Enhancement Chapter: Raven and Johnson's Biology, Sixth Edition

Chapter outline

45.5 The dinosaurs were the dominant terrestrial vertebrates until they abruptly disappeared 65 million years ago.

45.6 Dinosaurs had a variety of unique skeletal and physiological characteristics.

45.7 Dinosaurs can be categorized into six basic body designs.


Figure 45.46
The most successful terrestrial vertebrates.
This Deinonychus, had one of the largest brain for body size of any dinosaur, was a highly effective predator, and killed its prey with a large razor-sharp claw on its rear feet.

Dinosaurs are the most successful of all terrestrial vertebrates (figure 45.46). They dominated life on land for 150 million years, an almost unimaginably long time---for comparison, humans have been on earth only 1 million years. During their long history, dinosaurs changed a great deal, because the world they lived in changed---the world’s continents moved, radically altering the earth's climates. Thus we cannot study dinosaurs as if they were a particular kind of animal, describing one "type" that is representative of the group. Rather, we have to look at dinosaurs more as a "story", a long parade of change and adaptation.

In this overview, we will examine a variety of very different kinds of dinosaurs, animals that lived at different times, adapted to very different worlds. First, we will trace the history of the dinosaurs, and try to get a sense of the evolutionary forces that so changed the dinosaurs as they evolved.

45.5 The dinosaurs were the dominant terrestrial vertebrates until they abruptly disappeared 65 million years ago.

The Triassic: Origin of Dinosaurs

Dinosaurs are reptiles. The first dinosaurs evolved from thecodonts, a crocodile-like group of now-extinct meat-eating reptiles (figure 45.47). The oldest dinosaurs of which we have any clear evidence left their fossils in late Triassic rock in Argentina, some 235 million years old (figure 45.48). Only 1 foot long, one of the first dinosaurs was a bipedal (that is, it stood up and ran on its hind feet) carnivore named Eoraptor. Almost as old is Herrerasaurus, a bulky 13 foot carnivore with sharp pointed teeth and a sliding jaw joint that let it slice its victims as it bite into them. These early dinosaurs, all meat-eaters, were the first vertebrates to exhibit the key dinosaurian improvement in body design-their legs are positioned directly under the body, allowing them to run swiftly after prey. By the end of the Triassic, small carnivorous dinosaurs were very common, most of them slightly-built and bipedal, like the long-necked Coelophysis.

Figure 45.47
Dinosaurs evolved from thecodonts.
Crocodiles, dinosaurs, and birds all arose from early reptiles called thecodonts. Together, crocodiles, dinosaurs, birds, and thecodonts are called archosaurs.

Figure 45.48
Early dinosaurs.
(a) Euparkeria was less than a meter long, but was an active carnivore. (b) Coelophysis was a long-necked, bipedal, carnivorous dinosaur of the late Triassic. (c) Returning to the sea, marine reptiles, such as this placodont, had large bodies, short necks, and paddlelike limbs. They were likely the dominant marine predators.

During the late Triassic, all the continents were together in a single super-continent called Pangaea (figure 45.49). There were few mountain ranges over this enormous stretch of land, the interior of which was arid and dry, with widespread deserts. Coastal climates were much the same all over the world-quite warm, with a dry arid season followed by a very wet monsoon season. By the end of the Triassic, 213 million years ago, dinosaurs had become common, replacing the thecodonts and surviving amphibians.

Figure 45.49
Migration of the major land masses.
The continents were not in their present positions when dinosaurs lived. In the Triassic (a), all of the continents were connected into a huge supercontinent, Pangaea. This landmass began to break up during the Jurassic (b). By the Cretaceous (c), today's continents began to be recognizable, although Australia and Antarctica did not separate until 50 million years ago, 15 million years after the dinosaurs became extinct.

There are at least three reasons why dinosaurs were so successful:

  1. Leg Mechanics. Positioning the legs directly under the body gave dinosaurs a key advantage over their reptile ancestors, the thecodonts---it allowed them to be more agile, faster runners.
  2. Drought Resistance. Pangaea had an arid continental interior in the late Triassic, conditions to which dinosaurs are superbly adapted. The mammals of the late Triassic, by contrast, were not "water-tight" enough for such climates.
  3. Luck. At the end of the Triassic a large meteor impacted in northeastern Canada (today the site is still visible, the Manicuoagan Crater) and appears to have been responsible for a great loss of diversity at the end of the Triassic. Thecodonts and many other species became extinct, leaving the "large animal" niche vacant for dinosaurs to fill.

