Say Tyrannosaurus rex and everyone knows what you are talking about, but say Guanlong or Alioramus and you might be met with a blank stare. The tyrannosaurs were amongst the last and most specialised of the theropod groups, and they can be roughly divided into two further groups. The most primitive members of the group were the tyrannosauroids (members assigned to just the more general Tyrannosauroidea) and looking here we can see how the tyrannosaurs began to evolve into the dominant predators of the northern hemisphere during the Cretaceous. By the end of the Cretaceous the tyrannosaurids held domain over the landscape. Tyrannosaurids can still be called tyrannosauroids since they still belong to this group, but they can be further separated into their own group (the Tyrannosauridae, which itself includes a further sub group called the Tyrannosaurinae) unique from the other tyrannosauroids. These members represent the pinnacle of tyrannosauroid evolution that began at least as far back as the Jurassic, and which was only cut short by the mass extinction of the dinosaurs at the end of the Mesozoic.
Where did the tyrannosaurs come
The first tyrannosaurs to be discovered were the larger members of the Tyrannosauridae that lived in the final stages of the Cretaceous. However, to get a better understanding of the tyrannosaurs as a larger group you need to go all the way back to the early Cretaceous and even late Jurassic where the earliest members of the Tyrannosauroidea are currently known from.
In the early days of tyrannosaur study the tyrannosaurs were usually assumed to be later relatives of dinosaurs such as large theropods like Allosaurus. The reasoning was simple, large theropods alive in the late Cretaceous must be descended from large theropods of the late Jurassic because they were both the largest predators in their ecosystems of the times. Not all palaeontologists agreed with this idea though, and with the advent of new discoveries and more in depth analysis of specimens, the conclusion that tyrannosaurs were later descendants of carnosaurs like Allosaurus is now confirmed as wrong.
This discovery centres around the idea that tyrannosaurs were actually an evolutionary offshoot of the coelurosaurs, smaller predatory theropods that were around at the same time as carnosaurs like Allosaurus, but not directly related to them other than the fact that both groups are both tetanuran (stiff tailed) theropods. The link was first proposed when similarities between coelurosaurs and tyrannosaurs were noted in the formation of certain bones of the skeleton. While carnosaurs were the dominant theropod predators of the Jurassic, the coelurosaurs were active predators in their own right, but in an ecological niche that saw them hunting only smaller prey, while staying out of the predatory scope of larger theropods.
A lineage between the tyrannosaurs and the coelurosaurs really started to be pieced together in the early years of the twenty-first century. In 2001 an early Cretaceous tyrannosuroid from England named Eotyrannus gave a glimpse at early tyrannosaur body forms. Although built like a tyrannosaur, Eotyrannus had much longer arms and hands than later tyrannosaurs of the Tyrannosauridae, as well as three fingered hands and proportionately longer legs. So, while the body was on the way to being like a tyrannosaur, the limbs were more in proportion with the earlier coelurosaurs.
Three years later in 2004 the description of Dilong from China was published. Also dating from the early Cretaceous, Dilong also has long limb proportions and an estimated body size of around two metres long when adult (the holotype is of a smaller juvenile), more in keeping with coelurosaur sizes. Dilong also had three fingered hands further confirming its primitive place in tyrannosaur evolution. Interestingly though the upper bones in the snout of Dilong are fused together, a trait associated with the later and much tyrannosaurids.
Two years after this in 2006 yet another primitive tyrannosauroid from China was named; Guanlong. A little larger than Dilong and with a fancy looking crest on its snout, Guanlong pushed the boundaries of tyrannosaur evolution all the way back into the Oxfordian stage of the Jurassic forming a line that now joins the coelurosaurs of the Jurassic to the tyrannosaurids of the late Cretaceous. At the time of its description Guanlong was widely reported the world over as the oldest known tyrannosaur (as a member of the Tyrannosauroidea as opposed to the Tyrannosauridae) but there is now some controversy as to what is exactly the oldest known tyrannosauroid.
Back in 1926 the palaeontologist Friedrich Von Huene named Proceratosaurus (from remains that were originally described as a species of Megalosaurus in 1910). The name means ‘first horned lizard’, a reference to the fact that it had a nasal horn and lived before the more famous Ceratosaurus. Proceratosaurus was considered to be a coelurosaur but from the late twentieth to early twenty-first century it has been considered by some to be a basal tyrannosauroid. Others however insist that Proceratosaurus is not a tyrannosauroid and is a coelurosaur, though so far it cannot be clearly defined as one either, or at least not as well as some better known coelurosaur genera. A possible third option is that Proceratosaurus might be part of an evolutionary offshoot that began to diverge away from the coelurosaur form but otherwise remained separate from the tyrannosaurs.
There is now sufficient evidence to state that the tyrannosaurs evolved from the coelurosaurs, but what about the tyrannosaurids at the end of the Cretaceous? The tyrannosauroids diverged away from the coelruosaurs in the late Jurassic and by the early Cretaceous they had already grown to medium theropod sizes. In 2012 the description of Yutyrannus revealed that some tyrannosauroids in the early Cretaceous were already approaching nine meters long, though this was still smaller than other theropod types of the time such as carcharodontosaurs (i.e. Acrocanthosaurus). It was probably the appearance of new prey types such as horned ceratopsians and armoured nodosaurs and ankylosaurs in the earlier stages of the late Cretaceous that drove the tyrannosauroids to develop the specialised adaptations of the tyrannosaurids. Unfortunately the lineage between the end of the early Cretaceous and the end of the late Cretaceous is still a little murky at this time. Most tyrannosaurid specimens at the time of writing are known from the last two stages of the Cretaceous, the Campanian and the Maastrichtian.
