A Beginner’s Guide to Theropods, Part 4: TYRANNOSAURS!
The tyrannosaurids of the late Late Cretaceous were in a unique position. While earlier theropods had spread freely across the continents, tyrannosaurs lived in a world of fragmented continents and inland seaways. In Asia and western North America, where they were restricted, rising mountains and ebbing seaways formed a huge diversity of habitats, inhabited by a huge diversity of hadrosaurs and ceratopsians. For the first time in theropod history, there were not only no other giant apex predators, but the next biggest carnivorous dinosaurs - dromaeosaurs and troodontids - were over an order of magnitude smaller than they were (Holtz). This meant they were free to not only inherit the role of big-game hunters but, throughout their ontogeny, to maintain their ancestral roles as mid-sized, long-legged pursuers of small, fast animals.
Left: Proportions in juvenile and adult Tyrannosaurus, and the animals they hunted. Right: Theropods in Tyrannosaurus’ environment, with non-predators highlighted (Holtz).
By counting rings in bone laid down at regular intervals (called “lines of arrested growth”), researchers can estimate that, while nearly all dinosaurs grew quickly, most spent much of their lives below their maximum adult size. Tyrannosaurs spent as much as half their lives as juveniles before undergoing rapid and enormous growth spurts, reaching reproductive maturity, leveling off, and dying shortly after. In terms of age, T. Rex's growth curve looks much like that of a human—if you can imagine an 1800-kg preteen putting on another 600 every year!
Tyrannosaur growth curves (Erickson) compared to those of other archosaurs (Hone and Mallon- scaled to proportion of total lifespan).
This growth period was not only fast; it involved some radical morphological changes (Carr). The skull grew deeper, accommodating massive jaw muscles. The teeth on the sides of the mouth grew thicker - almost conical - and deeper-rooted. Skull bones fused, air sacs expanded, and species-specific crests formed on the eyes and snout. These changes are so great, especially in Tyrannosaurus, that some researchers have considered certain “morphs” to be their own species. It's important to note that no known specimen of “Nanotyrannus” is mature, and Carr argues that whatever features aren't already known from individual variation are either found in younger juveniles or older adults. Likewise, a proposal that Tyrannosaurus represents three species, using many characters that had previously been hypothesized as sexually dimorphic, more probably reflects continuous variation in a diverse, wide-ranging species (Carr et al).
While young tyrannosaurs and their parents were certainly not eating together, there is contentious evidence that they may have lived together. Three Daspletosaurus of different ages have been found in a Montana bonebed, surrounded by scattered, chewed-up remains of hadrosaurs. Another bed contained remains of 26 Albertosaurus! Some (Currie) have suggested these are evidence of mixed-age packs, but others (Roach and Brinkman) think they're more likely opportunistic, Komodo-dragon-like feeding frenzies. We know they must have fought over food, territory, or mates, because individuals of several species show healed bite wounds from other tyrannosaurs (Brown et al), and some (Dalman and Lucas) even show evidence of cannibalism!
Because of their bone-crunching adaptations, we know more about tyrannosaur feeding habits than any other theropods’. Tail vertebrae from an Edmontosaurus have been found with a T.rex tooth embedded in the bone, with new bone growing around it (dePalma et al). Likewise, a Triceratops horn with bite marks on it shows it was partly broken off, but its owner lived to heal (Happ and Carpenter). Specimens of the Asian hadrosaur Saurolophus (Hone and Watabe) and ornithomimosaur Deinocheirus (Bell et al) show that Tarbosaurus was a selective feeder, using punctures, scrapes, and gouges on different parts of the bone to get as much out of it as possible. By simulating forces required to produce these marks (Erickson et al), along with models of the jaw musculature (Bates and Falkingham), several teams have estimated that Tyrannosaurus could bite down with up to 12,000 pounds of force, three times as strong as a saltwater crocodile and that even juveniles were strong enough to puncture bone (Peterson et al). But like most large predators, tyrannosaurs would not have habitually targeted large, dangerous adults, typically going after lower-risk juveniles or scavenging, chewing up bone whenever they had the chance (Hone and Rauhut).