The Jurassic: Golden Age of the Dinosaurs

During the early Jurassic, the period that followed the Triassic, vast deserts still covered much of Pangaea, especially in what was later to become western North America, which was covered by a vast sand sea during much of this time. Few ferns survived into these drier climates-the dominant trees were cycads, plants that resemble palm trees and are well-adapted to arid climates.

Change, however, was in the air: The great super-continent of Pangaea was breaking up. Long fingers of ocean began to separate the northern part, called Laurasia (the future continents of North America, Europe, and Asia) from the southern part, called Gondwana (the future continents of South America, Africa, Australia, and Antarctica), and the two land masses became fully separated by the end of the Jurassic. World sea levels began to rise, and much of Laurasia and Gondwana began to be inundated by sea water, forming shallow inland seas. The world climate became even warmer, and because so much of the land was nearer to the oceans conditions became progressively less arid.

The Jurassic is called the "Golden Age" of dinosaurs, because of the variety and abundance of dinosaurs that lived during this time, among them the largest land animals of all time, the giant sauropods (figure 45.50). Sauropods such as Brachiosaurus were the dominant herbivores (plant eaters) of the Jurassic. Some weighed 55 tons, stood 35 feet tall, and were over 100 feet long, longer than a football field! They had enormous barrel-shaped bodies with heavy column-like legs, and very long necks and tails.

Figure 45.50
A sauropod.
Sauropods, such as this Mamenchiasaurus, were the largest land animals ever to have lived.

The next-largest Jurassic herbivores, the size of a big pickup truck, were the stegosaurs. Stegosaurus weigh about a ton and was 15 feet long. Stegosaurs were bottom browsers that ate low to the ground. They had a row of narrow plates along their backs, and on the tip of their tail sharp spikes over a foot long.

By the late Jurassic, very sophisticated carnivorous dinosaurs (flesh-eating dinosaurs are called theropods) had evolved (descendants of Herrerasaurus) that preyed on the large herbivorous dinosaurs. Bipedal, with powerful legs, short arms, and a big head, Allosaurus had the body design typical of theropods, well-suited for rapid running and a quick, slashing attack.

The Cretaceous: Triumph of the Chewers

The Jurassic ended 144 million years ago, followed by the Cretaceous, a period of profound change in the history of dinosaurs. Both Laurasia and Gondwana fragmented during the Cretaceous into the continents we now know. Sea levels continued to rise, so that by the mid-Cretaceous sea levels had reached an all-time high and the interior of North America was a vast inland sea. Much of the world's climate was tropical, hot and wet, like a greenhouse. Most importantly, the early Cretaceous saw the first appearance of flowering plants, angiosperms. The appearance of this new kind of plant had a profound effect on herbivorous dinosaurs. Stegosaurs and most sauropods become extinct, replaced by a totally different kind of plant eater better adapted to consuming these tough versatile plants.

Sauropods and stegosaurs do not chew the plants they eat. They simply shred the leaves and stems and swallow the shreds whole-rocks within their stomachs then batter the swallowed plant material to a pulp. A good strategy with cycads, which have soft, pulpy interiors, this approach doesn't work well with angiosperms, which are much woodier and tougher. With the rise of angiosperm plants at the beginning of the Cretaceous, sauropods and stegosaurs were replaced by iguanodonts, which had "chewing" teeth (figure 45.51). The jaws of iguanodonts contain enormous batteries of grinding teeth that shred, pound, and grind even the toughest angiosperms. Even bigger than stegosaurs, Iguanodon was as heavy as an elephant.

Figure 45.51
Stegosaurs were common herbivores in the Jurassic, while iguanodonts, ceratopians, and armored dinosaurs were more common in the Cretaceous.

Later in the Cretaceous, iguanodonts are replaced in turn by a diverse array of equally large chewing herbivores: hadrosaurs (the so-called "duck billed" dinosaurs) with bony crests on their heads, ceratopians like Triceratops with bony head frills and horns, and armor-plated dinosaurs called ankylosaurs, the most numerous and diverse of all kinds of dinosaurs. As long as 30 feet long and weighing up to 5 tons, many of these chewing dinosaurs were larger than a Sherman battle tank.

