Tyrannosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Tyrannosauridae

Genus: Tyrannosaurus

Species: Tyrannosaurus rex

Introduction

Tyrannosaurus rex is one of the largest theropod dinosaurs ever found in the fossil record. It is also one of the world's most iconic and recognizable, characterized by its massive size, powerful jaws, and fearsome reputation.

Tyrannosaurus rex existed during the Late Cretaceous, between 70 and 66 million years ago, long after the separation of Pangaea into the supercontinents Laurasia and Gondwana, and their subsequent separation into the five large regions of East, Central, and West Laurasia, and East plus West Gondwana.

Evidence from paleontological studies has indicated that several mass extinction episodes have taken place. This includes the K/T extinction event that occurred between the Cretaceous and the Tertiary (Paleogene) periods some 66 million years ago, which was responsible for wiping out the dinosaurs, including Tyrannosaurus rex, and 50 percent of all other life.

The cause of the Late Cretaceous dinosaur extinction has been one of the most contentious issues in paleontology, though there are two basic schools of thought regarding it. Catastrophists argue that the extinction occurred very rapidly, while gradualists state it happened very slowly. Though many theories have been expressed, ranging from credible to outlandish, most tend to center on meteoric impact, sea regression, volcanism, and climate change. It is certain that the occurrence of any or all of these events and processes would have significantly modified ocean currents, rainfall, vegetation and food availability, and global temperatures, all of which could have pushed declining species into extinction.

Classification

Paleontologists employ two main classification methods for naming and grouping dinosaur species: the Linnaean system and cladisitics. The Linnaean system, developed by Carl Linnaeus in the mid 1700s, attempts to classify dinosaurs by placing them into hierarchical groupings based on their similarities to each other. It follows the generalized pattern, that is, kingdom, phylum, class, order, family, genus, and species, until a single organism remains in the final group.

Cladistics is the second classification system used and is favored by many paleontologists. First introduced in the 1950s, cladistics highlights relationships between organisms in regards to kinship and ancestry. Cladistics classifies organisms into a “clade” based on the premise that organisms with characteristics in common also have common ancestors. In dinosaur cladistics, the major dinosaur groupings are Theropoda, Sauropodomorpha, Ornithopoda, Thyreophora, and Marginocephalia, all of which comprise several different clades.

In 1887, British paleontologist Harry Seeley determined that dinosaurs could be divided into one of two orders, Ornithischia (or bird-hipped dinosaurs) and Saurischia (or lizard-hipped dinosaurs). However, by the end of the second decade of the twenty-first century, some scientists had begun to challenge this traditional division. All tyrannosaurs belong to the Saurischia order, which in turn comprises two major groups (or suborders)—the large herbivorous Sauropodomorpha and the Theropoda, which includes bipedal carnivores such as Tyrannosaurus rex. Although theropod dinosaurs have historically been defined as specifically carnivorous species, further research has indicated some of these species may have had more varied diets.

Theropods are an extremely diverse dinosaur group, which includes the largest clade called the Tetanurae and its two main divisions, the Carnosauria and the Coelurosauria. It might seem logical to place tyrannosaurs within Carnosauria, as has been the case historically, but further fossil research has demonstrated that Tyrannosaurus species actually belong to the Coelurosauria group, which contains those theropod species that have more in common with birds than carnosaurs.

Anatomy

With a 1.2 meter (4 ft) jaw, teeth of 15 to 30 centimeters (6–12 in) long, and a bone-crushing jaw force greater than that of any modern predator, Tyrannosaurus rex was a formidable species. Although Tyrannosaurus rex was very large and weighed an extraordinary amount, it possessed characteristics common to all tyrannosaurs, including short but deep jaws with stout, rounded teeth, forward-facing eye-sockets, long muscular hind limbs, and very short forelimbs.

As with many of the bipedal dinosaurs, Tyrannosaurus rex was traditionally mounted in an extreme upright position with its tail on the ground, somewhat like a modern-day kangaroo. Further research has demonstrated, however, that such a stance would have been physically impossible, resulting in the dislocation of several joints. Later depictions of stance and locomotion show the Tyrannosaurus had a much more bird-like posture, with their vertebral column (backbone) positioned horizontal to the ground and the tail in the air during locomotion.