Acrocanthosaurus vs. Tyrannosaurus (Holtz).
Practically everything about tyrannosaur anatomy seems designed to deliver the most powerful bite possible, from the robust, flexible neck - for a wide range of biting motions and angles - to the wide leg muscles and deep counterbalancing tail for carrying the massive head around (Snively and Russell). (Incidentally, if any dinosaur tried to stand in a kangaroo-like “tripod” pose, it would dislocate its hips and neck.) The back of the skull was wide, accommodating massive jaw muscles, but also giving them exceptional binocular vision (Stevens).
Unlike most theropods, which had bladelike slicing teeth, tyrannosaur teeth were built for puncturing and pulling. Two-thirds of the already long teeth were root, and they were curved and reinforced against pulling stresses. Also, unusually, they had a crocodile-like bony palate, allowing them to resist torsion from struggling animals and ripping flesh (Holtz). Even the notoriously tiny arms - too short to touch each other or reach the mouth - probably had some use, being robust and with wide areas for muscle attachment. These may have made powerful flexors, able to hold prey close without getting in the way of the jaws (Carpenter and Smith; Padian). They were sensitive, too: casts of their braincases show big olfactory bulbs and inner ears attuned to rapid head movements (Witmer and Ridgley). By studying patterns of tooth wear and bone texture, (Cullen et al) infer their long teeth - like those of most theropods - were covered with monitor-like lips, keeping them moist and protected from erosion. Some authors (Carr; Kawabe and Hattori) think these were covered with keratinized scales, protecting them from the bites of other tyrannosaurs. In crocodiles, scales like these cover bundles of sensory neurons, allowing them to sense temperature and make fine-scale movements for building nests and adjusting bites.
A Word on Size
How did so many dinosaurs reach such gigantic sizes? Why are no mammalian carnivores as big as Tyrannosaurus and no herbivores the size of sauropods? There are a number of possible reasons, each of which may be necessary but not sufficient to reach truly giant sizes. First, it's important to note the biggest animal ever, the blue whale, is alive today. But to be big on land, all animals need certain adaptations. These include upright limbs to support the weight of the body (cheated by some water-living crocodilians) and a powerful 4-chambered heart to generate the pressure required to move it (Pontzer et al). Environment plays an important role, too: although they fluctuate throughout its 180-million-year span, levels of both oxygen and carbon dioxide were generally higher throughout the Mesozoic (Price et al). This meant that plants were more productive and, with their uniquely poised system of air sacs distributing oxygen around the body, dinosaurs could be too.
But perhaps the most important factor, as we've already seen, is growth. Unlike large mammals, in which babies are already large animals, dinosaur hatchlings were tiny (the biggest sauropods laid eggs smaller than a basketball - any larger and the shell would be too thick for the hatchling to breathe). Unlike the big crocodilians and sea turtles they lived alongside, dinosaurs grew quickly and died young, with most reaching reproductive maturity while still growing. Because dinosaurs spent so much of their lives growing, they were able to effectively take on the roles of multiple modern animals, skewing their adult size distribution higher than any other group of animals (O'Gorman and Hone).
How could they afford such fast-paced lives? On average, each individual of any animal has only one offspring that survives to adulthood and reproduces: any more and populations would grow unsustainable. Any fewer, and they'd decline to the point of extinction. This can be achieved through different reproductive strategies: while large mammals bear small numbers of large young, most dinosaurs were r-strategist reproducers. This means they grew to reproductive age quickly, laid a lot of eggs, invested less in each individual offspring, and could afford higher chances of each one not making it to adulthood. In some cases, they were able to fend for themselves straight after hatching. Large mammals, by contrast, need to live long and invest in their offspring heavily, because the length of their gestation means the loss of one might take years to replace! The bigger the mammal, the longer the gestation, the greater the risk of extinction (Holtz). As we'll see later, some dinosaurs managed to reverse this strategy, committing to fewer offspring and retaining small body sizes throughout their lives, effectively colonizing those niches for good.