Tyrannosaur body form
Although definitely tyrannosaurs, the earlier tyrannosauroids were quite different to the later tyrannosaurids on a number of areas. Most obvious are the limb proportions with both the arms and the legs of adult tyrannosauroids being proportionately longer than the limbs of the tyrannosaurids (particularly adult tyrannosaurids). The hands of tyrannosauroids in particular had three fingers while the tyrannosaurids had two fingered hands. The teeth of tyrannosauroids were not as well adapted for crunching tough prey, but it should be remembered that the tyrannosauroids first evolved to hunt smaller prey, and the armoured prey of the late Cretaceous had not evolved yet. A shared feature amongst all tyrannosaurs though is the way the tip of the snout rounds off to be fairly blunt.
While the teeth of early tyrannosauroids were more similar to other smaller coelurosaurian teeth, the teeth of larger tyrannosauroids began to evolve along with changing prey types. The culmination of these were the round conical teeth of the tyrannosaurids which were some of simplest yet at the same time some of the most specialised teeth of any large theropod. The larger tyrannosaurids had access to some ‘soft’ prey such as hadrosaurs and even sauropods depending upon the region, but large amounts of prey in the late Cretaceous were tougher, and in some cases, armoured, and these required teeth that could punch through the armour to pierce the internal organs and blood vessels within. Conical teeth as opposed to laterally compressed teeth were perfect for these jobs since they had no weak planes upon which they could break. By being deep rooted (as much as two thirds of the tooth was rooted into the maxilla) and being driven by incredibly strong jaw closing muscles, these teeth had no problem punching through thinner armour and breaking bones.
While the tyrannosaurids did not have the largest skulls of the large theropods (for even bigger skulls see the carcharodontosaurs), they probably did have the strongest. The upper snout bones of the skull were fused together whereas in other similar skulled large theropods they were unfused (though some ceratosaurs and abelisaurs are noted to have fused frontal skull bones). Tyrannosaurids were also the only known large theropods that had hard palates in their mouths. These features are not only reflection of the colossal bite forces in use by these dinosaurs, but also of the types of prey animals such as large ceratopsians and ankylosaurs which were inherently tougher and bonier than the prey animals that other large theropods had to deal with.
The vision of the later tyrannosaurids can be summed up in one word; Excellent. The rearward portion of the skull of most theropods is wider than the snout so that the arcs of vision from both eyes can face forwards and overlap. This overlap causes both eyes to register the same image so that the brain receives a three dimensional representation of an object which can then be used to gauge distance (in layman’s terms, depth perception). In terms of proportions the tyrannosaurids had some of the widest rear skull portions of any of the large theropods, giving them a much higher degree of depth perception than earlier large theropods. In the highly popular 1993 film Jurassic Park it was suggested that if you stay perfectly still a Tyrannosaurus won’t ‘see’ you and will simply leave you alone. To date there is no conclusive scientific basis for this, and the notion remains as simply a plot device for a film.
In terms of overall size the tyrannosaurs had very humble beginnings. During the Jurassic period the tyrannosauroids only seem to have ranged between 2 and 3 meters long. Tyrannosauroids of the early Cretaceous were sometimes small such as the two meter Dilong, but Eotyrannus in Europe was already between 4 and 5 meters long, while Yutyrannus could grow to at least 9 meters long when fully grown. Around nine meters seems to have been a stable size for tyrannosaurs as a group since even the earlier tyrannosaurids like Daspletosaurus seem to have been limited to around 9 to 10 meters long at most. It was not until the last stages of the Campanian to Maastrichtian that tyrannosaurids grew bigger. To date the largest known tyrannosaur is Tyrannosaurus with the most complete specimen being some 12.8 meters long, while isolated bones and fragments from other individuals suggest a possible 13 meters long. Tyrannosaurus also used to be the largest known theropod dinosaur, but today other large theropods such as Giganotosaurus, Spinosaurus and possibly Carcharodontosaurus are recognised as being bigger (though definitions vary depending upon weight, length, etc.).
While the tyrannosauroids had functional long fore limbs with three fingered hands, the short stubby two fingered arms of the tyrannosaurids were certainly not as versatile as their ancestors arms were. This has given rise to some ridicule for tyrannosaurids since at first impression it would be very easy to say that they were useless. More modern analysis however indicates that the small arms were far from useless, and were in fact capable of offering secure holds onto struggling animals. It is not entirely out of the question that if a tyrannosaur failed to kill an animal with the first bite, it could hold on long enough to reposition its mouth for a better bite. The arms may have also been useful for holding onto a mate during mating. The arms of tyrannosaurids were also not the smallest of the known large theropods; the abelisaurs that lived in the southern hemisphere at the same time that the tyrannosaurids lived in the north, took arm reduction to even further extremes.
Like all theropods tyrannosaurs were bipedal, which means that they always moved about on their two rear legs as opposed to all four limbs (quadrupedal). Because they are closer to their coelurosaurian ancestors, the tyrannosauroids have proportionately longer legs than their tyrannosaurid descendants as an unofficial rule. Compared to some dinosaurs, tyrannosauroids would have likely been amongst the fastest predators in their ecosystems. As animals get larger though they get heavier which means that the legs have to adapt to support this increased weight, and it’s the tyrannosaurids that best reflect this. There are currently far more specimens of tyrannosaurids than tyrannosauroids and of differing ages, and these have revealed a startling pattern in how tyrannosaurids changed as they grew. Younger tyrannosaurids that were still small and lightweight had proportionately much longer legs than adults. As they grew older and heavier, the legs did not grow at the same rate as the rest of the body so they ended up being proportionately shorter than they were when a specific individual was very young. This means that even though a larger adult could cover more ground with each stride, they could never be as potentially fast as they were when juvenile. This change could likely mark a change in hunting behaviour as well, with smaller and faster juveniles hunting prey too swift for the adults (as well as maybe staying out of their way) to adults hunting slower but far more powerful prey.
