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Tuesday, 23 September 2014

Ten Things We Don’t Know about Tyrannosaurs

Tyrannosaurus rex and its closest relatives, the tyrannosaurs, are among the best known and most popular dinosaurs - and yet there is still plenty we don’t know about these fascinating creatures...

Photograph of "Sue", a Tyrannosaurus Rex at the Field Museum of Natural History in Chicago, IL.
Despite its name, we don't know if the T-rex we know as "Sue" was male or female. Dinosaurs aren’t sexually dimorphic, including T. rex; their skeletons provide no clue as to their gender. The only evidence we have of a particular specimen's sex comes from either finding eggs inside of a skeleton, or finding medullary bone in long bones. Medullary bone has been found in only one T. rex so far. Image credit: Heather Paul (CC-BY-ND)

1. What age could T. rex live to?

It's possible to work out how old a tyrannosaur was when it died, by looking at growth rings inside its bones - just like counting the rings of a tree. The oldest T. rex yet examined in this way has been nicknamed Sue, and is on display at the Field Museum. It’s thought that Sue was 28 years old[1] when it died. Only about a dozen skeletons have been cut up to determine their age, and there are other T. rex’s that look like they might be older than Sue, but haven't had their growth rings counted. This means that we really don’t exactly know the maximum age of T. rex; it's possible that it will turn out to be much more than 28 years once the sample of adults has increased.

2. How were tyrannosaurs related?

Evolutionary trees are diagrams that can be drawn to show how animals are related to each other. Researchers gather data and use this to try to reconstruct the evolutionary history of a group of species - but it isn’t always simple. At the moment there are two versions of the evolutionary tree of tyrannosaurs[2][3] which differ in which species they include, and where they appear on the tree. As more data is collected, trees produced by different groups of researchers usually become more similar. It is likely that with more time and research we will, eventually, find a history that all of the available data supports. Until then though, how tyrannosaurs evolved remains something we don’t know.

3. What did their eggs, embryos, & hatchlings look like?

Despite the popularity of tyrannosaurs, we don’t know anything about the earliest growth stages of any tyrannosaur species. Currently, there are no skulls or skeletons of embryos or juveniles up to a year old. We don’t even know what a tyrannosaur eggshell looks like - very few embryos have been discovered inside fossilised eggs, which is the only way we could be certain of the species the egg belonged to, so the number of dinosaur species identified in this way is very low. It could be that tyrannosaur eggs have already been collected (among those that currently lack embryonic bones) but we just haven’t realised it yet! Hopefully this situation, at least for eggs and embryos, will change very soon as dinosaur eggs are being discovered all the time in places such as China.

4. Were there two groups of Tyrannosaurs in Laramidia?

In the Cretaceous period, what is now North America existed as 2 islands; Laramidia in the west, and Appalachia in the east, which were split by the Western Interior Seaway approximately 100 million years ago. Image courtesy United States Geological Survey (Public Domain)
It has been suggested that tyrannosaurs in the Late Cretaceous of western North America (Laramidia) can be divided into a northern group and a southern group[3]. However the fact that the fossil record from that period is incomplete throws doubt on this hypothesis. The far north and the far south of this region are virtually blank slates in terms of tyrannosaur fossils, and no fossils have yet been found from the 20 million years following the split of North America into the subcontinents of Laramidia and Appalachia. Once specimens have been found to fill the gaps, there is a chance that the proposed two groups will lose support and be replaced by something more complex.

5. What Tyrannosaurs lived in Appalachia?

It is extremely rare to find tyrannosaur fossils in the eastern region of North America, which existed as an island continent called Appalachia during the Late Cretaceous. In fact, only two species have been found, we only know of one skeleton for each of them, and neither is complete. We can tell from these that the eastern tyrannosaurs are more primitive than their western counterparts; they have shallow snouts and large arms, in contrast to the deep snouts and short arms seen in the Laramidian and Asian species. Finding out more about these animals would give clues as to what the ancestors of tyrannosaurs looked like in North America before it was split into two, but the rarity of these fossils means that Appalachia may remain the ‘dark continent’ of tyrannosaur history.

6. How long ago did the Nemegt species live?

The Nemegt Formation in the Mongolian People’s Republic is a rich source of fossils, and includes well-known tyrannosaurs such as Tyrannosaurus bataar (Tarbosaurus) and Alioramus. It is clear from looking at these fossils that they are closely related to those found in Laramidia, which suggests animals moved from one area to the other. Currently, however, we don’t know when, or in which direction, the exchange occurred. It is thought the Nemegt fauna occurred close to the end of the age of dinosaurs, but before the last slice of time that included T. rex. Often, the age of rocks can be determined by looking for the presence of certain radioactive materials, but unfortunately this hasn’t been possible in the Nemegt Formation as the materials aren’t present. This means we only have a rough idea of how long ago the rocks formed[4]. To narrow it down from the current estimate of 80-66 million years ago, we need to find a new area of rock that is possible to date but, unfortunately, this seems unlikely.

Overview of the Dragon’s Tomb site where the famous 'Nemegt' formation can be found, Altan Ula, Mongolia. Photograph by David Evans, University of Toronto.
In 1952, the “Dragon’s Tomb” in Mongolia’s Nemegt Formation was the site on which at least 15 specimens[8] of an until-then undiscovered species were discovered: Saurolophus angustirostris, a large hadrosaurine duck-billed dinosaur. Photograph ©David Evans, used with permission.

