Introduction to Tryannosaurus Rex Behavior
On a typical day near the end of the Cretaceous Period, more than 65 million years ago, a bloody battle rages along the bank of a shallow river in what is now South Dakota. Two gigantic Tyrannosaurus rex dinosaurs, the largest and most fearsome predators of their time, attack each other with clashing jaws and powerful kicks from tree-sized, clawed hind legs. Finally, one T. rex gains an advantage and clamps its long teeth onto the other combatant's head. Even the huge and thickly muscled skull of the victim cannot resist the power of the deadly, bone-crushing bite that follows. The vanquished dinosaur bellows in agony and then falls dead. The victor limps away into the dense forest bordering the river, leaving the torn carcass of its foe to rot at the river's edge.
Although the tyrannosaur's flesh soon decays, its skeleton is covered by sediment, layers of sand, mud, and fine mineral particles deposited by the river. Over millions of years, these layers are compressed by additional sedimentary deposits and eventually turn to stone. During this process, water seeps into the porous bones, depositing minerals that transform them into fossils. Over time, the climate changes. The river dries up, and the forest turns to grassland. Finally, winds, rains, and floods wear away the soil to create desolate “badlands.” And then one day, the relentless erosion starts to reveal the fossilized skeleton of the T. rex.
Although this scenario is imaginary, it may be close to the mark. In 1990, amateur fossil hunter Susan Hendrickson, working in the South Dakota Badlands, discovered the preserved bones of a T. rex that may have died in just such a battle and been fossilized in just such a way. Excavations by a team from the Black Hills Institute of Geological Research in Hill City, South Dakota, soon revealed the well-preserved remains of a Tyrannosaurus with head wounds that may have resulted from a deadly fight. The fossil, nicknamed Sue after its discoverer, was the most complete T. rex ever found, with almost 90 percent of the bones accounted for. In 1997, the Field Museum of Natural History in Chicago purchased the fossil of Sue, and in May 2000 a reconstruction of the skeleton went on display at the museum.
While the Field Museum paleontologists (specialists in ancient animal life) cleaned the rock from the skeleton and prepared the bones for exhibit, researchers elsewhere studied other T. rex remains to learn more about these awesome creatures. They studied bite scars on the fossilized remains of T. rex prey and the contents of fossilized tyrannosaur feces (solid body waste). The researchers also used high-tech imaging devices to learn more about the anatomy of the dinosaurs. And they returned to excavation sites to look for clues to how T. rex lived. Although questions remained in 2000, the combined efforts of many investigators had already led to a better understanding of these monsters of the prehistoric world.
The First Clues of An Ancient Monster
The name Tyrannosaurus rex means “tyrant lizard king,” and beyond a doubt these creatures reigned over the other land animals of the time. Today, 65 million years after they died out, they are one of the best-known animals in the world, living or extinct--a ferocious beast that has captured the fascination of the public in countless museum exhibits, news reports, movies, and books.
Scientific research on T. rex, as well as the public's fascination with it, began in 1902, when paleontologist Barnum Brown of the American Museum of Natural History in New York City led a fossil expedition in Dawson County, Montana. At a sandstone outcrop, Brown and his crew discovered and excavated the partial skeleton of a large, unknown dinosaur. Brown's team returned for more material in 1903 and again in 1905. All together, they recovered the dinosaur's lower jaw, parts of the skull, the spine, shoulder, pelvis, hind limbs, and other fragmentary remains. They were unable to find the animal's forelimbs or neck bones.
Brown's boss at the American Museum, paleontologist Henry Osborn, published the first scientific description of the dinosaur and chose the name Tyrannosaurus rex. Brown's fossil became the tyrannosaur type specimen, the fossil that was used as the basis for describing the species. In 1906, Osborn determined that another partial specimen unearthed by Brown in 1900 was also a T. rex. Then in 1908, Brown and his colleagues excavated a more complete T. rex in Dawson County. Although the American Museum initially intended to prepare and display both skeletons, it sold the type specimen to the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, where it was still on display in 2000.