Head crests and ornamentation
Many dinosaurs had elaborate display devices on their bodies, and even some of the tyrannosaurs got in on this action as well. Currently the best known example is the tyrannosauroid Guanlong which has a large single crest that rises up from the top of its snout. In the past features such as this have been interpreted as additional weapons, but the crest of Guanlong is so gracile that it could never be feasibly used as a weapon. Instead it was likely used to identify members of the same species and during breeding time it may have become even more brightly coloured in order to attract a mate.
Another example of head ornamentation is the Asia tyrannosaurine Alioramus. Noted for being a tyrannosaurid of more gracile build, Alioramus has a series of low ridges that run across its snout. Again these were most likely for display since they are simply too small to have served any other conceivable purpose.
Probably over ninety-nine percent of the time that fossilised remains of extinct animals are found, all we have are fragments of bone to at most collections of some of the bones of a given individuals skeleton. Bones are good, depending upon how well preserved they are we can at least get an idea to the overall form of a creature as well as establishing things like muscle attachment points to which we can then base ‘fleshed out’ reconstructions. It is only upon the rarest of the rare occasions that animals are fossilised that soft tissues such as skin, muscle and internal bone structures are revealed. Fortunately, so many tyrannosaur specimens have now been found that some of them actually do give us a glimpse into the internal biology of tyrannosaurs.
In 2005 a Tyrannosaurus femur (MOR 1125) was intentionally broken for transportation but this also allowed the internal structure of the bone to be studied. In a subsequent study by Mary Higby Schweitzer, signs of the internal blood vessels and bone matrix revealed that in terms of internal construction the femur of Tyrannosaurus was very similar to that of an Ostrich. Additionally microscopic structures which resemble blood cells were also found. In 2008 a study by Kaye et al however contested the claim that the internal structures represent tissues and were instead the remains of a biofilm left behind by bacteria. The original describer however later refuted this, stating that the branching patterns observed in the fossil were akin to internal structures, and that no known bacteria could produce them. Remains of what appears to have been collagen proteins have also been found in association with some Tyrannosaurus fossils, and if these do represent collagen (traces of bacteria is a counter explanation for the collagen-like structures) then this raises the exciting prospect of getting a glimpse at part of the Tyrannosaurus DNA chain by comparison with other known animals.
One known specimen of Tyrannosaurus shows the presence of medullary tissue. The only animals alive today that have this kind of tissue are birds, something that further supports the dinosaur to bird theory of evolution. Medullary tissue is essentially for the storage of calcium so that during egg production there is a ready supply of calcium within the problem that can be used to ensure the proper formation of an egg shell. Tyrannosaurus is not the only dinosaur known to have had medullary tissue however, the orntihischian ornithopod Tenontosaurus is also known to have had medullary tissue as well.
As previously mentioned above, many specimens of tyrannosaurids are known to be juveniles, and there are now so many it is possible to establish a growth pattern amongst tyrannosaurs. The average lifespan of a large tyrannosaurid such as Tyrannosaurus has been established as thirty years (based upon study of the growth patterns of bone across numerous specimens). A 2004 study by Erickson et al established a pattern where for the first five years of life, growth in a tyrannosaurid individual would be very slow. Between five and ten years the rate of growth would begin to accelerate before exploding to the period between ten and twenty years of age. This growth spurt between the ages of ten and twenty would see an individual going from a small juvenile to an almost fully grown adult. After twenty years of age growth would continue but at a much slower rate, with the rate of growth decelerating all the way to the age of thirty. Such a fast rate of growth has also been interpreted as many to represent a metabolism akin to a warm blooded animal.
The study of the metabolism of dinosaurs has been the subject of a lot of debate and controversy. Back when study upon dinosaurs first began in the mid nineteenth century, naturalists of the time interpreted them as nothing more than exceptionally large lizards. Even when more accurate reconstructions began to come forward, dinosaurs were still treated as lizards and were therefore assumed to have cold blooded metabolisms like them. This means that dinosaurs like tyrannosaurs were thought of as sluggish during the early morning and not hunting until they had warmed themselves up in the sun. The ready acceptance of this, but what is essentially antiquated notion is today challenged by a lot of palaeontologists, mainly because we now know a vast amount more about the inner workings of animals than what our forebears did some two hundred and more years ago.
First, metabolisms that operate at a warm blooded level can actually happen by accident simply by growing big. Called gigantothermy, animals which have a large body mass often end up with the outer tissue layers insulating the inner tissues and organs so that they operate nearer to a warm blooded level. This principal has actually been studied in many animals living today, including some that according to older schools of thought are cold blooded, such as the great white shark (Carcharodon carcharias).
Second, dinosaurs are not lizards; they are no more related to them other than that both types of animal can be classed as reptiles. Reptiles in general are cold blooded, but it should be remembered that birds are also evolved from reptiles (as are mammals, but this is beside the point here). Birds seem to have been descended from the maniraptorian dinosaurs, which have coelurosaurian ancestors, the same as the tyrannosauroids. Birds have metabolisms that are akin to those of mammals, they need to in order to be able to fly. Dinosaurian ancestors of birds did have feathers, and wider study indicates that these evolved for insulation, not flight, which itself denotes that they were trying to preserve a metabolism at a rate similar to what we would call a warm blooded animal. Assuming that this is correct, then the question would be how far back did this warm blooded metabolism go along the evolutionary line? Before we go completely off topic here, let’s just say that if that kind of metabolism could go back to the coelurosaurs, then it is very possible that it was passed down to the tyrannosauroids, and later the tyrannosaurids. One observation might confirm a similar internal structure between the tyrannosaurids and birds is the similar internal bone structure already mentioned above. This also leads us to another important topic about the presence of feathers.
