The Ultimate Guide To Tyrannosaurs

Author: Ashley Strack
Tyrannosaurus rex may be the most well known dinosaur to the general public. This massive cretaceous-era theropod dinosaur has dominated the public consciousness’ image of “dinosaurs” for over a century since its discovery, and for good reason. Standing at approximately 39-42 feet in length and a towering 13-15 feet high at the hips, this 6-9 ton carnivore was given a name meaning “Tyrant Lizard King”, with scientists assuming its status as the apex predator of its environment.

Tyrannosaurus rex was certainly impressive, however there were many others in its family, with an interesting and varied evolutionary history. Most large theropods by the late cretaceous were all part of the same ancestral lineage, Carnosauria. This comprised the comparably sized carcharodontosaurids, and the derived megalosaurids like Spinosaurus. However, the Tyrannosaurs were cut from a different cloth, so to speak. Instead of being a branch of Carnosauria, the Tyrannosaurs were a derived branch of the distinct, possibly older, coelurosaur family. The first true Tyrannosauroids evolved in the mid-Jurassic period of the Mesozoic era. By age, they appear to have originated in Europe, with one of the earliest known species, the 166 million year old Proceratosaurus bradleyi, being first found in Britain. In as few as 6 million years, the group’s geographic range had expanded all the way to china, and by the beginning of the cretaceous period, Tyrannosaurs existed on every continent in the northern hemisphere.

Possibly the only thing someone could describe as “small” on the later tyrannosaurs however, were their peculiarly sized arms. Yet in spite of their size, the two-taloned arms of tyrannosaurs retained a proportionately high level of strength, able to exert an estimated pressing force of nearly 400 pounds. Its arms were the least thing to worry about however, if you happened to be a cretaceous-era prey animal. With powerful hind legs, an adult Tyrannosaurus was estimated to run at about 12-20 mph, with higher estimates placing it in the realm of 25 mph. Perhaps not enough to outrun a modern cheetah, but certainly enough to keep pace with the large herbivorous stock of its day. And when it did catch them, its gaping maw, filled with 60 teeth, each nearly a foot in length from tip to root, could spell the end for even some of the most impressively armored animals in evolutionary history. With a bite force that conservative estimates place at 12500 psi, T.rex could crush bones in its jaws, giving it access to the highly nutritious marrow within.

T.rex was undoubtedly an impressive animal, but it was only the last and largest in a long line of tyrannosaurs, stretching back into their origins in the jurassic period. While none of its ancestors and relatives reached the same size as their most famous representative, the other members of this large family were still fascinating for a variety of other reasons. Many earlier and more distantly related members of the family have been found with evidence of feathers, sparking debate as to whether their larger counterparts may also have had them. Some scientists have suggested the larger tyrannosaurs, such as T.rex, may have been obligate scavengers, animals that only scavenge kills from other predators. Still others theorize that T.rex and its contemporary relative, Nanotyrannus lancensis, were actually adult and juvenile specimens of the same species, T.rex. As with all fossil animals, there remains much to be discovered.

Quick Facts

  • Tyrannosaurs originated in the Jurassic period of the Mesozoic era, with the earliest known member, Proceratosaurus bradleyi, dating back to 166 million years ago.

  • These early tyrannosaurs were smaller, with Proceratosaurus being about 6-7 feet long and 2-3 feet tall, a far cry from the 30-40 foot monsters their descendants would become.

  • Tyrannosaurus rex was the largest of the tyrannosaurs, and one of the largest theropod dinosaurs known to humans. Until the discovery of the unrelated theropod Giganotosaurus in 1995, it was considered the largest of all the carnivorous dinosaurs. Today, we know there are a handful of others, including giganotosaurus’ relatives Carcharodontosaurus and Mapusaurus, and the current record holder, Spinosaurus.

  • The tiny arms of tyrannosaurs were actually proportionately quite strong. Explanations for their purpose are varied, with proposals including a mobility aid while getting up from a sitting position, grabbing on to a mate during intercourse, and grabbing onto an ambushed prey animal in order to push them into a more vulnerable position.