Intelligence

One method used to measure animal intelligence is the encephalization quotient (EQ). Dinosaurs tend to rank on the lower end of the intelligence scale. However, with an EQ score falling between 1.0 to 1.5, Tyrannosaurus rex is considered one of the more intelligent dinosaur species. (Comparatively, humans have an EQ of 7.44, and dolphins measure a 5.31.) This follows the hypothesis that carnivorous dinosaurs tended to have higher intelligence than herbivorous species, a requirement for successful hunting. The relative intelligence of the Tyrannosaurus is based on a comparison of its brain-to-body size ratio to similarly sized animals.

Reproduction and Population

As is the case with many dinosaur species, information about reproduction and population numbers can only be inferred from fossil records. Given the relative rarity of fossilization, however, it is difficult to accurately measure the past population numbers of prehistoric animals.

Most dinosaur species are thought to have been oviparous, including the Tyrannosaurus, meaning that they laid several eggs per clutch (or batch). In almost all cases of fossilization, soft tissue is destroyed, which limits the ability of researchers to determine gender or sexual dimorphism accurately. In one very rare case, a group of researchers extracted bone tissue from the marrow cavities of fossilized Tyrannosaurus rex bones. This tissue was identified as medullary bone, which is considered unique to extant female birds. From this, researchers have inferred that the Tyrannosaurus rex specimen was female.

Later theory has also suggested that female dinosaurs had a more robust body form and wider pelvis to allow for the passage of eggs. Despite being oviparous like birds, however, it is considered very unlikely that this species would have exhibited contact incubation behavior, as their sheer size would have crushed the eggs. Although it is far from settled, research has indicated that it is unlikely that many dinosaur species practiced post-hatching care.

In 2021, research by Charles Marshall and others calculated that an estimated total population of 2.5 billion Tyrannosaurus rex individuals existed over the species' lifespan. They also estimated that 20,000 adults were living at any one time.

Diet

The fearsome reputation of the Tyrannosaurus rex stems from both its sheer size and its diet. This species of carnivorous dinosaur fed predominantly on other dinosaurs and was capable of eating up to 230 kilograms (500 lbs) of meat in one meal. Examination of fossil bones has provided evidence that the Tyrannosaurus bite was capable of penetrating bone, with puncture marks from Tyrannosaurus teeth found in the fossilized bones of species such as the three-horned Triceratops and the duck-billed Edmontosaurus. Evidence of dinosaur diet can also come in the form of coprolites (fossilized animal dung). One of the largest coprolites ever found has been identified as most likely belonging to Tyrannosaurus. Consisting of many crushed and pulverized bones, it again demonstrates the carnivorous nature of this species.

Although Tyrannosaurus teeth are impressive and the largest of any dinosaur, relative to the size of its jaw, the teeth are quite stout and rounded. As such, the teeth were not effective cutting instruments but were instead used to “grip-and-rip” prey, known as the “puncture-pull” feeding hypothesis. Such a feeding strategy meant the Tyrannosaurus skull had to withstand a tremendous bite force, which, at more than 3,000 pounds, was beyond that of any living animal. Research on Tyrannosaurus skulls has provided evidence that physical characteristics such as open skull sutures and thickened nasal bones acted like “shock-absorbers” during feeding.

Behavior

Prehistoric animal behavior is difficult to determine, with researchers inferring much from fossilized remains or from behavior seen in animals today. Although Tyrannosaurus is often depicted as a fast-moving predator in pop culture, some studies have determined that it would have been incapable of running at high speeds as that would require each leg to weigh 43 percent of its body mass, a physical impossibility. As such, Tyrannosaurus may have only moved at a maximum of 18 kilometers (11 mi) per hour. Based on scaling trends in modern day animals, however, other researchers state that this dinosaur could have reached speeds of 48 to 72 kilometers (30–45 mi) per hour. As the debate over the speed of Tyrannosaurus continued, in 2017 a paper published in the journal PeerJ discussed the findings of research on the topic conducted by a team affiliated with the University of Manchester. According to their analysis, which used uniquely more comprehensive and advanced biomechanical computer models, Tyrannosaurus likely moved at maximum speeds closer to around 19 kilometers (12 mi) per hour. They argued that the dinosaur's leg and feet bones would not have been able to withstand any faster speeds. Nevertheless, most paleontologists agree that whatever the actual speed, it would not have affected its ability to catch prey, as many of the larger herbivorous species were even slower.