Did tyrannosaurs have feathers?
The answer to this question actually does depend upon which genus you are talking about. Some skin impressions are known for some large genera such as Tyrannosaurus, and these reveal that at least some of the larger ones did not have feathers but bare skin covered in small pebble-like scales. The reason for this is usually explained away as gigantothermy, a biological principal where a body is so massive the outer tissue layers end up insulating the internal areas and organs. Since feathers first evolved to insulate the body from the cold, they would be unnecessary and in fact be a hindrance to such large creatures, hence explaining the loss through evolution.
But some tyrannosaurs did have feathers. The first tyrannosaur definitively proven to have feathers was Dilong from Asia. At just over one and half meters long for the holotype individual, and a upper size estimated to be up to two meters long, Dilong was not the giant predator that tyrannosaurs are usually portrayed as. As such, Dilong probably did not have to live with the effects of gigantothermy, therefore a covering of primitive insulating feathers meant that Dilong was more resilient to the cooling effects of the environment and time of day. This also raises speculation that other genera of small tyrannosaurs were also likely feathered, especially when tyrannosaurs are usually thought of as evolutionary advanced theropods that were superior to the older forms.
However in 2012 there was an upset to the notion that only small tyrannosaurs had feathers and this was the description of Yutyrannus. The description of Yutyrannus was born out of the discovery of three individual tyrannosaurs of differing ages from Aptian age rocks. All three had feathers and the largest of these, an adult, was estimated at around nine meters long. Not only did this make Yutyrannus the largest known feathered tyrannosaur, it also made it the largest known feathered dinosaur, comfortably beating the previous record holder, a therizinosaur named Beipiaosaurus. The discovery of insulating feathers upon such a large dinosaur has been interpreted as being an adaptation to living in a cooler environment, perhaps at a higher elevation than what most other kinds of large tyrannosaur lived at.
Unfortunately for those tyrannosaur genera that are preserved without skin or feather impressions we can only guess and a rule of thumb at the moment is that most of the larger genera didn’t, the smaller ones maybe. Another area of consideration is that most young tyrannosaurs may have had a downy covering of feathers when newly hatched, and then shedding them as they gradually matured to adult size and didn’t need them. This is analogous to how bird chicks have a downy feathery covering when they are developing before shedding them as their new adulthood plumage comes in.
When and where did tyrannosaurs
Tyrannosaurs, particularly the larger tyrannosaurids are usually associated with the ending stages of the Cretaceous, especially the Campanian and early Maastrichtian periods. Larger tyrannosaurs such as Yutyrannus were known from earlier in the Cretaceous however, and the current lack of tyrannosaur remains from slightly earlier Cretaceous periods is more of a reflection of the lack of suitable age appropriate rock formations and our lack of study of those which are available. Tyrannosaurs of course lived in the early Cretaceous and even the Jurassic, though these would have been smaller genera ancestral to the larger later forms. It of course should be remembered that not every individual dinosaur that ever lived would become fossilised, just a rare few.
The early distribution of tyrannosaurs seems to have been spread across Europe, Asia and North America. As the Cretaceous era progressed tyrannosaurs seem to have been more common in Asia and North America, which seems to have been the main areas for tyrannosaurs, especially the larger genera, all the way through to the Maastrichtian of the Cretaceous. Tyrannosaurs in Europe by contrast are at the time of writing unknown from the late Cretaceous, and it remains to be seen if tyrannosaurs were still here at the end of the Cretaceous, or if they were only present in the Jurassic/Early Cretaceous, as evidenced by Eotyrannus and Juratyrant.
So far tyrannosaurs remain unknown from the southern continents such as South America, Africa, Australia and Antarctica. It is not entirely out of the question that one day tyrannosaur remains may one day be found from some of these locations but current fossil evidence is lacking. Spinosaurids are known from South America, Africa and Asia at this time, and if they could reach Asia from Africa, then earlier, possibly smaller genera of tyrannosauroids might have migrated towards Africa. South America is also a tantalising, if remote possibility. The presence of saurolophine hadrosaurids (i.e. Willinakaqe and Secernosaurus) in South America towards the end of the Cretaceous might indicate a possible if brief faunal interchange between North and South America, two continents previously thought to have been totally isolated from one another at this time. The reasoning here is that if saurolophine hadrosaurs could have made it to South America from North America, then what if their predators followed them?
There is still one controversial genus called Santanaraptor that has been named after fossils discovered in Brazil. Although only named from a partial post cranial skeleton of the hind quarters with some soft tissues, some palaeontologists believe it to be the first tyrannosauroid known from South America. Other palaeontologists however disagree with this interpretation and instead consider Santanaraptor to be a coelurosaur based upon features of the femur.
Are all tyrannosaur genera valid?
One of the problems regarding the classification of tyrannosaurs is exactly what constitutes a different species or genus of tyrannosaur. Usually the five main genera involved in these discussions are Tyrannosaurus, Albertosaurus, Daspletosaurus and Gorgosaurus from North America and Tarbosaurus from Asia. All four of the latter genera have been considered to belong to Tyrannosaurus by some palaeontologists, perhaps as different species, while others maintain that they represent valid individual genera. The reasons for why are many for both opinions, and without having the fossils in front of you it is hard to clearly appreciate why, but we’ll cover a brief overview.