The flesh-eating theropods of the Cretaceous are more diverse and formidable than the allosaurs which dominated the Jurassic. Among the largest were the Tyrannosaurs. Never common, only a few dozen fossil Tyrannosaur skeletons are known. Many other smaller flesh-eating dinosaurs are known from the Cretaceous.

Extinction of the Dinosaurs

Towards the end of the Cretaceous, sea levels began to fall and the climate to cool. Many kinds of dinosaurs become less common-and then, suddenly, 65 million years ago, all dinosaurs disappear from the fossil record (figure 45.52). What cause the sudden extinction of the dinosaurs, after 150 million years of success? Most biologists now agree that the most likely cause was the impact of a gigantic meteor (it appears to have been 5-10 miles across!) off the coast of Yucatan. The thin line of sediment that marks the end of the Cretaceous in rocks today is rich in iridium (a mineral rare in the earth's crust but common in meteors), in tiny spheres of cooled molten rock, and in bits of quartz shocked by high-velocity impact (figure 45.53). The impact created a huge crater 185 miles in diameter, throwing massive amounts of material into the atmosphere that would have blocked out all sunlight for a considerable period of time, creating a world-wide period of low temperature. Insulated with feathers or fur, the endothermic birds and mammals (those whose body temperature is determined by the heat generated by the animal's metabolism) survived, and the ectothermic reptiles and amphibians (those whose body temperature is determined by the temperature of the environment) did too-ectothermic animals simply lower their activity levels.

Figure 45.52
Extinction of the dinosaurs.
The dinosaurs became extinct sixty-five million years ago in a major extinction event that also eliminated all the great marine reptiles as well as the largest of the primitive land mammals. Crocodiles, small lizards, and turtles survived, but reptiles never again achieved the diversity of the Cretaceous period.

Figure 45.53
Traces of iridium have been found in Cretaceous sediments worldwide.
A very large impact crater lies off the coast of the Yucatan peninsula in Mexico. The age of this crater coincides with the extinction of the dinosaurs at the end of the Cretaceous Period. Scientists have calculated that an impact large enough to create this crater would have thrown enough dust (which can be identified by its high iridium content) into the atmosphere to drastically reduce the amount of sunlight reaching the Earth's surface.

We can never be sure why the dinosaurs did not live through the deep cold. Disease might have killed them, or sudden intense competition from mammals have led to their extinction. However, the most reasonable and widely-accepted explanation is that the cold itself killed them. Most if not all Cretaceous dinosaurs appear to have been endothermic (figure 45.54), biologists now believe (we will discuss the evidence for this in the next section). Being endothermic created a critical dilemma in the period of intense cold after the meteor impact, for the dinosaur body had no insulation, no way to retain body heat. Endothermy made a great contribution to the success of the dinosaurs, but created an evolutionary dead end from which they could not emerge.

Figure 45.54
Cretaceous dinosaurs were likely endothermic.
Cretaceous dinosaurs included the carnivorous theropods, herbivores such as ceratopians, and pterosaurs.

45.6 Dinosaurs had a variety of unique skeletal and physiological characteristics.

Key Dinosaur Characteristics

Like their thecodont ancestors, dinosaurs are extinct diapsid reptiles, often-bipedal---that is, they are extinct reptiles with a pair of holes in the skull behind each eye (figure 45.55), many of which walked on two feet. The great marine (ocean-swimming) reptiles of those times, ichthyosaurs and plesiosaurs, were not dinosaurs, and neither were the flying reptiles, the pterosaurs---these other reptiles did not walk on land as dinosaurs and thecodonts did. Huge therapsids like the sail-back Dimetrodon did walk on land, but they were not dinosaurs, either---these ancestors of mammals were synapsids, with but one hole in the skull behind the eye.

Figure 45.55
Four types of reptiles.
Reptiles are put in four subclasses according to their skull design. Anapsids have no hole between the postorbital and squamosal bones. Synapsids have one hole displaced inferiorly and between these bones. Euryapsids have one hole displaced superiorly and between these bones, and diapsids have two holes between these bones, one superior and one inferior.

Crocodiles are also diapsid reptiles, like dinosaurs direct descendants of thecodonts. Are crocodiles dinosaurs? No. What makes dinosaurs unique, what sets them apart from both crocodiles and thecodonts, are key changes in how they stand and run.

Unique Characteristics of Dinosaurs

A variety of unique features are shared by all dinosaurs that set them apart from thecodonts or any other vertebrates, among them the shape of hand, foot, thighbone, and ankle. Two key anatomical changes are of particular importance, changes which improved speed and agility, providing an important evolutionary advantage to early predacious dinosaurs.