When Brown and Osborn first began studying T. rex, the fields of paleontology and geology were in their infancy. Osborn and his contemporaries knew that dinosaurs were reptiles but not lizards, as scientists had first described them in the early 1800's. (Out of tradition, however, paleontologists continued to use the Latin word saurus, meaning lizard, in naming newly discovered dinosaurs.) But Osborn made some incorrect judgments of his own about the newly discovered species that later needed to be revised. For example, his estimated age for the T. rex specimens, 3 million to 4 million years old, was off by more than 60 million years. In the decades after the initial discoveries, scientists learned much more about the age of the Earth and the anatomy of T. rex and other dinosaurs.
When Dinosaurs Lived
Paleontologists now know, for example, that dinosaurs lived throughout most of the Mesozoic Era (248 million to 65 million years ago), which scientists divide into three periods. Dinosaurs first appeared about 230 million years ago during the Triassic Period (248 million to 213 million years ago). The largest dinosaurs that ever lived, such as the plant-eating Brachiosaurus, lived during the Jurassic Period (213 million to 145 million years ago). T. rex did not appear until late in the Cretaceous Period (145 million to 65 million years ago) and was the largest member of a family of meat-eating dinosaurs called tyrannosaurids. The T. rex and its relatives died out with the last dinosaurs ever to walk the Earth.
Paleontologists and geologists have also learned that the world in which T. rex lived was very different from our world. Throughout most of the Cretaceous Period, a long seaway extending from the Arctic region to the Gulf of Mexico divided the continent of North America. The width of the sea varied with changes in the global sea level. During the reign of T. rex, the sea stretched as far west as the western part of present-day South Dakota and as far east as present-day Wisconsin. The region in which T. rex lived, along the western coast of the sea, was a lush, forested region with ferns, pine trees, redwoods, and flowering plants. And dinosaurs were not the only inhabitants of the region. Streams and riverbank communities included a wide range of fish, amphibians, lizards, crocodiles, and snakes. Small plant-eating mammals shared the forests with the dinosaurs.
During decades of excavations, paleontologists have also learned much about the characteristics of T. rex. By 2000, researchers had recovered the remains of some 40 tyrannosaurs, ranging from juveniles to adults. About 20 of the specimens were fairly complete, and the largest of them exceeded 12 meters (40 feet) in length. As a result of these finds, T. rex has become one of the best documented and best understood dinosaur species.
Much of our knowledge about tyrannosaurs comes from studies of their skulls, which are up to 1.6 meters (5 feet) long. The upper and lower jaws of T. rex skulls are still filled with the large pointed teeth that made the Tyrannosaurus such a potent killer. A T. rex's teeth, serrated on the edges somewhat like the blade of a bread knife, could exceed 30 centimeters (12 inches) in length. More than half of the length of the teeth consisted of the roots, which were anchored in a massive jaw, giving the teeth great strength. These characteristics made it obvious to paleontologists from the start that T. rex was a very formidable dinosaur.
Creating A Clearer Picture of T. Rex
Some aspects of the Tyrannosaurus, however, were not so obvious. For example, early paleontologists were not sure how T. rex moved. The first reconstructions of T. rex skeletons showed the dinosaur standing upright with its tail resting on the ground. Likewise, paintings in books and museum exhibits portrayed the giant creatures as fierce but thick-bodied and sluggish. By the 1970's, however, paleontologists had determined, from the shape and position of Tyrannosaurus pelvis and leg bones, that a T. rex moved about with its body parallel to the ground. The animal held its long tail up in the air as a counterbalance to its large head.
Other clues to the way T. rex moved came from studies of the leg bones where tendons had attached muscles to the bone. The size of the markings suggested that the leg muscles were quite powerful--not the muscles of a creature that plodded along. Far from being sluggish, paleontologists concluded, T. rex was actually quite agile. Similar findings about the anatomy of other dinosaur species led researchers to conclude that dinosaurs in general were active animals.