Covering the four North American genera first, Tyrannosaurus and Daspletosurus can be considered to be robust morphs, while Albertosaurus and Gorgosaurus are gracile morphs. For those not familiar with these terms, both Tyrannosaurus and Daspletosaurus were more heavily built than either Albertosaurus or Gorgosaurus. Daspletosaurus, Albertosaurus and Gorgosaurus all lived during the Campanian period of the Cretaceous, while Tyrannosaurus is confirmed to have lived slightly later in the early Maastrichtian.
Daspletosaurus is similar to Tyrannosaurus in its robust build, but so far fossils suggest that it grew a bit smaller than Tyrannosaurus as well as lived at an earlier time. For these reasons Daspletosaurus has been considered to be an earlier and smaller species to the Tyrannosaurus type species T. rex, but usually it is treated as a distinct genus by most others for these same reasons. Daspletosaurus has however also been considered to be the ancestor of Tyrannosaurus, the larger size of the latter genus being considered to be a reaction to larger types of prey dinosaurs that appeared from the Campanian to Maastrichtian periods.
Albertosaurus and Gorgosaurus however are a lot more blurry. Both have gracile builds and both are known from Campanian era deposits. The only immediate difference is that Albertosaurus seems to have been a touch larger than Gorgosaurus. Again, getting an answer about Gorgosaurus being the same as Albertosaurus will vary depending upon which palaeontologist you ask, yet so far Gorgosaurus usually remains listed as a distinct genus, even though the similarity to Albertosaurus is often mentioned. On a final note about these North American genera, Albertosaurus has also been considered to be the same as Tyrannosaurus, perhaps even a sexually dimorphic form of either a male or female. However continuing fossil discoveries have cast serious doubt upon this, and out of all of these genera, Alberotsaurus is the one widely considered to be the most valid and separate from Tyrannosaurus.
The genus that has the most debate centred around it is Tarbosaurus, a dinosaur that bears the nickname, the ‘Asian T-rex’. Those who consider Tarbosaurus to be the same as Tyrannosaurus do concede that it is not a perfect match for the type species, Tyrannosaurus rex, which is why it has been considered to represent a second species called Tyrannosaurus bataar. Indeed, this was in fact the first classification of the fossils that would later be called Tarbosaurus when they were first described by Evgeny Maleev in 1955. However, Tarbosaurus remains are only known from Asia, and are typically from an earlier time period (Campanian) than Tyrannosaurus (up to early Maastrichtian). Debate about whether Tarbosaurus should actually be considered a species of Tyrannosaurus has continued into more modern times, yet Tarbosaurus continues to be widely regarded as a genus that is closely related to but distinct from Tyrannosaurus by many sources. On a side note about Tarbosaurus, another Asian tyrannosaur named Alioramus was once considered to be the juvenile form of Tarbosaurus. Over the years however new fossil discoveries for these two tyrannosaurs have re-affirmed that both genera are distinct form one another.
There are of course those genera that have been referred to as ‘tooth taxons’ because they were established upon only the description of teeth. This include names that included Deinodon and Aublysodon, and most of these date back to a time when fossils were being dug out of the ground faster than palaeontologists could describe them. Tooth taxons have always proven troublesome when it comes to establishing their validity, even though there are occasional exceptions to this such as with the history associated with the dinosaur Troodon. Tyrannosaur tooth taxons are named from areas where more complete genera are also known, but the similarity between the teeth of tyrannosaur genera makes it impossible to establish which are distinct and which are not.
As for these two genera mentioned, Deinodon used to be a commonly portrayed dinosaur around the late nineteenth to early twentieth centuries, but today the teeth of this genus are considered to probably be those of either Daspletosaurus or Gorgosaurus (possibly both). Aublysodon however still continues to occasionally be mentioned as a valid tyrannosaur genera as late as the early twenty-first century despite the wider consensus that it is probably made up of Daspletosaurus fossils, possibly those of juveniles. Aublysodon fossils however also have links to Gorgosaurus, Tyrannosaurus, Tarbosaurus, Bistahieversor and Nanotyrannus, again revealing the difficulties encountered when dealing with genera only based upon teeth. Aublysodon teeth have also in the past been associated with Dromaeosaurus, Saurornitholestes, to even more surprisingly Ornithomimus and Struthiomimus, neither of which had teeth in their mouths!
How did tyrannosaurs kill other
The key killing apparatus of a tyrannosaur is the mouth. The type genus of the group for example, Tyrannosaurus, is considered to have one of the, if not the most powerful bite force of any dinosaur. To go one step further, only immensely large marine predators such as the giant shark C. megalodon seem to have had bite forces even greater than Tyrannosaurus. The teeth of tyrannosaurs, particularly the larger ones were also very robust and had a conical form to them rather than the laterally compressed (flattened) slicing teeth of other kinds of meat eating dinosaurs. This means that while the teeth of tyrannosaurs were not that suited to slicing, they were great for puncturing, and by being conical instead of flattened they would have been far less likely to break under the strain of the immense bite force.
These features are undoubtedly a result of continually evolving in an ecosystem which held the presence of many bony and armoured dinosaurs such as ceratopsians and ankylosaurs. These kinds of dinosaurs would have needed both the strong bite force as well as the robust teeth, as these would be the only way that a predator would be able to crunch through their bones and armour.
Hadrosaurid dinosaurs however would have also made up part of the diet of tyrannosaurs, as evidenced by multiple hadrosaur fossils with tyrannosaur tooth marks upon them. One specimen of an Edmontosaurus is even proof that not all of these attacks were successful since the damaged bones of a bite mark into its back actually healed over before death of the individual. This same specimen also indicates that tyrannosaurs would have to bite down on top of their prey, which is really not surprising when you consider that the head would have been the highest point of the body. Because of this the obvious target for a tyrannosaur to bite would be the spine since not only could this be easily reached, but severe damage to this could instantly paralyse and even kill (depending upon the location of the bite) a large dinosaur with the minimum of effort required to take it down.
