, Q.; Xing, X. (2011). A new tyrannosaurine theropod, Zhuchengtyrannus magnus is named based on a maxilla and dentary Cretaceous Research : 10.1016/j.cretres.2011.03.005

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.