Tyrannosaurus Rex: Butcher of the Cretaceous

Tyrannosaurus Rex: National Museum of Natural History. Washington, D.C

Since its discovery in the summer of 1902 by Barnum Brown, no other dinosaur has captured the human imagination quite like the Tyrannosaurus Rex. Upon its discovery, Barnum Brown wrote this to Henry Fairfield Osborn, friend, and curator of the American Museum of Natural History in New York. “It is as if a child’s conception of a monster had become real and was laid down in stone” (Kindall,2022). Though most of the skull and tail were missing, everything about this monster would overwhelm the human imagination. The specimen that Brown found stood 13 feet tall at the hips, its jaws measured over 4 ft in length, and would have weighed 6-8 tons. This was the only known specimen to science as was given the appropriate name Tyrannosaurus Rex by Henry Osborne in the fall of 1902. Tyrannosaurus which means “tyrant lizard” in Greek and “rex” which means “king” in Latin; Tyrannosaurus Rex, the king of the lizards, no other name would capture in two words the sheer power contained within this beast.

Who was this creature? What was the nature of this tyrant lizard? Without the restless adventuring spirit of Barnum Brown who unearthed the first T-Rex and captivated the imagination of people everywhere, dinosaurs would have remained nothing more than a novelty never inspiring people to devote the necessary resources and energy to understand this creature and the world in which it lived. Without Tyrannosaurus Rex to grace museum halls, only herbivores would represent the giants of the Mesozoic. Without Tyrannosaurus Rex there would be no Jurassic Park, no dinosaurs toys, no children in dinosaur pajamas, and no creature to stand at the intersections of popular culture and modern science. Without T-Rex, our world would look very different and our understanding of the past would be incomplete. The discovery of Tyrannosaurus Rex did more than draw the public to Natural History Museums, Tyrannosaurus Rex started a revolution in understanding our world. In this post, we will reach out with our imagination and touch the bones of this incredible creature from another time and place.

Tyrannosaurus Rex “Walter” biting down on a Triceratops. National Museum of Natural Hisotry, Washingron, D.C.

Tyrannosaurus Rex was the ultimate predator, the largest and deadliest animal to walk the earth. Ruling the American Midwest for millions of years, Tyrannosaurus Rex brought down massive armored prey Triceratops and Inkeylosaurs tanks of the Cretaceous. At an average of 40 feet long, 18 feet tall, and weighing 6-8 tons, Tyrannosaurus rex was a mouth on legs, the butcher of the Cretaceous. The skull of T-rex is the most impressive anti-tank weapon evolution ever produced. The jaws of this theropod are infamous, 5 feet long, and filled with 60 piercing and bone-crushing teeth with a bite force 16 times stronger than an alligator. Constructed of 64 bones, the skull, and attached 2 ft thick neck muscles could lift a hippo. Counterbalancing the massive skull is a tail weighing a ton combined with a pelvis and legs that anchor and account for half of the T-rex’s weight.

Ironically, the teeth of Tyrannosaurus Rex are the bluntest of all the Tyrannosauroid family, meaning that the oral weaponry of this colossal theropod was for bone crushing which brings into question adaptions for scavenging. It is clear from the braincase of Tyrannosaurus skulls that the olfactory region is prominent, indicating that the T-Rex had superb smelling capabilities that aided in sniffing out carcasses or kills from other carnivores. Contrary to Jurassic Park, Tyrannosaurus Rex could detect even the slightest movement. At 16 inches apart and 18 feet high, the eyes of T-Rex possessed stereoscopic binocular vision and could discern fine detail at a distance of 4 miles, six times further than a human with 20/20 vision. The human eye requires the aid of binoculars to see details at this distance.

If impeccable vision and olfactory senses were not enough, brain case studies of Tyrannosaurus show that this apex predator could hear sounds below 40 Hz at great distances. While unable to discern the chirps of birds or the buzz of insects, T-Rex could perceive the low grunt of a distant meal or the low-frequency call of a potential mate. If T-Rex could hear low-register sounds, it stands to reason that it could produce low-register sounds. Contrary to Cinema, Tyrannosaurus could not roar as often depicted in dinosaur thrillers. However, in the absence of a roar, Tyrannosaurus vocalizations would be felt as vibrations. The human ear is unable to discern sounds below 20 Hz, but we can perceive vibrations produced by sounds below this audible register. While unable to hear the vocalizations of the approaching T-Rex, we could sense the vibrations produced like rolls of distant thunder. While not as dramatic as cinema depictions of encounters, the experience of an ever-increasing vibration as the Tyrannosaurus Rex approached in the dark of night in a wooded environment would be terrifying. Millions upon millions of years in the making, Tyrannosaurus Rex is the ultimate apex creature of evolution.