Did tyrannosaurs hunt in packs?
This is one of the controversial questions concerning not only tyrannosaurs but predatory dinosaurs in general. Most apex predators that we know today are solitary creatures (though many exceptions are noted), and for a long time large predatory dinosaurs were only found as single individuals and it is for these reasons why the classic depiction of large predators like tyrannosaurs has always been of solitary hunters. However there are three well documented cases of multiple tyrannosaurs being found together.
The first is a discovery of three individual Daspletosaurus being found alongside the remains of five hadrosaurs. This is the weakest argument of the lot as it’s possible that five hadrosaurs had somehow managed to die together and then attracted three Daspletosaurs either as a group or individually before they themselves were somehow killed. Because this scenario could play out in more than one way, it cannot be taken as definitive proof, what would be needed instead is a predator only bone bed of several individuals of differing ages.
The discovery of exactly this kind of evidence was first made in 1910 when Barnum Brown discovered a bone bed made up of nothing but Albertosaurus remains. This bone bed was subsequently ‘forgotten’ until 1997 when it was re-discovered by Philip Currie who had been specifically searching for the bone bed for evidence to support his ideas about pack hunting dinosaurs. The bone bed in question was made up of the remains of at least twenty-two Albertosaurus of different ages. Critics have suggested that this may be the remains of a predator trap, yet this does not explain why only Albertosaurus were found there, surely there should be other meat eating creatures, as well as possibly stuck herbivores which may have lured others there if this were the case? It has also been interpreted as an ancient watering hole that multiple individual Albertosaurus wandered towards in a drought where they died of thirst when it finally dried up. This however still does not fully explain why only Albertosaurus were present and if several individuals could wander towards a watering hole, how do we know that they did not travel to said watering hole as a group?
As previously mentioned above, the description of Yutyrannus was born out of the discovery of three individuals of all different ages found with one another. Although not as numerous as the Albertosaurus bone bed, this still ticks the right boxes for potential evidence to support the idea that at least some genera of predatory dinosaurs, possibly tyrannosaurs, hunted in packs. Also while many palaeontologists are not convinced about the idea that dinosaurs could hunt in packs, it should also be remembered that just because a dinosaur skeleton is preserved on its own, it does not mean that it lived on its own. It is highly unlikely that a group of animals will be killed all at the same time as one another, but a certainty that they will die one at a time. An individual could die from sickness or severe injury and be left behind by the group as it moves on and when you add to the equation that not every member of a group will have its bones preserved, you get a scenario where the assumption that said creatures were only solitary is the easy conclusion to jump to.
It is also worth considering that the very definition of pack hunting can vary between people. Usually people associate the concept with how a pack of wolves or a pride of lions hunt and while this could be a possible scenario for dinosaurs, it is not the only one. One possible explanation is that some dinosaurs exhibited mobbing behaviour in the same way a group of birds can gang up to attack an individual. For dinosaurs it could simply be a case of several associated individuals charging a herd of plant eating dinosaurs at the onset of a trigger event such as one individual making a lunge and starting a panic. With the sick and injured individuals being slower, one or more of the predatory individuals could combine to attack and kill it. After the predators have their fill, possibly after asserting their dominance and feeding rights over one another, they then split to travel their own separate paths which may or may not involve stalking another herd. In such a scenario they would not be hunting by pack association, merely following the game trail where they knew they could find dinosaurs to hunt.
If tyrannosaurs did form packs made up of the same individuals, then it may have been a case of a group centred around familial links. With Yutyrannus for example there was one adult, one sub adult and a younger juvenile. it may be that the younger members were sticking with the adult while they learned to hunt, but when they were of an age to start having young of their own they split away from their parent group to start another small ‘pack’ of their own. This is very similar to what happens with some big cats that live in Africa today.
Altogether tyrannosaurs and other predatory dinosaurs are quite likely to have lived as either solitary hunters or even in groups, but it is also possible that some genera may have been more predisposed to one over the other. The ecosystems where they lived would have needed to be set up to support their mode of behaviour, but as long as there was plenty in the way of available prey species to support them, there really is no limit to how tyrannosaurs could have adapted to live.
The scavenger controversy
In the past the palaeontologist John (often alternatively written as Jack) R. Horner has been the man most often associated with the notion that Tyrannosaurus was an obligate scavenger, though other palaeontologists have also looked into this. Further, Tyrannosaurus is not the only tyrannosaur to have been labelled in this way, at times in the past all tyrannosaurs have been accused of being nothing but scavengers.
To put things within context, scavenging is where an animal eats the bodily remains of another animal that was already dead when found by said animal in the first instance. How the dead animal died is of no consequence here, the important point is that the animal that found the remains did not kill, or expend any energy in any way towards killing it. Scavenging is one of the oldest forms of foraging and has been around since the dawn of animal life when small organisms would browse upon the bodies and other organic matter that sank down to the sea floor.
The problem in defining scavengers comes when you make the observation that almost every single predatory animal on the planet will at times exhibit scavenging behaviour. Scavenging is one of the least risky and energy efficient forms of survival since there is little to no risk of injury in so doing, and energy only has to be spent in finding and moving to a carcass. The problem with obligate scavenging is that an animal that lives by this method is always dependent upon the skills and success of other animals that kill their own prey. This is the first problem with the ideas that tyrannosaurs were obligate scavengers; if they were not killing the larger prey dinosaurs, who was? Apart from occasional giant crocodiles (which would have likely dragged prey into the water beyond reach of meat eating dinosaurs), the only other common carnivorous dinosaurs were troodonts and dromaeosaurs, often much smaller than some of the larger plant eating dinosaurs that have been known to be eaten by tyrannosaurs as evidenced by tyrannosaur tooth marks on their bones. But aside from just this let’s look at some of the arguments for tyrannosaurs being scavengers.
