and its closest relatives are pop culture icons. Tyrannosaurs, the group of dinosaurian carnivores that includes Tyrannosaurus rex
, are also the most intensively studied extinct dinosaurs,Tyrannosaurus rex
was initially described 105 years ago, and for most of the 20th century tyrannosaurs were known almost solely from fossils of Tyrannosaurus rex
and four closely related species of large, multi-ton Late Cretaceous predators. Within the last decade, however, the diversity of tyrannosaurs has more than doubled, and during the past year alone, six new species were described, some of which are 100 million years older and 1/100 the size of Tyrannosaurus rex
A new paper reviewing recent research and the evolution of tyrannosaurs was published in the journal Science
of 17 September 2010.
Approximately 20 tyrannosauroid genera are currently known, 5 of which were described during the past year. Tyrannosaurs are a long-lived group that originated by the Middle Jurassic, ~165 million years ago. Until recently, the prevailing notion was that tyrannosaur body size gradually, and progressively, increased over time. However, new discoveries have led to a reassessment. Enormous size is restricted to the latest Cretaceous tyrannosaurids, some of which grew to lengths of 13 m and masses of 5 to 8 tons. For the first 80 million years of their history tyrannosaurs were mostly small- to mid-sized animals that lived in the shadow of other giant predators, and only during the final 20 million years of the Mesozoic did they develop into some of the largest terrestrial carnivores to ever live. The dominance of tyrannosaurs as megapredators was purely a latest Cretaceous phenomenon.
Arguably palaeontologists know more about tyrannosaur biology than that of any other dinosaurs. Much of this knowledge has been gained over the past 20 years, through the collection of skeletons of both adults and juveniles, bones of their prey with bite marks, coprolites (fossil faeces, or fossilised dung if you like), stomach contents, and specimens marked by diseases.
Bite marks, coprolites, and quantitative techniques have helped to reveal what tyrannosaurs ate and how they fed. Tyrannosaurid bite marks have been found on the bones of a wide diversity of species, including various other tyrannosaurs, demonstrating that they were generalists. Bite mark patterns show that tyrannosaurids characteristically bit deeply into carcasses, often through bones, and then pulled back, creating long cuts. Some Tyrannosaurus rex
bite marks and coprolites with bone chunks indicate that bone was fractured, ingested, and used for sustenance, a mammal-like attribute not seen in extant reptiles. The bite forces needed to crunch through bone would have been enormous. Biomechanical experiments have replicated the size and depth of fossilised bite marks and suggest that Tyrannosaurus rex
generated bite forces of at least 13.4 kN. (For comparison the bite force exerted by an adult Nile crocodile has been shown to measure 22 kN - if you have ever seen a crocodile snap its prey, you may have an idea about what that means).
Several signs indicate that tyrannosaurs changed their habits as they grew. In Late Cretaceous tyrannosaurids, the difference in form between the lightly built, fast and nimble juveniles and the larger, bulkier adults indeed suggests that foraging behavior and targeted prey size changed as they grew. The deep and muscular adult skull, with reinforced sutures and robust teeth, is well suited for sustaining high bite forces, whereas juveniles had none of these features. Furthermore, the longer and more slender hind limbs of juveniles indicate that they were relatively faster than adults. These differences could have promoted major size-related shifts in behaviour. It is plausible that adults preferentially attacked larger, but less mobile, prey than their younger counterparts. Such a shift from juvenile to adult behaviour is not seen in many familiar predators today (e.g., lions), but is present in extant crocodilians.
Whether Tyrannosaurus rex
and other large tyrannosaurs were scavengers or predators has generated much speculation and dispute. Tyrannosaur stomach contents indicate that tyrannosaurs were capable of active predation, but like most carnivores, tyrannosaurs probably both scavenged and hunted.
Tyrannosaur fossils have mainly been found in Asia and North America, but they were likely present on the southern continents during their early evolutionary history. It is likely that tyrannosaurs preferred wetter habitats (isn’t that what we have always been told?), but this assumption may reflect a sampling bias.Tyrannosaurus rex
is just the tip of the iceberg of tyrannosaur diversity, and is quite abnormal when compared with other members of the group. Reference
Brusatte et al.Tyrannosaur Paleobiology: New Research on Ancient Exemplar Organisms
Science 17 September 2010:
Vol. 329. no. 5998, pp. 1481 - 1485