  • Some members of the family, such as Yutyrannus huali, have been found with preservation indicating a coating of fluffy, down-like protofeathers on their bodies. Some have suggested that this indicates larger members of the family may have had feathers as well, but evidence for this is scant at best. Several preserved skin impressions of T.rex in particular show that it was covered in a coat of small, pebbly scales, most under an inch in diameter.

  • Yutyrannus is also the largest known dinosaur found with a covering of feathers, changing scientists’ perceptions that feathers were selected against in larger sized theropods.

  • Tyrannosaurs came into prominence as apex predators in the last 20 million years or so of the Mesozoic era. Prior to this, the tyrannosaurs were kept smaller by the evolutionary pressure exerted by existing large predators, the allosaurids, and their descendants the carcharodontosauridae.

  • Description

    Like other theropods, Tyrannosaurs were bipeds, standing on two legs with their forelimbs reduced for non-locomotor function. Their necks were S-shaped, like modern birds, with their heads held above and ahead of the body. They had long, muscular tails which, in addition to aiding in balance, also assisted the legs in pushing the animal forward, and in the larger Tyrannosaurs, absorbed the shock during more aggressive charges.

    Tyrannosauroids, from the largest to the smallest, can be recognized by a suite of several key shared traits (In phylogeny, traits like these shared from a common ancestor are called “synapomorphies”). Their snouts were taller and blunter than other theropods, with teeth in the premaxilla, the front of the upper jaw, being differently and distinctly shaped from the rest of the teeth. These teeth have a distinct D-shaped cross section, aiding in piercing and crushing. Their surangular bones (a pair of bones in the lower jaw), were ridged, creating a stronger site for muscular attachments. Tyrannosaurs also had proportionally longer hindlimbs than other similarly sized theropods, evidence of a faster, pursuit lifestyle in the smaller, more ancestral forms, though it is highly unlikely they maintained speedy lifestyles in the larger, more derived specimens.

    Earlier tyrannosauroids, like Guanlong and Proceratosaurus, had 3 fingers on each hand, and arm proportions in line with other similar sized theropods. Their skulls were lightly built and narrow, and their legs were long, keeping with their lithe builds. They were small to mid-sized pursuit predators hunting smaller prey in the shadow of their large, Jurassic contemporaries. As time went on, distribution of large predators shifted southward, leaving open ecological niches in Asia and North America for the earliest nascent tyrannosaurids. The family grew in size from there, jumping from under-10 foot pursuers to truly giant ambush predators that grew anywhere from 25 to 40 feet in length. As they grew, their forelimbs shrunk and their skulls enlarged, adapting to become ambush predators with an emphasis on incredibly powerful bites.
    While some older tyrannosauroid species are known to have had feathers, their descendants likely had little to none. Fossilized remains of T.rex integument indicate that it had a covering of pebbly scales, meaning that any feathering it had would have been sparse, if it indeed had any feathers to begin with.

    Some have suggested that tyrannosaurs, in particular T.rex may have had lips that would have protected their teeth from the elements. Evidence that has been put forth in favor of this hypothesis includes the rugosity of the skull around the mouth suggesting a greater concentration of soft tissue and blood vessels. However, comparisons to extant crocodilians, whose skulls are similarly rugose near the mouth, notably do not have such lips. Additionally, the closely related older species, Daspletosaurus horneri, had its skull studied in intense detail, finding that the bone closest to the jaw resembled lipless crocodilians more than it did other lipped reptiles.

    Specialization of Skull and Teeth

    The skulls of Tyrannosaurs, especially the derived Tyrannosaurs of the late Cretaceous, were heavier than other large theropods, with long, thick teeth. The premaxilla (the front most bone of the upper jaw) was raised, with fused nasals, making it reinforced to handle greater stresses. CT scans of the skull reveal a brain with very large optical and olfactory reception centers. Their sense of smell was so strong, in fact, some scientists have gone so far as to suggest it may have been the most powerful sense of smell of any terrestrial animal to ever exist. Likewise, their binocular vision would have been on par with that of modern birds of prey.