One commonly referred piece of ‘evidence’ for the obligate scavenger theory is that tyrannosaurs were scavengers because the teeth in their mouths were all sharp and never worn. This idea goes further because they are not worn they could have only been used to eat prey that was not struggling but already dead. The truth of the matter however is that the reason why tyrannosaurs seem to only have sharp teeth is that they like all other toothed dinosaurs constantly replaced their teeth throughout their entire lifetime. These replacement teeth can be seen on certain skull specimens where damage to the maxilla reveals the rows of smaller teeth ready to grow and replace the older worn teeth as they fall out. If not visible by the naked eye, the replacement teeth can be revealed through modern technology with a CAT scan machine. This is why no serious palaeontologist or student of palaeontology would ever lend credence to this idea; there are simply thousands of pieces of fossil evidence that disprove it. Regardless of the evidence however, there are still some people who are either ill-educated or wilfully ignorant of the evidence who continue to establish this theory as ‘proof’ that tyrannosaurs were scavengers to simply further their own personal agendas.
Alternative arguments associated with the teeth include how the front teeth were better suited for scraping, such as taking meat off the bone. However this does not explain why the other teeth were not better adapted for this purpose or for crushing bones.
It cannot be argued that when compared to earlier theropods, the arms of tyrannosaurs were puny. How much of an aid they were in hunting and killing dinosaurs is impossible to be certain, but the main theory here is that past speculation has been that the arms were so small that they could be of no practical use in killing another dinosaur and therefore tyrannosaurs must have been scavengers. In brief, although the arms were small, modern analysis of the arm mechanics has revealed that they were not as puny as they may first seem, however it is not the actual mechanics that should concern us here but how likely they were to even be used.
The basis of this comparison is born out of the comparison of tyrannosaur arms to the arms of other theropods such as dromaeosaurs. Dromaeosaurs have long arms that are very flexible with dexterous hands for gripping and holding onto things like struggling prey. Because tyrannosaurs are not as capable of the same feats of flexibility as dromaeosaurs, they have been considered to be useless in killing and therefore the arms of obligate scavengers. Before we go further on this, let’s look at two modern types of birds; eagles and herons.
Eagles and herons both are both predators yet they look very different from one another. Here we are going to focus upon one area of their bodies, the feet. Eagles have powerful feet equipped with strong sharp talons (which is why falconers need to wear tough gauntlets to protect their arms and hands). When an eagle drops down onto prey such as a rabbit, the strong feet drive the talons through the soft tissue and into the internal organs, bringing swift death to the victim. Herons by contrast have very delicate feet adapted for wading along lake and river edges. If a heron tried to kill a rabbit with just its feet then it probably would not do much more than annoy the rabbit. So in this context, if eagles and herons were only known to us by fossils of their feet and stomach contents, we could surmise that eagles were predators while herons were scavengers. Fortunately we can see both eagles and herons alive and in their natural habitats, and while herons don’t kill like eagles, they do kill; they just use their beaks instead.
Back to tyrannosaurs, they don’t have the dromaeosaur arms, but they do have extremely strong and powerful mouths. These mouths are far more powerful than anything possessed by a dromaeosaur and it is feasible that with such powerful mouths the arms became small not only because they were unnecessary for killing, but reducing in size would also reduce the weight of the arms, compensating for the heavy skull and muscle tissue in and around it, helping to maintain balance at the hips between the fore and aft halves of the body. So in short, tyrannosaurs didn’t have long powerful arms because they had other equipment for killing.
-Position of tooth marks
We know that tyrannosaurs ate other dinosaurs there is just some controversy over when. There are an increasing number of recovered fossils from both plant and meat eating dinosaurs that show punctures, chips and scrapes that match the form of tyrannosaur teeth. Some of these fossils such as the hip region of a Triceratops have been taken as proof that tyrannosaurs were scavengers because the tooth marks were on what would have been the underside of the bone when the Triceratops was alive and standing on all fours. The problem with this interpretation is that we don’t know what killed the Triceratops, be it illness, old age, drowning in flood, predator attack or even if it was killed by the tyrannosaur known to have fed upon it after death. All we can say is that the Triceratops was obviously dead and that most of the flesh and organs of the belly flanks and thighs must have been removed in order for the teeth to mark the bone.
The only way tooth marks could prove hunting by tyrannosaurs is if the attack was unsuccessful and that the wounds healed, simply because the bodies automatic healing processes stops soon after death. Fortunately there is now one very well documented specimen that ticks this final box. A specimen of the hadrosaur Edmontosaurus was a bite in the neural spines of its vertebrae that closely matches the form of a tyrannosaur maw. More importantly the damaged vertebrae healed, indicating that the Edmontosaurus was not only alive at the time of the attack, but must have lived for quite some time afterwards for the bones to heal like they did. This is a valuable piece of so called ‘smoking gun’ evidence since by the very definition of the word ‘obligate’, an obligate scavenger would not be attempting to take down its own prey.
-Tyrannosaurs had exceptional
smell for locating carrion
Brain reconstructions of tyrannosaurids indicate that the sense of smell for these theropods was incredibly well developed, to the point that they could smell food from well beyond visual range. Unfortunately this neither proves scavenging or active hunting since a good sense of smell is a trademark of most animals that live by eating meat, whether by predation or scavenging.