  1. Strong knee and ankle joints. In bipedal thecodonts (and crocodiles) the knee and ankles flex in a swiveling motion, supporting a swinging walk. Flexing is possible in their ankle because the bones of the left and right sides are connected by a swiveling peg-and-socket joint. In dinosaurs (and birds) the knee is a simple hinge, and peg-and-socket ankles are gone---the foot gives up its grasping design to become a stiff propulsive level, the dinosaur ankle bones firmly attached to the shin bone. These new knee and ankle structures provide the greater strength and stability necessary to support a bulky body (figure 45.56).
  2. Upright stance. Thecodonts stood with their legs angled out, in a "Vee" stance. The top of the leg bone (the femur) juts at an inward angle into a shallow groove on the hipbone (the ileum) to form the hip socket. Because bone builds where force is applied, the ileum is thick behind the groove. Dinosaurs stood upright, with legs positioned directly beneath the body. The top of the femur is angled sharply inward, with a ball-shaped tip that fits into a lip on the socket of the ileum. Because there is no pressure sideways, the side of the ileum has no bone at all---there actually is a hole in the side of the socket, where the ileum meets the two other pelvic bones.

Figure 45.56
Ankle joints of (a) bipedal thecodonts and crocodiles and (b) dinosaurs and birds.
The ankle bone in dinosaurs was firmly attached to the shin bones, and this simple hinge construction allowed for a much stronger joint.

A dinosaur's foot is quite different from a human one. The long bones of the foot are not parallel to the ground, as yours are, but bunched together and running upward to the ankle joint, like a horse's do---so, like horses, dinosaurs walk on their toes at all times. Long slender toes give the dinosaur foot a firm grip on the ground. Nearly all dinosaurs have only three walking toes on the foot. Their footprints are very similar to those of birds!

There Are Two Orders Of Dinosaurs

Early in the evolution of dinosaurs, the structure of the hip evolved in two distinct directions (figure 45.57). There are three bones that meet at the dinosaur hip socket, with large powerful leg muscles attached to each of them. This distinction in hip structure is so clear that it is used to define two orders of dinosaurs---saurischians and ornithischians (see table 45.3).

Figure 45.57
The two orders of dinosaurs differ in their hips.
Ornithischian ("bird-hipped") dinosaurs had a rear-pointing pubis bone, while Saurischian ("lizard-hipped") dinosaurs had a forward-pointing pubis. Birds, however, evolved from the lizard-hipped dinosaurs!

Saurischians. In one kind of dinosaur, the three hip bones radiated in different directions. Because that is the way a lizard's do today, these are called lizard-hipped dinosaurs. Many of the largest of dinosaurs were lizard-hipped, including enormous vegetarian sauropods and meat-eaters such as Tyrannosaurus rex.

Ornithischians. In the other kind of dinosaur, the forward-projecting of the three hip bones is turned back parallel to the rear-projecting one. Because that is the way it is in birds today, these are called bird-hipped dinosaurs. The names do not imply relationships---birds, for example, actually evolved from lizard-hipped dinosaurs!

Table 45.3 Orders of Reptiles
Order Typical Examples . Key Characteristics Approximate Number of Living Species
Ornithischia Stegosaur Dinosaurs with two pelvic bones facing backward, like a bird's pelvis; herbivores, with turtlelike upper beak; legs under body Extinct
Saurischia Tyrannosaur Dinosaurs with one pelvic bone facing forward, the other back, like a lizard's pelvis; both plant- and flesh-eaters; legs under body Extinct
Pterosauria Pterosaur Flying reptiles; wings were made of skin stretched between fourth fingers and body; wingspans of early forms typically 60 centimeters, later forms nearly 8 meters Extinct
Plesiosauria Plesiosaur Barrel-shaped marine reptiles with sharp teeth and large, paddle-shaped fins; some had snakelike necks twice as long as their bodies Extinct
Ichthyosauria Ichthyosaur Streamlined marine reptiles with many body similarities to sharks and modern fishes Extinct
Squamata, suborder Sauria Lizards Lizards; limbs set at right angles to body; anus is in transverse (sideways) slit; most are terrestrial 3,800
Squamata, suborder Serpentes Snakes Snakes; no legs; move by slithering; scaly skin is shed periodically; most are terrestrial 3,000
Chelonia Turtles, tortoises, sea turtles Ancient armored reptiles with shell of bony plates to which vertebrae and ribs are fused; sharp, horny beak without teeth 250
Crocodylia Crocodiles, alligators, gavials, caimans Advanced reptiles with four-chambered heart and socketed teeth; anus is a longitudinal (lengthwise) slit; closest living relatives to birds 25
Rhynchocephalia Tuataras Sole survivors of a once successful group that largely disappeared before the dinodaurs; fused, wedgelike, socketless teeth; primitive third eye under skin of forehead 2

Dinosaur Physiology

Were Dinosaurs Endothermic?