Tyrannosaurus Rex “Walter” tail. The tail of T-rex weighed a ton to counterbalance the large 5 foot skull. National Museum of Naturl History. Washington, D.C.

To fully understand Tyrannosaurus Rex, we need to understand its origins. The apex predator of the late Cretaceous, Tyrannosaurus Rex was the final and one of the largest carnivores that evolution ever produced. What adaptions took place along T-rex’s lineage to create such an amazing creature? Why did Tyrannosaurus have tiny arms and why was the skull so massive? All the answers to these questions lie in T-rex’s evolutionary history starting with the Permian Extinction, which ushered in the first dinosaur.

Tyrannosaurs began 100 million years ago and like all dinosaurs, they started as small, underdogs living in the shadows of the other apex predators at the beginning of the Triassic, behemoths such as postosuchus apex archosaur, and other carnivores of the Triassic. 240-230 million years ago, dinosaurs like Herrerasaurus and Eoraptor evolved from their cat-size dinosaurmoprhs ancestors when the earth hosted the once giant supercontinent Pangea. Although one united land mass, Pangea was a challenging environment for primitive dinosaurs to both live and evolve. Dryland extended from pole to pole, but on the other side was an open ocean-Panthalassa. Because currents could travel from the equator to the poles without interference, low latitudes were warm preventing ice caps from forming. The Arctic and Antarctic were tropical with summer temperatures a temperate 70 to 80 degrees year-round and winter temperatures barely below freezing. Because Pangea was centered on the equator, and the other half was cooling down in the winter, this land orientation caused violent air currents to traverse the supercontinent. These air masses triggered megamonsoons, bringing torrential downpours to the land mass, and causing flooding and deadly storms. (Brusatte, 2018)

The global megamonsoons divided Pangea into regions based on varying amounts of precipitation, winds, and temperatures. Within this landmass were the mid-latitudes. These regions were cooler with a moist and wet climate that was hospitable to life. Here Herrerasaurus, Eroraptor, and other dinosaurs lived and thrived. Pangea with its extreme weather and dangerous unpredictability was the evolutionary stage set out for the dinosaurs. From the ashes left by the Permian Extinction, dinosaurs evolved in this harsh world with many challenges but they weren’t alone.

Herrerasaurus: Image Credit

Evolving alongside the dinosaurs were other creatures that were larger and stronger. One of these adversaries was the metopsoaurus a giant amphibian. Metoposaurus was a monster with a head the size of a coffee table and jaws with hundreds of piercing teeth. Metospsoaurus was the ancestor of today’s frogs, toads, newts, and salamanders and it dominated the shore regions of many of Pangea’s lakes and rivers, particularly in the midlatitude humid belts. Small primitive dinosaurs such as Eoraptor were on the menu and had to approach the shore regions with great caution. Dicynodont, a therapsid, pig-like mammal that ate roots, leaves, and insects completed with the primitive dinosaurs for food and habitat. Saurosuchus a crocodile cousin and mightly apex predator and one of the largest Rauisuchians was a tyrant forcing the dinosaurs into their role as underdogs in the ecosystem.

When life in Triassic Pangea seemed bleak and antagonistic towards the primitive dinosaurs, two important things happened that gave them an edge. First, the humid belt region that was dominated by the rhynchosaurus and dicynodonts began to see their numbers decrease and finally disappear. (Brusatte, 2018). It’s not understood why these creatures faced extinction but the effect was in the dinosaur’s favor. The niches vacated by these large herbivores gave the primitive sauropod dinosaurs such as Panphagia and Saturnalia a new and abundant food source. Plateosaurus, a well-documented primitive sauropod thrived 225-215 million years ago during this time. Second, around 215 million years ago, the first dinosaurs began inhabiting the subtropical arid regions of the Northern Hemisphere, now the American South West (Brusatte, 2018). It is thought that climate change and monsoon patterns made these regions more tolerable, allowing the dinosaurs access to these once-arid regions.

Plateosaurus: Image Credit

Among these primitive dinosaurs that exploited the newly unoccupied and available food sources and once arid regions of Pangea was Coelophysis. A dog-size, lightweight, fast-running, sharp-toothed Triassic dinosaur who was the earliest member of a theropod dynasty that would one day produce Velociraptors, Birds, and Tyrannosaurus Rex-the largest carnivore to ever walk the Earth (Brusatte, 2018). This is where the story of T-rex began, the humblest of beginnings in the Triassic arid regions of Pangea.

