Growth Paleontologist
Gregory Erickson and colleagues have studied the growth and life history of tyrannosaurids. Analysis of bone
histology can determine the age of a specimen when it died. Growth rates can be examined when the age of various individuals are plotted against their size on a graph. Erickson has shown that after a long time as juveniles, tyrannosaurs underwent tremendous
growth spurts for about four years midway through their lives. After the rapid growth phase ended with
sexual maturity, growth slowed down considerably in adult animals. A tyrannosaurid growth curve is S-shaped, with the maximum growth rate of individuals around 14 years of age.
T. rex juveniles remained under until approximately 14 years of age, when body size began to increase dramatically. During this rapid growth phase, a young
T. rex would gain an average of a year for the next four years. This slowed after 16 years, and at 18 years of age, the curve plateaus again, indicating that growth slowed dramatically. For example, only separated the 28-year-old "Sue" from a 22-year-old
Canadian specimen (
RTMP 81.12.1). Medullary tissue is found only in female birds during ovulation, indicating that "B-rex" was of reproductive age. Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes. Compared to albertosaurines,
Daspletosaurus showed a faster growth rate during the rapid growth period due to its higher adult weight. The maximum growth rate in
Daspletosaurus was per year, based on a mass estimate of in adults. Other authors have suggested higher adult weights for
Daspletosaurus; this would change the magnitude of the growth rate, but not the overall pattern. Some other specimens of different sizes has been found, but their age at death has not been determined.--> The discovery of an embryonic tyrannosaur of an as-yet-unknown genus suggests that tyrannosaurids developed their distinctive skeletal features while developing in the egg. Furthermore, the size of the specimen, a dentary from the lower jaw found in the
Two Medicine Formation of Montana in 1983 and a foot claw found in the
Horseshoe Canyon Formation in 2018 and described in 2020, suggests that neonate tyrannosaurids were born with skulls the size of a mouse or similarly sized rodents and may have been roughly the size of a small dog at birth. The jaw specimen is believed to have come from an animal roughly while the claw is believed to belong to a specimen measuring around . While eggshells have not been found in association with either specimen, the location where these neonate tyrannosaurids were uncovered suggests these animals were using the same nest sites as other species they lived with and preyed upon. The lack of eggshells associated with these specimens has also opened up speculation to the possibility that tyrannosaurids laid soft-shelled eggs as the genera
Mussaurus and
Protoceratops are believed to have done. Fossil footprints from the Wapiti Formation suggest that as tyrannosaurids grew, the feet became wider with thicker toes to support their weight. The broader feet suggest that adult tyrannosaurids were slower-moving than their offspring.
Life history The end of the rapid growth phase suggests the onset of
sexual maturity in
Albertosaurus, although growth continued at a slower rate throughout the animals' lives. and large dinosaurs as well as in large mammals, such as humans and
elephants. By tabulating the number of specimens of each age group, Erickson and his colleagues were able to draw conclusions about life history in tyranosauridae populations. Their analysis showed that while juveniles were rare in the fossil record, subadults in the rapid growth phase and adults were far more common. Over half of the known
T. rex specimens appear to have died within six years of reaching sexual maturity, a pattern that is also seen in other tyrannosaurs and in some large, long-lived birds and mammals today. These species are characterized by high infant mortality rates, followed by relatively low mortality among juveniles. Mortality increases again following sexual maturity, partly due to the stresses of reproduction. While this could be due to preservation or collection
biases, Erickson hypothesized that the difference was due to low mortality among juveniles over a certain size, which is also seen in some modern large mammals, like
elephants. This low mortality may have resulted from a lack of predation, since tyrannosaurs surpassed all contemporaneous predators in size by the age of two. Paleontologists have not found enough
Daspletosaurus remains for a similar analysis, but Erickson notes that the same general trend seems to apply.
Albertosaurus have been found in aggregations that some have suggested to represent mixed-age
packs that filled separate roles during hunting based on age class, with the more cursorial juveniles driving prey towards the adults.
Locomotion Locomotion abilities are best studied for
Tyrannosaurus, and there are two main issues concerning this: how well it could turn; and what its maximum straight-line speed was likely to have been.