The suggestion that dinosaurs may have been endothermic has long been an area of spirited disagreement among paleontologists (those who study fossil animals). Because dinosaurs have been traditionally grouped among the reptiles, and living reptiles are all ectothermic, it has been common practice to simply assume that dinosaurs were ectothermic too. But dinosaurs actually resemble endothermic birds far more closely than they do ectothermic lizards, so arguments have been advanced that dinosaurs were in fact endotherms, animals whose body temperature is determined by the heat generated by their own metabolism (figure 45.58). Among these arguments are the following:

  1. Growth rate. Examination of the fossil bones of babies in dinosaur nests indicate that dinosaur babies grew very fast. This is typical of endotherms (a German shepherd dog goes from one-pound puppy to a 120-pound adult in one year), but unheard of for ectotherms (a python takes 20 years to go from five pounds at birth to 120 pounds).
  2. Bone texture. Slow-growing ectotherms like lizards make bones slowly, laying down regular layers of collagen one on top of the other, and need few Haversian canals to transport nutrients into the slow-growing bone. Fast-growing endotherms (mammals and birds) lay down collagen in their fast-growing bones so quickly there isn't time to form neat parallel rows, and an abundance of Haversian canals are present to bring lots of nutrients to the hard-working bone-building cells. When examined with a microscope, dinosaur bones prove to resemble those of endotherms, with a loosely packed jumble of collagen and many Haversian canals.
  3. Isotope ratios. The toes and tails of ectothermic animals are colder than their torsos (its bulk keeps the body's core warmer), while in endotherms the toes and tails are usually the same temperature as the rest of the body. If dinosaurs were ectothermic, the bones in their extremities would have been formed at a lower temperature that those of their torso, just as in lizards today. This can be tested directly! Bone contains two isotopes (different forms) of oxygen atom, oxygen-16 and oxygen-18, and at high temperature 16O is taken up preferentially into new bone. The bones in the toes and tail of a ectothermic crocodile contain less much 16O than the rib bones for this reason, while the toe bones of an endothermic cow contain the same proportion of 16O as its ribs because its feet are at the same temperature as its torso. When six Cretaceous dinosaurs were examined in 1992, the proportion of 16O in toe, tail, and rib were as similar as in a mammal---these dinosaurs maintained their own body temperature! Jurassic dinosaurs are now being examined.

Figure 45.58
Dinosaurs and endothermy.
In truly endothermic animals, the extremities are kept at the same temperature as the rest of the body. By analyzing the pattern of oxygen isotopes laid down in the fossilized bones, scientists have found that some dinosaurs in the Cretaceous exhibited a pattern of oxygen isotopes that indicates an endothermic heat distribution. Sauropods, however, may have been inertial homeotherms, maintaining a constant body temperature due to their size. While their extremities may have contained less heat than the main bulk of the body, sauropods likely maintained a high body temperature simply because their large body volume allowed them to retain heat.
Child-Rearing Among Dinosaurs

Maternal care is rare among reptiles, while almost universal among mammals and birds. Among the living reptiles, only crocodiles care for their young. Dinosaurs laid their eggs in nests, and many kinds of dinosaurs then abandoned their eggs to hatch untended, leaving the hatchlings to fend for themselves, much as a turtle does today. However, in the last 15 years paleontologists examining the nests of hadrosaurs have come across clear evidence that hadrosaurs cared for their young: egg shells within the nests were broken into tiny bits, as if trampled underfoot by hatchlings for a long time; young of different ages died within single nests; dead hatchlings had worn teeth, and so must have been fed in the nest; leg bones of hatchlings are not yet fully formed at birth, and so are not yet ready for walking. These findings indicate the young hadrosaurs were reared in their nests for a considerable time after hatching, fed by their parents. Hadrosaurs appear to have nested in colonies, huge herds migrating across North America in the late Cretaceous, returning each season to nesting grounds to rear young.