Coelophysis: Image Credit

Coelophysis was first discovered in 1889 from a massive bone bed found on Ghost Ranch in New Mexico. During the bone wars (a fierce rivalry between Edward Drinker Cope and Othniel Charles March) these men discovered a vast number of dinosaur specimens naming many of the most well-loved dinosaurs throughout their rivalry. It was Cope who later named Coelophysis in 1889. The Ghost Ranch bone bed dates back to 220 million years ago when a Triasic megamoonson flood overtook a herd of Coelophysis and buried them so rapidly that their bodies were protected and fossilized in the sediment.

As the Triassic continued, the primitive dinosaurs along with the dominant archosaurs evolved, multiplied, and occupied more regions of the Pangea supercontinent. The dinosaurs continued to diversify and thrive as they occupied vacant niches left by the waning populations of dicynodonts and some Rauisuchian species. Then about 201 million years ago, the earth began to rumble. Miles below the surface, plate tectonics, the engine that drives the continents came to life.

The supercontinent Pangea began to split, and North America separated from Europe and Africa. The Atlantic Ocean occupies the region that these continents once claimed. But before the continental divorce was finalized, the Earth hemorrhaged lava unlike anything today. Massive volcanic eruptions raged for 600,000 years, and megavolcanoes erupted along what is the Atlantic Seaboard today. Unlike vulcanism today, these mega volcanoes erupted in four violent pulses siring the edges of the continents, followed by the flood basalts of the Central Atlantic Magmatic Providence (CAMP). CAMP is a milestone in Earth’s history, a gravestone, and the cause of the Triassic Extinction. CAMP is the largest igneous province in the solar system, an estimated 11 million kilometers, and the eruptions belched tidal waves of lava and flames similar to the Siberian Traps. Waves of lava flowed across the land, incinerating everything in the path. Like the Permian Extinction, massive amounts of carbon dioxide and sulfur dioxide were released into the atmosphere causing intense global warming followed by cooling. The oceans are acidified due to the temperature fluxations, starving the waters of oxygen and triggering an ecosystem collapse on land and in the water. An estimated 30 percent of species died, including all archosauromorphs, crocodylomorphs, pterosaurs, and some dinosaurs. Other groups that died out were aetosaurs, phytosaurs, and rauiuschids. After the dust settled and the volcanoes subsided, the dinosaurs became great survivors of the global meltdown.

As Pangea unzipped, the earth hemorrhaged lava: Image Credit

Underdogs and forced to remain small in the shadow of the much larger and dominant archosauromorphs, crocodylomorphs, and pterosaurs, the dinosaurs were better able to scratch out a living in the aftermath of the Pangea supercontinent breakup. Inheriters of the Earth and now a dominant force, the dinosaurs responded by getting bigger, much, bigger, ushering in the time of the Age of the Dinosaurs and the Jurassic Giants.

In dinosaur evolution, there are two major clades of Dinosauria categorized by a forward or backward-facing pubis bone. Saurichians (“lizard-hipped”) dinosaurs have a forward-pointing pubis and (Ornithischians (“bird-hipped”) dinosaurs have a backward-facing pubis bone. Saurischia contains all theropod and sauropod dinosaurs. Ornithiscia contains armored and horned dinosaurs such as Triceratops, Ankylosaurus, and all Hadrosaurs such as Edmontosaurus. Another distinguishing feature is that Saurinchia possesses air sacs, spaces within the bones that make the skeletons lighter. It is, for this reason, that titanosaurs could raise their long necks and birds can achieve flight by efficiently utilizing oxygen in their bodies.

Forward Facing Pubis of Tyrannosaurus Rex: Image Credit
Backward-facing Pubis Bone of Edmontosaurus (Hadrosaur): Image Credit

Tyrannosaurus Rex, is a tyrannosaur, a clade of theropod tyrant dinosaurs. Once thought that Tyrannosaurs were descendants of Allosaurus, a member of a Carcharodontosauroid clade that produced the largest theropod to ever walk the earth, Giganotosaurus. However, physical characteristics in the skulls, hips, forelimbs, and hindlimbs separated the tyrant dinosaurs from this carnosaur group. Tyrannosaurs have large skulls relative to body size, solid roofs in their mouth which increased their lethal bite, blunt snouts, eyes aimed forward producing stereoscopic vision, and scraper teeth in the front of their upper jaws, a trait unique to tyrannosaurs. Then in 2004, a little dinosaur from the Early Cretaceous was found in Chona’s Yixian Formation.