7. Did advanced tyrannosaurs have scales?

We have all seen pictures and models of T. rex looking like a giant lizard, but in reality we don’t know what covered the skin of advanced tyrannosaurs. More and more types of dinosaurs, and their relatives the flying pterosaurs (including Pterodactylus), are being found with feathers and hair-like structures on their skin, which would make them appear very different to the images in popular media. However, in some lineages such as sauropods, duckbilled dinosaurs and horned dinosaurs, feathers were lost, and scales reappeared. When it comes to tyrannosaurs, the picture is less clear. The most ancient tyrannosaurs were feathery in species big (Yutyrannus) and small (Dilong), but the presence of scales is known from patches in a handful of specimens of advanced tyrannosaurs, although these have only been reported in passing without detailed description[5]. Most specimens aren’t well enough preserved to get a full picture of their external appearance. If, however, tyrannosaur mummies are found one day, this would cast light on this aspect of their appearance.

8. What happened in the middle Jurassic period?

Unfortunately, we can only discover what the fossil record allows, and there are periods of time where there is little to work with. One of these is the Middle Jurassic, which was between 176 and 161 million years ago. This means there is a gap in our knowledge of tyrannosaur evolution, and it is difficult to determine how they developed from so called ‘basal’ forms to the more ‘advanced’ tyrannosaurs, or if these forms are even related. Hopefully, the discovery of more fossils from this interval will help fill in the gaps and give us a better picture of the history of tyrannosaurs.

9. Why did the alioramins have such long, low snouts?

Tyrannosaur workers have recently seen the addition of a new lineage, the alioramins[6]. The species in this group, Alioramus and Qianzhousaurus, have long and low snouts, in contrast to the short and deep snouts seen in, say, T. rex. T rex used its snout to deliver powerful bites to its prey, so while it is likely the longer snouts of the alioramins are an adaptation to a different kind of prey, its exact function is presently unknown.

10. Were there any tyrannosaurs in Siberia?

So far, fossils of several types of Late Cretaceous dinosaurs have been found in the far northeast of Russia, but no tyrannosaurs[7]. It is thought that dinosaurs dispersed across Northeastern Siberia and the north slope of Alaska, when moving between between Asia and Laramidia during the Late Cretaceous, so it is likely there are specimens here waiting to be found. Whatever tyrannosaur species are found in Siberia, they will be entirely new to science and they will certainly jostle the family tree.

This article was written by Dr Thomas Carr PhD, a vertebrate paleontologist who specializes in the growth and evolution of tyrannosaurs. He has named three new genera of tyrannosaurs, namely Appalachiosaurus (Alabama), Bistahieversor (New Mexico), and Teratophoneus (Utah), and he was part of the team who named Alioramus altai (Mongolia). Dr. Carr was the first to publish growth series of dinosaurs using cladistic analysis, a method usually used for recovering evolutionary relationships, for Tyrannosaurus rex and Albertosaurus sarcophagus. He collects dinosaur fossils with his students and volunteers each summer on federally regulated lands in southeastern Montana. Dr. Carr is an Associate Professor of Biology at Carthage College (Kenosha, WI), the Director of the Carthage Institute of Paleontology, and the Senior Scientific Adviser to the Dinosaur Discovery Museum (Kenosha, WI). You can visit Dr. Carr's blog, Tyrannosauroidea Central, and follow his tweets at @Tyrannosaurcarr.

why don't all references have links?

[1] Erickson, Gregory M et al. "Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs." Nature 430.7001 (2004): 772-775. doi:10.1038/nature02699
[2] Brusatte, S. L., Norell, M. A., Carr, T. D., Erickson, G. M., Hutchinson, J. R., Balanoff, A. M., Bever, G. S., Choiniere, J. N., Makovicky, P. J., and Xu, X. 2010. "Tyrannosaur paleobiology: New research on ancient exemplar organisms." Science 329: 1481-1485.
[3] 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
[4] Shuvalov, V. F. 2000. "The Cretaceous stratigraphy and paleobiogeography of Mongolia." In Benton, M. J., Shichkin, M. A., Unwin, D. M., and Kurochkin, E. N. (eds.) The Age of Dinosaurs in Russia and Mongolia, pp. 256-278. Cambridge University Press, Cambridge.
[5] Currie, P. J. 2004. "Theropods, Including Birds." In Currie, P. J. and Koppelhus, E. B. (eds.) Dinosaur Provincial Park: A spectacular ancient ecosystem revealed; pp. 367-397. Indiana University Press, Bloomington and Indianapolis.
[6] Lu, J., Yi, L., Brusatte, S. L., Yang, L., Li, H., and Chen, L. 2014. "A new clade of Asian Late Cretaceous long-snouted tyrannosaurids." Nature Communications 5, article number: 3788. doi:10.1038/ncomms4788
[7] Weishampel, D. B., Barrett, P. M., Coria, R. A., Le Loeuff, J., Xing, X., Xijin, Z., Sahni, A., Gomani, E. M. P., and Noto, C. "Dinosaur Distribution." In Weishampel, D. B., Dodon, P., and Osmolska, H. (eds.) The Dinosauria Second Edition, pp. 517-606. University of California Press, Berkeley.
[8] Horner, John R.; Weishampel, David B.; Forster, Catherine A (2004). "Hadrosauridae". In Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 438–463. ISBN 0-520-24209-2.