    All this is on top of their massive biting strength. As stated previously, an adult Tyrannosaurus Rex’s bite force was a staggering 12500 pounds per square inch. This is enabled by a combination of factors. For one, The bite profile of a tyrannosaur was U-shaped towards the snout, widening towards the back of the jaw. This allowed the animal to press more force across a broader area than the V-shaped bite profiles of other large theropods. Their teeth were also markedly different from other theropods. Almost every other theropod had flat, triangular teeth made for tearing chunks of flesh off their prey. Instead, the teeth of Tyrannosaurs were more robust, with the teeth of the premaxilla having a distinct D-shaped cross-section, and every tooth having strong serrations on both edges. Additionally their teeth were strong and very resistant to breaking and wear. Dinosaurs regrew replacement teeth their entire lives, but for a large adult carnivore like Tyrannosaurus rex, it would have taken longer.

    Evolutionary History

    Tyrannosaurs were, at all stages of their evolutionary history, active predators who also engaged in opportunistic scavenging. Early Tyrannosauroids were lithely built, and had proportionally longer legs than other theropods. This would indicate a body plan well suited for pursuing smaller prey animals, and weaving through the densely wooded landscapes that covered large portions of the Earth in the Jurassic period. The niche of large terrestrial predator at that time was already occupied by the successful allosauroid carnosaurs, such as Allosaurus. As such, the early tyrannosauroids were kept small and nimble. As time went on, the forests receded as the Earth’s temperature cooled going into the cretaceous period. The existing large carnosaurian predators had undergone further derivation, becoming the carcharodontosaurs and the spinosaurids. Additionally, as habitats changed, so did their range, shifting southward to chase their more specialized large prey animals. The more generalist tyrannosauroids, however, were more easily able to adapt to the changing environment, and swiftly filled many open niches in the northern hemisphere, from Europe to Asia, eventually crossing over a land bridge in the late Cretaceous to North America. The many adaptations they had for subduing prey quickly helped them gain the upper hand once they grew in size, bringing devastating power to the family.

    Hunting and Feeding Behavior

    As stated prior, all known Tyrannosaurs were active predators, however the hunting strategies used by early, basal Tyrannosauroids was very different from the lifestyle of the more derived Tyrannosaurids. Their small bodies and nimble builds suggest that they were adapted for quickly pursuing smaller prey. Their jaws were able to swiftly bring an end to the small animals that made their home in the Jurassic underbrush. Interestingly, because the more derived Tyrannosaurs of the Cretaceous were so large, it is thought that young and juveniles of this time probably fed in a similar manner, with these large tyrannosaurs filling multiple different predator niches throughout their life history. Young Tyrannosaur specimens indicate that their skulls were more gracile compared to their older counterparts, and that around the 12-20 year range, the Tyrannosaurs experienced a rapid growth spurt, with their skulls and jaws becoming much more robust.

    For a short period from the 1990’s through the 2000’s, there was debate as to how such a large animal like Tyrannosaurus rex would be able to chase down prey, with paleontologist John R. Horner suggesting that the famous animal was in fact an obligate scavenger. However, since then, significant evidence suggesting a lifestyle of active predation has been found. This includes T.rex teeth found in the skeletal remains of herbivores, as well as wear on bones that showed signs of having healed, indicating that the prey animal got away from a failed tyrannosaur attack.

    Tyrannosaurs also had strong binocular vision, a trait most commonly found in actively predatory animals, as it allows them to focus on a singular target and identify them with a greater degree of stereoscopy and depth perception. Their strong sense of smell would have aided in locating prey, as well as keeping track of injured hunts over long distances. An herbivore that had escaped a tyrannosaur attack with a grievous injury would likely find that its escape only delayed the inevitable. Tyrannosaurs also had robust, heavy bodies compared to other similarly sized theropods, meaning that they could likely have endured more in direct confrontations.

    Additionally, if the large tyrannosaurs like T.rex *were* pure scavengers, then that would mean that contemporaneously, there would be no large predators in the area to prey on the large herbivores. The only other known carnivores from the same habitats as T.rex were dromaeosaurs; smaller dinosaurs in the “raptor” family, the dromaeosauridae. While these dinosaurs were pack hunters and quite ferocious in their own right, it is highly unlikely that they would have been able to hunt adult herbivores like Triceratops and Ankylosaurus.