Because of this new interpretation, paleontologists began to rethink their assumptions about the physiology of dinosaurs. They had long assumed that all dinosaurs were cold-blooded, like modern reptiles. A cold-blooded animal's body temperature changes with the temperature of the environment, and its level of activity changes with its body temperature. A lizard, for example, is active when its body is warm and sluggish when it is cool. Warm-blooded animals, such as birds and mammals, maintain relatively constant body temperatures, tend to be more active, and--like dinosaurs--have anatomies that allow for active lives.
A Scavenger or A Predator?
Although some disagreements remain about whether or not dinosaurs were warm-blooded, most paleontologists now accept the idea that dinosaurs were active creatures. And most researchers agree that the evidence for an agile, swift T. rex demonstrates that the Tyrannosaurus was primarily a predator, a meat-eating animal that kills for food. Although T. rex was always recognized as a meat-eater, some paleontologists have argued that it was only a scavenger, a creature that feeds on the flesh of already-dead animals. Some researchers take strong stands on this point, but others have noted that among modern meat-eating animals, very few are strictly predators or scavengers. Lions, for example, are predators, but they also scavenge for food. Likewise, vultures are scavengers, but they also kill prey. Consequently, most paleontologists by the 1990's did not attempt to use a narrowly defined label to describe the Tyrannosaurus. Instead, much of their research on the dinosaur attempted to construct a clearer image of how it may have hunted for prey.
If T. rex was a predator, it had to be at least as fast as its prey. To determine how fast a T. rex could run, paleontologist James Farlow at Purdue University in Fort Wayne, Indiana, studied the preserved footprints of several theropods, the group of dinosaurs (including T. rex) that walked only on their hind legs. Farlow calculated the animals' probable top speed by combining such factors as the length of their stride, the length of their leg bones, and their estimated weight. Some theropods, Farlow concluded, could run as fast as 40 kilometers (25 miles) per hour.
In 1995, Farlow and Purdue physicist John Robinson also calculated the running ability of T. rex. The Tyrannosaurus had relatively small arms-about 1 meter (3.3 feet) long-that would have been too short to break the animal's fall if it lost its balance. If a T. rex fell while moving at a speed of 40 kilometers per hour, the researchers argued, the force with which it hit the ground would have caused fatal injuries. They estimated, therefore, that tyrannosaurs probably did not run faster than 35 kilometers (22 miles) per hour, but that would have been fast enough to catch most of the plant-eating dinosaurs of the time.
The small arms of the Tyrannosaurus have raised other questions about how the dinosaur hunted its prey. On a 40-meter body, 1-meter arms appear virtually useless. And indeed, paleontologist John Horner of the Museum of the Rockies in Bozeman, Montana, contends that T. rex's arms were too short to help the animal capture prey. Horner and a few other paleontologists have cited this limitation as evidence that T. rex was a scavenger rather than a predator.
Other researchers note, however, that even though the Tyrannosaurus's arms were small in relation to the body, they were not necessarily useless. Paleontologist Kenneth Carpenter of the Denver Museum of Natural History reported evidence in 1990 that the arms were quite strong. Carpenter and his colleagues examined the markings on fossil arm bones where tendons had connected the biceps to the bone. The researchers estimated that a T. rex's arm could bear a weight of about 180 kilograms (400 pounds). Most paleontologists argue, therefore, that if the arms were not useful in the initial capture of prey, they could have been used to help subdue the victim or to manipulate the carcass.
Mighty Jaws, Sharp Eyesight, Keen Sense of Smell
In any case, most researchers agree that the size of T. rex's arms was irrelevant to the animal's status as a predator. Its speed, agility, and mighty jaws would have been more than sufficient for it to capture and kill prey. In 1996, paleontologist Gregory Erickson and his colleagues at the University of California at Berkeley created a hydraulic replica of T. rex jaws and tested their force on bones from cattle. They wanted to determine how much force it would take to create T. rex bite marks similar to those found on the fossil remains of a Triceratops, a plant-eating dinosaur of the Cretaceous Period. Erickson's team determined that the strength of a T. rex bite could have been as much as 13,400 newtons (3,015 pounds of force). Such force, applied by the animal's strong, serrated teeth, would have made the bite of a T. rex deadly.