45.7 Dinosaurs can be categorized into six basic body designs.

Many different kinds of dinosaurs evolved over the long 150 million year reign of the dinosaurs (figure 45.59). While each kind of dinosaur has its own peculiarities, many can be thought of as variants of one of six basic body designs: sauropods, theropods, stegosaurs, ornithopods, armored dinosaurs, and horned dinosaurs (figure 45.60). Each body design is driven by the requirements of a particular life-style, determined in large measure by what the dinosaur ate, how it digested its meal, and how it avoided being eaten.

Figure 45.59
A dinosaur cladogram.

Figure 45.60
Dinosaurs are categorized into two main groups based on hip design.


Sauropods, the dominant dinosaurs of the Jurassic, were giant herbivores, the largest land animals to have ever lived. All had massive barrel-shaped bodies supported by four pillar-like legs, and very long necks and tails. There were two basic kinds.


Backbone Design. The secret of supporting the massive sauropod body lay in the construction of the backbone. Great chunks of bone were hollowed out from the sides of each vertebra, leaving a backbone made of thin sheets and struts of bone, lightweight but immensely strong.

Neck Design. Sauropods have extremely long necks, as much as half their length. These 35-foot necks did not have more vertebrae than other dinosaurs (typically 12; for comparison, your neck has nine), but each was elongated to three times the length of a back vertebra. While some sauropods may have fed on treetops, many ate low vegetation, and it is not clear why they had such long necks. How did the neck muscles support this 35-ton weight? They didn't. Front and back legs act like the towers of a suspension bridge. A groove runs along the top of the backbone, holding a massive tendon that runs like a cable between tail and neck. The cable allows the weight of the long neck to be supported by a counterbalancing tail---you get the same effect if you hold your arms outstretched from your sides, a rope passing tautly from one wrist to the other over your shoulders---your arm muscles don't have to work to keep your arms up, because their weight is going down your shoulders to your legs.

Heart Design. If Brachiosaurus stood with its head up, its brain would be 25 feet higher than it heart. The weight of a column of blood this tall would produce enormous pressure in the heart. If it were a typical reptile heart, with an incomplete division of the ventricle, blood would push into the pulmonary circuit and blow out the thin-walled capillaries of the lungs. This argues that sauropods must have had four-chambered hearts, like modern birds and mammals do, with a complete separation of the ventricle.

Stomach Design. Sauropods were vegetarians that fed on cycads, palm-like plants with mushy interiors. They shredded the plants with thin spoon-shaped or pencil-like teeth, and ate the shreds without grinding. The grinding necessary to prepare the plant material for digestion took place within the stomach, in a gizzard where stones swallowed by the dinosaur mashed the plant material to a pulp. The cellulose plant material was then digested by microbes within stomachs that acted as huge digestion vats. The bigger the stomach and its rocks, the more efficient the grinding, which is one reason the sauropods grew so large.

Leg Design. Sauropods were lizard-hipped. The immense weight of the body was supported by four pillar-like legs that did not bend when walking. Each leg ended with a broad, round "elephant-like" foot. Like all dinosaurs, sauropods stood on their toes---a wedge-shaped heel pad supported the great weight.

Tail Design. Their tracks tell us that sauropods did not drag their tails along the ground, but rather held them stiffly out behind. This, and the fact that the neck bones of most sauropods butt up against each other squarely, suggests to many paleontologists that sauropods held their necks out straight, rather than "up" as often illustrated.

Typified by this Brachiosaurus, these largest of sauropods had serrated, chopping teeth and front legs longer than the rear ones, to raise the body up to crop the tops of trees. A Brachiosaurus was 75 feet long and 40 feet tall (the shoulders were 20 feet off the ground!), weighing an incredible 89 tons (more than 12 African bull elephants or three thousand students). The neck was 32 feet long!


Typified by this Diplodocus, these sauropods had smaller bodies than the top-eaters (Diplodocus weighed 11 tons), and even longer necks and tails (the tail of Diplodocus is 46 feet long!). From tip of nose to tip of tail, Diplodocus was 86 feet long, the length of a subway train. Other bottom browsers were even longer. Opposite of top eaters, the hind legs of bottom browsers were longer than the front legs. They had slender, peg-like teeth, good for browsing at the surface. Apatosaurus was a somewhat larger member of this group, weighing 33 tons.