Tyrannosaurus may have been slow to turn, possibly taking one to two seconds to turn only 45° – an amount that humans, being vertically oriented and tail-less, can spin in a fraction of a second. The cause of the difficulty is
rotational inertia, since much of
Tyrannosauruss mass was some distance from its center of gravity, like a human carrying a heavy timber. Scientists have produced a wide range of maximum speed estimates, mostly around , but a few as low as , and a few as high as . Researchers have to rely on various estimating techniques because, while there are many
tracks of very large theropods walking, so far none have been found of very large theropods running—and this absence
may indicate that they did not run. Jack Horner and Don Lessem argued in 1993 that
Tyrannosaurus was slow and probably could not run (no airborne phase in mid-stride). However, Holtz (1998) concluded that tyrannosaurids and their close relatives were the fastest large theropods. Christiansen (1998) estimated that the leg bones of
Tyrannosaurus were not significantly stronger than those of elephants, which are relatively limited in their top speed and never actually run (there is no airborne phase), and hence proposed that the dinosaur's maximum speed would have been about , which is about the speed of a human sprinter. Farlow and colleagues (1995) have argued that a 6- to 8-ton
Tyrannosaurus would have been critically or even fatally injured if it had fallen while moving quickly, since its torso would have slammed into the ground at a deceleration of 6
g (six times the acceleration due to gravity, or about 60 metres/s2) and its tiny arms could not have reduced the impact. However,
giraffes have been known to gallop at , despite the risk that they might break a leg or worse, which can be fatal even in a "safe" environment such as a zoo. Thus it is quite possible that
Tyrannosaurus also moved fast when necessary and had to accept such risks; this scenario has been studied for
Allosaurus too. Most recent research on
Tyrannosaurus locomotion does not narrow down speeds further than a range from , i.e. from walking or slow running to moderate-speed running. A computer model study in 2007 estimated running speeds, based on data taken directly from fossils, and claimed that
T. rex had a top running speed of . (Probably a juvenile individual.) Studies by Eric Snively
et al., published in 2019 indicate that tyrannosaurids such as
Tarbosaurus and
Tyrannosaurus itself were more manoeuvrable than allosauroids of comparable size due to low rotational inertia compared to their body mass combined with large leg muscles. As a result, it is hypothesized that tyrannosaurids were capable of making relatively quick turns and could likely pivot their bodies more quickly when close to their prey, or that while turning, they could "pirouette" on a single planted foot while the alternating leg was held out in a suspended swing during pursuit. The results of this study potentially could shed light on how agility could have contributed to the success of tyrannosaurid evolution. Additionally, a 2020 study indicates that tyrannosaurids were exceptionally efficient walkers. Studies by Dececchi
et al., compared the leg proportions, body mass, and the gaits of more than 70 species of theropod dinosaurs including tyrannosaurids. The research team then applied a variety of methods to estimate each dinosaur's top speed when running as well as how much energy each dinosaur expended while moving at more relaxed speeds such as when walking. Among smaller to medium-sized species such as dromaeosaurids, longer legs appear to be an adaptation for faster running, in line with previous results by other researchers. But for theropods weighing over , top running speed is limited by body size, so longer legs instead were found to have correlated with low-energy walking. The results of the study further indicated that smaller theropods evolved long legs for speed as a means to both aid in hunting and escape from larger predators while larger predatory theropods that evolved long legs did so to reduce the energy costs and increase foraging efficiency, as they were freed from the demands of predation pressure due to their role as apex predators. Compared to more basal groups of theropods in the study, tyrannosaurids showed a marked increase in foraging efficiency due to reduced energy expenditures during hunting and scavenging. This likely resulted in tyrannosaurs having a reduced need for hunting forays and requiring less food to sustain themselves as a result. Additionally, the research, in conjunction with studies that show tyrannosaurs were more agile than other large-bodied theropods, indicates they were quite well-adapted to a long-distance stalking approach followed by a quick burst of speed to go for the kill. Analogies can be noted between tyrannosaurids and modern wolves as a result, supported by evidence that at least some tyrannosaurids such as
Albertosaurus were hunting in group settings.