Predators, in addition to being adept at catching and killing prey, tend to have superior vision, and paleontologists think T. rex's eyesight was probably well adapted to hunting. The Tyrannosaurus's eyes were about the size of a softball, which most paleontologists agree would have given the dinosaur sharp eyesight. In addition, the T. rex skull was narrow and the eyes faced forward. Consequently, the eyes' fields of vision would have overlapped, most likely giving T. rex depth perception, a valuable characteristic for chasing prey.
T. rex must have also been good at sniffing out its prey, because it had an enormous cavity for the olfactory bulb, a region at the front of the brain that is responsible for the sense of smell. John Horner, in defense of the scavenger theory, contends that T. rex used this keen sense of smell to find dead animals. But other researchers argue that smell would be equally important in hunting. In 1999, researchers at the Field Museum in Chicago got a better look at how strong T. rex's olfactory sense might have been. The museum sent Sue's skull to a Ventura County, California, laboratory owned by the airplane manufacturer Boeing. Researchers at the laboratory created images of the skull with an X-ray technique called computed tomography. The images enabled scientists to examine the entire skull inside and out, one thin cross-section at a time. With this unobstructed view of the skull's interior, the researchers could see that the space for the olfactory nerves had been much larger than paleontologists had predicted.
Other Findings About T. Rex
The fossil remains of other dinosaurs have also provided evidence for the predatory habits of T. rex. Commercial paleontologist Peter Larson, who led the excavation of Sue, also unearthed the skeleton of a plant-eating dinosaur called the Edmontosaurus (one of several species commonly known as the duck-billed dinosaurs) that bore healed scars from a T. rex bite. The fact that the wounds had healed, Larson pointed out, showed that the animal continued to live after receiving them. Thus, the bites were inflicted not by a scavenger but by a predator that had failed to make a kill. Moreover, Larson noted that the bones of Sue and another Black Hills Institute Tyrannosaurus specimen, called Steven, contain healed scars from T. rex bites in their tail bones. These creatures had survived attacks from one of their own. These findings indicated that the tyrant king may have preyed upon its own kind or at least fought other tyrannosaurs for territory or mates.
The bite marks on T. rex victims are a good example of the wealth of clues from secondary sources that paleontologists investigate in order to better understand Tyrannosaurus behavior. In 1992, Gregory Erickson studied the T. rex bite patterns on Edmontosaurus and Triceratops fossils to draw conclusions about how T. rex ate. He concluded that long, deep gouges in the bones were made when the Tyrannosaurus bit deep into the flesh and ripped it from the bone. Smaller parallel scars in tight places along the backbone, Erickson thinks, resulted from T. rex using only its front teeth to nip away pieces of flesh.
Another useful indirect source of information on T. rex are coprolites, or fossilized feces. The contents of a coprolite give clues to the diet and digestive system of the animal from which the feces came. In 1998, paleontologist Karen Chin of the United States Geological Survey and her colleagues reported their findings from the analysis of a T. rex coprolite. Within the large coprolite, the scientists discovered the crushed bones of plant-eating dinosaurs. They also noted that the bones were pitted and the broken edges were rounded off, changes that were probably caused by strong acids in the tyrannosaur's stomach.
Evidence From T. Rex Relatives
Paleontologists have also examined the fossil remains of dinosaurs that were closely related to T. rex--other members of the tyrannosaurid family or other theropods--to fill in gaps about T. rex. If researchers find a rare piece of evidence linked with one member of the tyrannosaurid family, they can make educated guesses about characteristics of other members of the family. For example, paleontologists have found no direct fossil evidence that tells them what T. rex skin looked like. They do know, however, that the Gorgosaurus, a Cretaceous relative of T. rex, had skin with hexagonal scales of various sizes, because they have found fossilized skin impressions of that dinosaur. Most researchers assume that T. rex skin was similar to that of the Gorgosaurus.