    Large Tyrannosaurs would have likely engaged in ambush predation, waiting out of sight and singling out weak or sick members of a herd before striking at an opportune moment. Because of their large size, they would not have been able to run at excessive speeds. Prolonged chases would also have been dangerous, as a fall for an animal the size of T.rex had a much higher chance of causing a debilitating injury. With its powerful jaws and legs however, short dashes leading to devastating bites would be the most likely attack strategy.

    Some evidence exists, however, of pack behavior in tyrannosaurs. The smaller, older tyrannosaurid, Albertosaurus sarcophagus, has been found in a group of approximately 26 individuals, indicating that they were all in the same spot when some event, such as a flash flood, killed them all. It is highly unlikely that all the individuals being in the same spot can be accounted purely by happenstance, and so the likely conclusion is that they were at least a temporarily formed group. The group was also not just adults, but juveniles and younger individuals as well. This suggests that younger individuals in packs like this aided in hunting prey, with some scientists positing that the younger, faster individuals would harass and herd large prey animals into the larger, more powerful, but slower adult members of the pack.

    While feeding, it is thought that Tyrannosaurs would tear of chunks of flesh with strong, wrenching motions of their head and neck. Their teeth were serrated but built much more robustly than that of other theropods. Scientists theorize that they functioned more like holdfasts than blades, letting a tyrannosaur keep its clamp-like jaws shut tight around its prey’s flesh, then used its powerful jaw and neck muscles to rip apart its prey. Their powerful, crushing jaws would have also allowed them to break apart bones, giving them access to very nutritious marrow inside.

    Fossilized remains of “headless” ceratopsians such as Triceratops have led some scientists to hypothesize that Tyrannosaurs had evolved specialized feeding behaviors after subduing these well-defended herbivores. The hypothesis suggests that, after killing their prey, Tyrannosaurs would grab the armored frills of ceratopsians in their mouths and tear off their heads in order to get at the nutritious neck meat and cervical vertebrae.

    Life History

    For large tyrannosaurs, growing up would have been tough. It has been suggested that Tyrannosaurs were likely good caretakers of their nests, but there is little to no tangible evidence to substantiate these claims. It certainly would have been prudent for them to do so, as the many large herbivores that Cretaceous Tyrannosaurs shared their habitat with would likely be quite hostile to any tiny tyrants they would encounter. And at the beginning of their lives, they would certainly have been quite small. Fragmentary remains of an embryonic tyrannosaur found from the late cretaceous of Canada indicate that a newly hatched tyrannosaur (in this case, likely Albertosaurus or Gorgosaurus) would only have been a tenth the size of its parent. At 2.5 feet, that may seem quite large, but their 30 foot parents would easily dwarf them. For additional context, a newborn modern elephant is only a quarter the size of its parent. These tiny newborns would likely have hunted small lizards, mammals, and insects.

    Growing somewhat quickly, Juvenile Tyrannosaurs would have been lankier and much more like their ancestors than their adult forms. They would have likely hunted in a similar manner as well, swiftly taking down small prey animals, including other dinosaurs. It is possible that young tyrannosaurs may have hunted together to take down larger prey, but this is hypothetical. These young tyrannosaurs would have competed with contemporary dromaeosaurs such as Deinonychus.

    Studies on the ontogeny, the skeletal structure and growth, of known Tyrannosaur specimens place their average natural life expectancy at around 25 years, with the largest known Tyrannosaurus rex skeleton being approximately 28 years old at the time of its death. Younger tyrannosaurs are rare, however many skeletal specimens are known from all physical stages of its life history. These specimens show that around the 13 year mark, young tyrannosaurs began a dramatic shift from their more nimble bodies into the robust, more strength oriented builds of their adulthoods.

    Extinction Event

    T.rex was the last in the tyrannosaur lineage, and a very successful predator in its niche. However, no adaptation could have prepared it, or the majority of the other large dinosaur species, from the chicxulub impactor meteorite. The devastation wrought by this enormous meteorite, which some scientists estimate as being nearly the size of Mt.Everest, marked the end of the Cretaceous period, and the end of the Mesozoic era. Without plant life to sustain them, the large herbivores that predators like T.rex would have fed upon would swiftly have gone extinct, and the tyrannosaurs shortly after.