Dilong, early Tyrannosauroid directly related to Coelurosaurs. Image Credit

Dilong, a little theropod no more than 5 feet long looked like a much earlier Coelurosaur with a few traits that are only found in Tyrannosaurs – a large skull for body size, a blunt snout, and little scraper teeth in the front of its jaws useful for nipping and scraping meat from bones. The discovery of this little dinosaur removed Tyrannosaurus as a distant relative of Allosaurus and placed T-Rex in Coelurosauria, a clade of theropod dinosaurs directly related to birds. T-Rex is no longer considered a Carcharodontosauroid but rather an overgrown Coelurosaur or giant chicken.

Then in 2010, a smaller dinosaur was discovered in Siberia. Kileskus is the oldest tyrannosaur found in rocks dating back to about 170 million years ago, firmly in the middle part of the Jurassic. This new tyrannosaur was small, 7 feet long, a few feet tall, and weighed less than 100 pounds with large nostrils and sinuses and a solid roof, many of the characteristics seen in Tyrannosaurus Rex. 

Kileskus: Image Credit

Kileskus and Guanlong (another tyrannosaur) were about the same size and had similar tyrannosaur characteristics, three-fingered hands, and head crests utilized for show. These primitive tyrannosaurs are a good picture of how the early tyrannosaur clade looked and behaved. While nowhere near the likeness of T-Rex, these small tyrannosaurs thrived in their ecosystem hunting bugs, small mammals, and other small things they could catch. They were fast, had sharp teeth, and feared Sinraptor and Monolophosaurus, cousins of Allosaurus, the large apex carcharodontosaur carnivores of the time.

Kileskus linked to an earlier discovery in 2009 that firmly locked another tyrannosaur puzzle piece into place. Sinotyrannus, a large tyrannosaur measuring 30–33 ft and weighing 2.5 metric tons was the oldest large basal tyrannosaur known. This discovery proved that tyrannosaurs gradually increased in size throughout the Jurassic and Early Cretaceous.

In 2012 the most remarkable tyrannosaur discovery was announced. A nearly complete skeleton of a new dinosaur found in China dating to 125 million years ago with protofeathers, Yutyrannus. Yuthrannus (YOO-tie-RAN-us), a coelurosaur- the theropod clade that contains T-Rex and birds-raised the prospect that coelurosaurs had feathered including the most famous tyrannosaur, Tyrannosaurus Rex.

Yutyrannus: Image Credit

Knowing T-Rex’s origin is only part of the question, the other question to answer is how did Tyrannosaurus Rex become so big. Not a lot is known about this period, the fossil record has yet to yield those secrets. What we do know is that about 94 million years ago the climate began to change. Temperatures spiked and sea levels oscillated and acidification starved the oceans of oxygen. Similar to the Permian Extinction but not as severe. During this time the large theropods of the time the carcharodontosaurs and spinosaurs died off leaving a niche that the tyrannosaurs filled. Tyrannosaurus Rex the remaining tyrannosaur responded by growing larger and starting the reign as the Tyrant King.

T-Rex’s small arms are a defining trait of this famous theropod, where did they come from? T-Rex is not the only dinosaur to have small arms, many theropods have small arms compared to the size of their bodies. There have been many theories presented to explain the evolutionary adaption of small arms, from protection during group feeding to the reduction of harm when fighting over mates or territory. The most likely scenario comes from a paper published in Acta Palaeontologica Polonica, suggesting that instead of the arms shrinking, theropods simply outgrew their arms making them appear smaller compared to body size.

Argentinaosaurus (Titanosaur) and Giganatosaurus (Thereopod); Image Credit

During the Jurassic, an arms race took place between herbivores and carnivores. As the herbivores increased in size, so did the carnivores. The giants of the Jurassic were the Titanosaurs, the largest land animals ever recorded in Earth’s history roamed from place to place in search of food. The large carnivores of that time, Allosaurus and Giganotosaurus stalked the herds of Titanosaurus as they marched across the land in yearly migrations. To take on one of these giants, theropods had to be large. Becoming large is calorically expensive and traits not necessary for survival are not selected. In the case of theropods, as their bodies and skulls became larger, arms did not follow suit. Evolution selected large jaws with powerful muscles yielding a lethal bite over arms in the arms race between herbivores and carnivores. By the time T-Rex arrived on the scene in the Cretaceous, arms were all but useless with evolution favoring the large boxy skulls with jaws full of 6-inch serrated steak knife teeth as the preferred mode of survival.