Works Cited
Holtz, Thomas (2021). Lectures for GEOL-104. Dinosaurs: A Natural History. University of Maryland. youtube.com/playlist?list=PLzO5aKfW25o1YGvDbUJC8yT9mD0TQn3MX
Erickson, Gregory M., GM; Makovicky, Peter J.; Currie, Philip J.; Norell, Mark A.; Yerby, Scott A.; Brochu, Christopher A. (2004). "Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs". Nature. 430 (7001). DOI: 10.1038/nature02699
Hone, D. W., & Mallon, J. C. (2017). Protracted growth impedes the detection of sexual dimorphism in non‐avian dinosaurs. Palaeontology, 60(4). 10.1111/pala.12298
Carr, T. D. (2020). A high-resolution growth series of Tyrannosaurus rex obtained from multiple lines of evidence. PeerJ, 8. https://doi.org/10.7717/peerj.9192
Carr, T. D., Napoli, J. G., Brusatte, S. L., Holtz, T. R., Hone, D. W., Williamson, T. E., & Zanno, L. E. (2022). Insufficient evidence for multiple species of tyrannosaurus in the latest cretaceous of North America: A comment on “The tyrant lizard king, Queen and emperor: Multiple lines of morphological and stratigraphic evidence support subtle evolution and probable speciation within the North American genus tyrannosaurus.” Evolutionary Biology, 49(3), 327–341. https://doi.org/10.1007/s11692-022-09573-1
Currie, Philip J.; Trexler, David; Koppelhus, Eva B.; Wicks, Kelly; Murphy, Nate (2005). "An unusual multi-individual tyrannosaurid bonebed in the Two Medicine Formation (Late Cretaceous, Campanian) of Montana (USA)". In Carpenter, Kenneth (ed.). The Carnivorous Dinosaurs. ISBN 978-0-253-34539-4.
Roach, Brian T.; Brinkman, Daniel T. (2007). "A reevaluation of cooperative pack hunting and gregariousness in Deinonychus antirrhopus and other nonavian theropod dinosaurs". Bulletin of the Peabody Museum of Natural History. 48 (1). doi:10.3374/0079-032X(2007)48[103:AROCPH]2.0.CO;2
Brown, C. M., Currie, P. J., & Therrien, F. (2021). Intraspecific facial bite marks in tyrannosaurids provide insight into sexual maturity and evolution of bird-like intersexual display. Paleobiology, 48(1). https://doi.org/10.1017/pab.2021.29
Dalman, S. G., & Lucas, S. G. (2017). New evidence for cannibalism in tyrannosaurid dinosaurs from the late cretaceous of New Mexico. New Mexico Geological Society, 2017 Annual Spring Meeting. https://doi.org/10.56577/sm-2017.482
DePalma, R. A., Burnham, D. A., Martin, L. D., Rothschild, B. M., & Larson, P. L. (2013). Physical evidence of predatory behavior in Tyrannosaurus rex. Proceedings of the National Academy of Sciences, 110(31). https://doi.org/10.1073/pnas.1216534110
Happ, John; Carpenter, Kenneth (2008). "An analysis of predator–prey behavior in a head-to-head encounter between Tyrannosaurus rex and Triceratops". In Carpenter, Kenneth; Larson, Peter E. (eds.). Tyrannosaurus rex, the Tyrant King (Life of the Past). ISBN 978-0-253-35087-9.
Hone, D. W. E.; Watabe, M. (2011). "New information on scavenging and selective feeding behaviour of tyrannosaurids". Acta Palaeontologica Polonica. 55 (4). doi:10.4202/app.2009.0133.