One of the most contentious issues in paleontology, however, relates to the feeding behavior of Tyrannosaurus rex. For nearly a century, scientists have debated whether this species was an active predator or a scavenger. The main advocate of the scavenger theory, American paleontologist Jack Horner, argues that with only two fingers and very short forelimbs, the Tyrannosaurus would have been incapable of catching and grasping prey. This coupled with its large olfactory organs means that it was likely a scavenger. Jaw morphology and dentition, however, provide ample evidence to support the active predator hypothesis. In 2013, researchers argued in a paper published in Proceedings of the National Academy of Sciences of the United States of America that they had found physical fossilized evidence in support of the theory that Tyrannosaurus had been predatory. After studying a fossil belonging to a duck-billed dinosaur in which the remains of a tooth they identified as belonging to Tyrannosaurus rex had been preserved, they claimed that the Tyrannosaurus rex must have broken off its tooth after biting down hard on the duck-billed dinosaur before it managed to get away. The majority of paleontologists believe that the answer most likely lies somewhere in the middle, with Tyrannosaurus rex being an active hunter and opportunistic scavenger.

Habitat and Other Life Forms

The Cretaceous period was characterized by the emergence and evolution of many new plant and animal species and although it ended with a mass extinction event, it was a particularly successful time for dinosaurs.

The tyrannosaurs species lived during the Late Cretaceous, which is the younger of the two Cretaceous epochs. Although the Late Cretaceous experienced a general cooling trend, its climate has been described as a “greenhouse climate” with little change in the thermal gradient from the equator to the poles. Many areas in the Northern Hemisphere still exhibited gymnosperm (seed plants) and fern-dominated vegetation. The Late Cretaceous was also characterized by the rapid radiation and diversification of angiosperms (flowering plants). Tyrannosaurus, for example, favored the wide forested river valleys of Northern America, which featured trees such as bald cypress, metasequoia, sequoia, sycamore, and magnolia.

Research

Barnum Brown discovered the first fossil in the Hell Creek Formation in America in 1902, and Henry Fairfield Osborn, a vertebrate paleontologist working for the American Museum of Natural History, was the first to describe Tyrannosaurus rex in 1905.

Although tyrannosaurs dominated the North American regions and were considered the top predators of the time, similar species have been found on the Asian continent. Of particular interest is Tarbosaurus bataar, found in Mongolia and closely related to Tyrannosaurus rex. Based on their similar anatomy, it is thought that they each occupied the same predator niche and played the same ecological role in their respective communities. In fact, some paleontologists claim that the two species should belong to the same genus.

In 2009, scientists unveiled what they believed to be the oldest known ancestor of T. rex. Unearthed from an ancient lakebed in northwestern China, the specimen was later found in a private collection in America and was donated to the University of Chicago. A joint American and Chinese team examined the specimen, naming it Raptorex kriegsteini and describing it as a miniature version of the Tyrannosaurus rex, almost identical in appearance but approximately 100 times smaller. However, later analysis brought this finding into question, as some scientists argued that the specimen, which was immature and therefore unreliable, was possibly a juvenile Tarbosaurus.

Several years after it had first been discovered in Saskatchewan, Canada, in the early 1990s and extracted for study, it was announced in 2019 that researchers working on the analysis had determined that the Tyrannosaurus rex fossil known as Scotty was the heaviest Tyrannosaurus rex discovered by that point. Also establishing that Scotty had been older upon its death, possibly in its early thirties, this study, which was based on measurements of the largely complete skeleton's leg, hip, and shoulder bones, found that this specimen offered further insights into the development of the species. At the same time, some paleontologists pointed out the relativity of such claims of size, arguing that the skeleton of the Tyrannosaurus rex discovered in South Dakota long considered the biggest, Sue, was even more intact and seemed to be only slightly different.

Partly because of its size and its greater public perception, the Tyrannosaurus rex remained one of the more heavily studied dinosaurs, and tyrannosaurs in general continued to be of great interest to paleontologists well into the twenty-first century. In 2020, a study was published that focused on whether Tyrannosaurus had an advantage related to the energy efficiency of its legs and walk.

In 2022 a dispute emerged among paleontologists over the classification of T. rex. In March of that year, a group of researchers published a study in Evolutionary Biology claiming that the dinosaurs classified as T. rex should actually be reclassified into three separate species: the typical T. rex, the larger T. imperator, and the skinnier T. regina. However, many paleontologists disputed this finding, and in July 2022, a different group of researchers refuted the March 2022 study's conclusions. They cited a number of issues with the three species hypothesis, including the study's limited sample size and the use of incorrect statistical techniques. Their conclusions confirmed that T. rex would indeed remain classified as a single species.

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