As the Jurassic yielded to the Cretaceous period, the large theropods that competed with the Tyrannosaurs died out. Ceratosaurus, Allosaurus, and Torvosaurus were gone by the beginning of the Cretaceous. Moving into the Cretaceous were two main theropod clades, Carcharodontosauria represented by Giganotosaurus (Southern Hemisphere), Tyrannosaurs represented by Abelisaurus (Southern Hemisphere), and Tyrannosaurus Rex (North Hemisphere). Together these clades of theropods stalked herds of herbivores and hadrosaurs in their respective regions. Then 90 million years ago, Giganotosaurus representing Carcharodontosaurs was out-competed by the tyrannosaurs and became extinct, leaving the rule world to the tyrannosaurs. For the remaining millions of years spanning the Late Cretaceous, Tyrannosaurus Rex reigned as King of the Lizards till one day 66 million years ago when the Age of the Dinosaurs came to an end.

Tyrannosaurus Rex was the apex predator of all time, the pillar of evolution. From the humblest of beginnings in the once arid regions of Pangea embodied in Coelophysis to the King of the Lizards, T-rex was a feat of evolution produced by millions upon millions of years of adaptions that created the most iconic creature of the Mesozoic. It seems as if Tyrannosaurus Rex was unstoppable, with nothing to fear or almost nothing. 66 million years ago T-Rex witnessed one of the worst days the world has ever seen. A rock 6 miles wide fell out of the sky smashing into the earth with unimaginable force bringing the reign of T-Rex and the Age of the Dinosaurs to a close. For 66 million years Tyrannosaurus Rex lay entombed in the sedimentary rock of the Hell Creek Formation in the American Midwest, exposed for the first time in 1902 by Barnum Brown, and changed our world forever.

No other creature has inspired the human imagination quite like Tyrannosaurus Rex. Since its discovery, T-Rex stands at the intersection between popular culture and modern science inspiring countless people, paleontologists, and dinosaur enthusiasts like myself to understand the world from which it lived. Tyrannosaurus changed our world in ways we may not realize. Without T-Rex, there would be no Godzilla, no dinosaur toys, no children in dinosaur pajamas, no Jurassic Park or Jurassic World, no inflatable dinosaur costumes, no creature to grace museum halls, and no counterpart to the countless herbivores from the Mesozoic. Without T-Rex our understanding of the Mesozoic would be incomplete, our heritage starting from cynodonts to primates would look very different, and we would look very different. T-Rex forced our rat-like ancestors to remain small, living under the feet of the dinosaurs in burrows or above their heads in the canopy. Without T-Rex, our evolutionary line would be forever changed. It is clear. I and my fellow man owe our existence to that chance rock that fell out of the sky 66 million years ago causing the great extinction event that brought the age of the dinosaur to a close. But life is resilient, our mammal ancestors rose out of the ashes left by T-Rex just like Coelophysis rose out of the ashes of the Permian. T-Rex’s story is our story, and our story can only begin when their story ends.

Our story begins because their story ends. ~ Noelle K. Moser ~ Sketched by me.

Barnum Brown’s T-Rex still stands on the fourth floor in Manhattan, New York surrounded by the sounds of one of North America’s largest cities. Visitors look up and gaze at the largest carnivore to ever walk the earth 66 million years ago. Frozen in time, Tyrannosaurus Rex is the witness of a world that we will never see but can only experience through bone.

Tyrannosaurus Rex found by Barnum Brown. American Museum of National History. New York, New York. Image Credit

I regret that I will never see a living Tyrannosaurus Rex, my closest encounters will be standing at the feet of these amazing creatures, looking up, reaching out, and touching bone. The only link I have to their world is through my backyard chickens, in their veins pump the blood of dinosaurs as they are coelurosaurs and directly related to the mighty T-Rex. I observe in their behavior the majesty that T-Rex possessed and can imagine how he walked and sounded. The crow of my roosters is a sound from another world, an ancient world that I will never see. But it is clear, that if Tyrannosaurus Rex had not perished in that chance asteroid impact, I would not be here. He stands as a testament and witness to that earth-shattering day. I am here because my small rat-like ancestors rose out of the ashes of the dinosaurs and ushered in the age of mammals.

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Brusatte, Steve. The Rise and Fall of the Dinosaurs: A New History of Their Lost World.New York: HarperCollins Publishers, 2018

Randall, David K. The Monster’s Bones the Discovery of the T. Rex and How It Shook Our World. New York: W.W Norton & Company, 2002.

Tyrannosaurus Rex, “Walter” and I. National Museum of Natural History Washington, D.C. (August 2022)