Integument An ongoing debate in the paleontological community surrounds the extent and nature of tyrannosaurid integumentary covering. Long
filamentous structures have been preserved along with skeletal remains of numerous coelurosaurs from the Early Cretaceous
Yixian Formation and other nearby
geological formations from
Liaoning, China. These filaments have usually been interpreted as "protofeathers,"
homologous with the branched feathers found in birds and
some non-avian theropods, although other hypotheses have been proposed. A skeleton of
Dilong was described in 2004 that included the first example of "protofeathers" in a tyrannosauroid. Similarly to
down feathers of modern birds, the "protofeathers" found in
Dilong were branched but not
pennaceous, and may have been used for
insulation. Based on the principle of
phylogenetic bracketing, it was predicted that tyrannosaurids might also possess such feathering. However, a study in 2017 published by a team of researchers in Biology Letters described tyrannosaurid skin impressions collected in Alberta, Montana, and Mongolia, which came from five genera (
Tyrannosaurus,
Albertosaurus,
Gorgosaurus,
Daspletosaurus and
Tarbosaurus). Although the skin impressions are small, they are widely dispersed across the post-cranium, being collectively located on the abdomen, thoracic region, ilium, pelvis, tail, and neck. They show a tight pattern of fine, non-overlapping pebbly scales (which co-author Scott Persons compared to those seen on the flanks of a crocodile) and preserve no hints of feathering. The basic texture is composed of tiny "basement scales" approximately 1 to 2 mm in diameter, with some impressions showing 7 mm "feature scales" interspersed between them. Additional scales can be seen in tyrannosaurid footprints. Studies find that the facial integument of tyrannosaurids had scales on the dentary and maxilla, cornified epidermis and armor-like skin on the subordinate regions. Bell
et al. performed an ancestral character reconstruction based on what is known about integument distribution in tyrannosauroids. Despite an 89% probability that tyrannosauroids started out with feathers, they determined that scaly tyrannosaurids have a 97% probability of being true. The data "provides compelling evidence of an entirely squamous covering in Tyrannosaurus," the team wrote, although they conceded that plumage may have still been present on the dorsal region where skin impressions haven't been found yet. Bell
et al. hypothesizes that the scale impressions of tyrannosaurids are possibly reticula which are secondarily derived from feathers though evidence is needed to support this. It has yet to be determined why such an integumentary change might have occurred. A precedent for feather loss can be seen in other dinosaur groups such as
ornithischians, in which filamentous structures were lost, and scales reappeared. Although gigantism has been suggested as a mechanism, Phil R. Bell, who co-authored the study, noted that the feathered
Yutyrannus overlapped in size with
Gorgosaurus and
Albertosaurus. "The problem here is that we have big tyrannosaurs, some with feathers, some without that live in pretty similar climates. So what's the reason for this difference? We really don't know."
Vision The eye-sockets of
Tyrannosaurus are positioned so that the eyes would point forward, giving them
binocular vision slightly better than that of modern
hawks. While predatory theropods in general had binocular vision directly in front of their skull, tyrannosaurs had a significantly larger area of overlap.
Jack Horner also pointed out that the tyrannosaur lineage had a history of steadily improving binocular vision. It is hard to see how
natural selection would have favored this long-term trend if tyrannosaurs had been pure scavengers, which would not have needed the advanced
depth perception that
stereoscopic vision provides. In modern animals, binocular vision is found mainly in predators (the principal exceptions are
primates, which need it for leaping from branch to branch). Unlike
Tyrannosaurus,
Tarbosaurus had a narrower skull more typical of other tyrannosaurids in which the eyes faced primarily sideways. All of this suggests that
Tarbosaurus relied more on its senses of smell and hearing than on its eyesight. In
Gorgosaurus specimens, the
eye socket was circular rather than oval or keyhole-shaped as in other tyrannosaurid genera.
Facial sensitivity Based on comparisons of
bone texture of
Daspletosaurus with extant
crocodilians, a detailed study in 2017 by
Thomas D. Carr et al. found that tyrannosaurs had large, flat
scales on their
snouts. At the center of these
scales were small
keratinised patches. In
crocodilians, such patches cover bundles of
sensory neurons that can detect mechanical, thermal and chemical
stimuli. They proposed that
tyrannosaurs probably also had bundles of
sensory neurons under their facial
scales and may have used them to identify objects, measure the
temperature of their
nests and gently pick-up
eggs and
hatchlings.
Tyrannosaurus rex itself was claimed to have been
endothermic ("warm-blooded"), implying a very active lifestyle. Since then, several paleontologists have sought to determine the ability of
Tyrannosaurus to
regulate its body
temperature. Histological evidence of high growth rates in young
T. rex, comparable to those of mammals and birds, may support the hypothesis of a high metabolism. Growth curves indicate that, as in mammals and birds,
T. rex growth was limited mostly to immature animals, rather than the
indeterminate growth seen in most other
vertebrates. Later they found similar results in
Giganotosaurus specimens, who lived on a different continent and tens of millions of years earlier in time. Even if
Tyrannosaurus rex does exhibit evidence of homeothermy, it does not necessarily mean that it was endothermic. Such thermoregulation may also be explained by
gigantothermy, as in some living
sea turtles. ==Paleoecology==