Bell, P. R.; Currie, P. J.; Lee, Y. N. (2012). "Tyrannosaur feeding traces on Deinocheirus (Theropoda:?Ornithomimosauria) remains from the Nemegt Formation (Late Cretaceous), Mongolia". Cretaceous Research. 37. doi:10.1016/j.cretres.2012.03.018.
Erickson, G. M., Kirk, S. D., Su, J., Levenston, M. E., Caler, W. E., & Carter, D. R. (1996). Bite-force estimation for tyrannosaurus rex from tooth-marked bones. Nature, 382(6593), 706–708. https://doi.org/10.1038/382706a0
Bates, K. T., & Falkingham, P. L. (2012). Estimating maximum bite performance in Tyrannosaurus rex using multi-body dynamics. Biology Letters, 8(4), 660–664. https://doi.org/10.1098/rsbl.2012.0056
Peterson, Joseph E.; Tseng, Z. Jack; Brink, Shannon (2021). "Bite force estimates in juvenile Tyrannosaurus rex based on simulated puncture marks". PeerJ. 9: e11450. doi:10.7717/peerj.11450.
Hone, D. W.; Rauhut, O. W. (2010). "Feeding behaviour and bone utilization by theropod dinosaurs". Lethaia. 43 (2). doi:10.1111/j.1502-3931.2009.00187.x.
Snively, Eric; Russell, Anthony P (2007). "Craniocervical feeding dynamics of Tyrannosaurus rex". Paleobiology. 33 (4). doi:10.1666/06059.1.
Stevens, K. A. (2006). Binocular vision in theropod dinosaurs. Journal of Vertebrate Paleontology, 26(2). https://doi.org/10.1671/0272-4634(2006)26[321:bvitd]2.0.co;2
Carpenter, K.; Smith, M. (2001). "Forelimb Osteology and Biomechanics of Tyrannosaurus rex". In Tanke, D. H.; Carpenter, K. (eds.). Mesozoic vertebrate life. ISBN 978-0-253-33907-2
Padian, K. (2022). Why tyrannosaur forelimbs were so short: an integrative hypothesis. Acta Palaeontologica Polonica, 67. https://doi.org/10.4202/app.00921.2021
Witmer, L. M., & Ridgely, R. C. (2009). New insights into the brain, braincase, and Ear Region of tyrannosaurs (Dinosauria, Theropoda), with implications for sensory organization and behavior. The Anatomical Record, 292(9). doi.org/10.1002/ar.20983
Cullen, T. M., Larson, D. W., Witton, M. P., Scott, D., Maho, T., Brink, K. S., Evans, D. C., & Reisz, R. (2023). Theropod dinosaur facial reconstruction and the importance of soft tissues in paleobiology. Science, 379(6639). https://doi.org/10.1126/science.abo7877
Carr, Thomas D.; Varricchio, David J.; Sedlmayr, Jayc C.; Roberts, Eric M.; Moore, Jason R. (2017). "A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system". Scientific Reports. 7: 44942. doi:10.1038/srep44942
Kawabe, S., & Hattori, S. (2021). Complex neurovascular system in the dentary of tyrannosaurus. Historical Biology, 34(7). https://doi.org/10.1080/08912963.2021.1965137
Pontzer, H., Allen, V., & Hutchinson, J. R. (2009). Biomechanics of running indicates endothermy in bipedal dinosaurs. PLoS ONE, 4(11). DOI: 10.1371/journal.pone.0007783
Price, G. D., Twitchett, R. J., Wheeley, J. R., & Buono, G. (2013). Isotopic evidence for long term warmth in the Mesozoic. Scientific Reports, 3(1). DOI: 10.1038/srep01438
O’Gorman, E. J., & Hone, D. W. (2012). Body size distribution of the dinosaurs. PLoS ONE, 7(12). https://doi.org/10.1371/journal.pone.0051925