-Tyrannosaurs used their size to
intimidate smaller predators
This theory isn’t bad but it does not give a full explanation either. Tyrannosaurs as a group were made up of genera of different sizes, though the late Cretaceous genera do seem to have been amongst if not the biggest predators of their ecosystems. Predators today can also be seen to be driven away from their kills from more powerful rival predators, and there is absolutely no reason to assume that this did not happen in late Cretaceous ecosystems as well. But specialised scavengers that bully predators into giving up their kills are very rare. Perhaps the most popular example in the fossil record of such a creature is the giant short faced bear Arctodus, a huge bear bigger than any known meat eater in its ecosystem. However despite its size, Arctodus is seen as a proportionately lightweight and gracile animal adapted for energy efficient locomotion over long distances. This does not fit the profile of many larger tyrannosaurs whose leg proportions actually changed for slower and less energy efficient locomotion as they reached adulthood.
-Tyrannosaurs were too slow to
Because the tyrannosaurs were not as fast as some predators like the dromaeosaurs and troodonts, they have been accused of being too slow to catch anything. In popular fiction tyrannosaurs have been shown to be so fast that they can chase after cars, but analysis and bio-mechanic models all come to the same conclusion; tyrannosaurs were not that fast. In truth though a predators only need to be just fast enough to outpace its target prey over a given distance. Prey in question could have been relatively fast hadrosaurs to mid-range ceratopsians to slower ankylosaurs that probably couldn’t do that much more than amble from danger. What matters is how fast tyrannosaurs were in relation to these types of dinosaurs, and if they were slower, they could have been caught by a tyrannosaur.
A further consideration is that no predator chases after prey as soon as it sees it, ambush tactics are almost always used. By sneaking close and striking at the most opportune time, even a slow predator can cover most of the distance between it and its prey before it is even seen by its victim. Remember that even the fastest land vertebrate predator we know the today, the cheetah, can only maintain its incredible speed for short bursts, and must first sneak into striking range of its prey before attacking.
In conclusion tyrannosaurs almost certainly scavenged the remains of other animals from time to time, this is common behaviour seen throughout vertebrate predators. It is unlikely though that tyrannosaurs were obligate scavengers, they neither have suitable adaptations and no evidence exists to conclusively prove that they were reliant upon scavenging. Unfortunately even as late as the twentieth-first century some people still post the antiquated theory about tyrannosaurs being obligate scavengers that never killed their own prey as definitive fact.
Did tyrannosaurs actually have
The earlier and smaller tyrannosaurs would have been at risk from other types of larger theropods that lived during their times, as well as possibly other similar sized theropods. Even smaller juveniles of the larger tyrannosaurs would have been at risk from other theropods should they have ever encountered them without the protection of their larger relatives.
When it comes to the larger tyrannosaurs the only other predators that seem capable of taking them on would members of their own species. Most predators will come into conflict with one another be it for territory control, dominance, feeding rights to access to females. Holes in tyrannosaur skulls could be signs of inter specific face biting for dominance, assuming that the alternative theory of them being caused by a parasite can be ruled out. More compelling are scrape marks on tyrannosaur bones that match up to what we would expect to be created by objects matching the form of tyrannosaur teeth. However while there is easily enough evidence to indicate that tyrannosaurs were cannibalistic, no one can yet say for certain if they actively hunted one another. Indeed, the evidence that is currently available is only indicative of one tyrannosaur feeding upon another, which may more simply be a case of opportunistic scavenging. This idea comes from the observation that the tooth marks are on the bones of the lower legs, an area that would have been very difficult for a tyrannosaur to clasp its mouth on unless the other tyrannosaur was already dead.
Probably the most spectacular evidence for tyrannosaurs actually been hunted comes from the holotype specimen of Appalachiosaurus which seems to have been attacked by the giant crocodile Deinosuchus. The tail of this Appalachiosaurus was damage to a vertebra which corresponds to the form of a Deinosuchus tooth, but it also reveals that the injury healed which indicates that while a Deinosuchus attacked a tyrannosaur, it did not kill one. The circumstances of this attack are unknown, but it could have been a mistimed strike that allowed the Appalachiosaurus an opportunity to get away, or it may simply have been one predator warning another to get out of its territory.
Tyrannosaurs will always be one of the most popular groups of dinosaurs, and further discoveries from North America and Asia are virtual certainties, it is only a matter of time. Further discoveries in Europe are also very possible, though it should be remembered that during the Mesozoic, Europe was more a collection of Islands rather than a single continent as it is today. Many palaeontologists also concede that we may one day find evidence that tyrannosaurs also lived on other continents such as Africa and even South America, though in the past dinosaur genera that have been speculated to be tyrannosaurs from these parts have later been more widely considered to be other types of theropods.
Since the early twenty-first century new discoveries have cast serious doubts upon the older preconceived notions about tyrannosaurs. New tyrannosauroids will help to fill in a still incomplete lineage between the first tyrannosauroids and the last tyrannosaurids. New feathered tyrannosaurs can also be expected to be discovered, especially from places like the Yixian Formation of China. We may also get further glimpses at social behaviour amongst tyrannosaurs, as in did they form hunting groups, or were they really just the solitary predators as has so long been perceived of them. One thing which is absolutely certain is that new discoveries have surprised us many times already, and the next new discovery may be the one to radically change our perceptions of tyrannosaurs, and by extension dinosaurs in general again.
‘?’ denotes a genus with questionable validity as a tyrannosaur. Tooth taxons are named after teeth making if near impossible to be certain about adding further skeletal elements. The teeth of these genera may be examples of other named genera represented by more complete remains.
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