The discovery in Mongolia of the skin impressions of another tyrannosaurid dinosaur, a creature called the Tarbosaurus, was reported in 1997 by Kenneth Carpenter. The impressions indicated that the Tarbosaurus had a fold of loose skin under its jaws. Carpenter theorized that this anatomical feature may have been a sort of pouch that was used to hold large chunks of flesh before the dinosaur swallowed them. He also suggested that the skin could have been a wattle, a loose piece of skin much like the ones on many modern animals, including a number of reptiles. The wattles of reptiles are often brightly colored, and they can be important to the animals in attracting mates and intimidating territorial rivals. If the loose skin under the jaw of the Tarbosaurus was a wattle, it may have been used for the same purposes. And the Tarbosaurus's cousin, T. rex, may have had a wattle (or meat pouch if that's what it was) of its own.
No Longer A Loner
Evidence from other dinosaur finds have provided additional clues to T. rex's modes of behavior. Paleontologist Robert Bakker of the Tate Museum in Casper, Wyoming, reported in 1998 on the excavation of a nest belonging to an Allosaurus, a theropod ancestor of T. rex that lived in the Late Jurassic and Early Cretaceous periods. Bakker found the remains of prey at the nest, evidence that the parents brought food to their young. Bakker argued that as ferocious as the Allosaurus and T. rex undoubtedly were, they may have lived in families, rather than as lone hunters, and cared for their young.
Peter Larson's excavations in South Dakota have supported that theory. At the site where Larson's team discovered Sue, they also found other T. rex individuals, including another adult and two juveniles, that may have been part of a group.
Paleontologist Philip Currie of the Royal Tyrrell Museum in Drumheller, Canada, has developed theories about tyrannosaurid group behavior based on investigations of the Albertosaurus, a Cretaceous dinosaur that was similar to T. rex. In 1997 in the Canadian province of Alberta (from which Albertosaurus gets its name), Currie rediscovered a site with numerous dinosaur fossils that Barnum Brown had begun to excavate in 1910. Currie found the fossils of 10 albertosaurs, ranging from juveniles about 5 meters (16 feet) long to adults about 10 meters (33 feet) long. Currie concluded that the Albertosaurus--and other tyrannosaurids--lived in groups and hunted together in packs. He argued that the younger, leaner juveniles would have been swifter than the full-grown adults and therefore useful in “rounding up” potential victims.
In 1999, Bakker excavated the first nearly complete juvenile T. rex skeleton, a discovery that added weight to the idea that the younger animals were fierce predators in their own right. The juvenile T. rex, nicknamed Tinker, had teeth about the same size of those in a full-grown adult. On the other hand, though Tinker was about two-thirds the length of an adult when it died, its bones were relatively slender, suggesting that the young dinosaur was only about one-fifth the weight of a full-grown T. rex. Thus, Tinker must have been a lean, fast, and deadly predator--a perfect member of a pack of T. rex hunters.
Other evidence of tyrannosaurid cooperation includes indirect evidence of caring for the injured. Larson noted that Sue had a broken leg bone, which had healed but must have incapacitated the dinosaur for some time. Larson suggested that the T. rex could only have survived with the help of other individuals who brought it food while the leg healed.
Images of group cooperation, family life, and nesting habits are a far cry from the first descriptions of T. rex in the early 1900's and the long-held assumptions about the tyrant king as a solitary hunter. The traditional portrayals of T. rex in films and novels can make it difficult to imagine this terrifying beast in any other way. And as the towering skeleton of Sue demonstrated at the opening of the Field Museum exhibit in 2000, T. rex will always arouse a sense of amazement at the animal's deadly fierceness--apparent even in bare bones. Nonetheless, our evolving understanding of the Tyrannosaurus as a social creature as well as a deadly predator make it seem even more lifelike.