Theropods were bipedal meat-eaters with short grasping forearms and sharp slicing teeth. All theropods were lizard-hipped, and shared the same basic body design. There were three basic kinds, differing mostly in size.


Large theropods---typified by Allosaurus in the Jurassic and this Tyrannosaurus rex (the largest of all meat-eating dinosaurs) in the Cretaceous. Allosaurus, the chief carnivore of the Jurassic, weighed about 2 tons and stood some 15 feet tall, while Tyrannosaurus (which evolved late in the Cretaceous and was never common) weighed fully 8 tons and stood 20 feet tall, with sharp ripping teeth each 6 inches long.


Medium-sized theropods with a lethal 5" sickle-shaped claw on each foot for ripping prey. About the size of a human, they had the light speedy body of a coelurosaur with the massive strong head of a carnosaur---typified by Velociraptor and this Deinonychus. Cretaceous dinosaurs, raptors had among the largest brains for body weight of any dinosaur. The arms were long for a theropod, with three grasping fingers with long sharp claws.


Small, fast running descendants of Coelophysis, with long necks and hollow, delicate bones, built for speed. Typified by this chicken-sized (8 pound) Compsognathus, coelurosaurs often hunted in packs.

Common in the late Jurassic, they are thought to be the direct ancestors of birds.


Tooth design. Theropod teeth are very different from a lion or wolf, which have deep-penetrating fangs. Theropods have long rows of curving dagger-like teeth, designed to slash through flesh, not hold it. Prey are literally sliced open: each tooth has a blunt front and a tapered sharp trailing edge. Their backward curve drives the line of teeth into a wound, slashing it open as the teeth saw backwards, while the blunt fronts stops forward movement of the teeth and so prevents struggling prey from wriggling free.

Jaw Design. The largest teeth are the upper ones, on the skull. During a bite, the jaw teeth hold the prey while the skull is pulled down and back by the neck. The jaw joint allows the skull to slide, shifting aft during a bite, so that retracting the powerful neck muscles after a bite pulls the skull back, raking the upper teeth backward through the prey. The jaw joints of theropods are quite flexible, allowing a very large bite. The lower jaw is hinged in the middle and at the center. In Cretaceous carnosaurs like T. rex, the skull is solid and the jaw moves in a grove along the skull. In Jurassic carnosaurs like Allosaurus, the jaw joint is like that of a teleost fish, connecting the jaw to the skull with a strut of bone---when the strut swings outward, it splays the jaw to the sides, creating a huge bite.

Leg Design. Theropods are bipedal, with short forearms and long powerful hind legs. The forelimbs of most theropods have 3 sharp claws (or in the case of T. rex, two) for seizing prey.

Tail Design. All theropods had long tails to counterbalance the weight of the body forward of the hips, giving them great agility. Tails of many theropods were stiffened by bony rods which ensheath the tail bones---this extra support is necessary to withstand the whiplash forces put upon the tail by powerful movements of the neck.

Backbone Design. In order to pivot their body weight directly over the hips, the vertebrae where the hip bones attach are fused into a strong rigid beam. The rigid backbone is balanced by an interlocking array of bone and muscle running down the front of the body from shoulder to lower hip. Called "belly ribs", this bodice of bone acted as an effective shield to protect the belly.

Skull Design. The skull of Jurassic theropods is not a solid cranium, like a lion or bear, but a highly flexible "rod and sheet" construction---struts, ball-in-socket joints, and sliding articulations, all bound together with ligaments. The braincase is the strong central core of the skull, only loosely bound to the other parts. In Cretaceous theropods, the skull is more solidly constructed, culminating in the T. rex skull, a single massive structure of bone


Ornithopods were large bipedal herbivores, one of several kinds of "chewing" plant-eaters dominated the Cretaceous period. Like stegosaurs, the ornithopods were bird-hipped. The great innovation of the ornithopods, which led to their replacing sauropods and stegosaurs, was the invention of chewing teeth. Unlike other herbivores of the Cretaceous, ornithopods lack any obvious defense against theropods. They must have had them, but their nature is unknown---perhaps they sprayed awful-smelling perfumes on their enemies, like a skunk! The two most important kinds of ornithopods were:


Iguanodonts, typified by this Iguanodon, were very large herbivores, 30 ft long, 16 feet tall, and weighing 5 tons---as big as a small house! Iguanodon had massive legs, and walked on all four legs most of the time. They evolved in the mid-Jurassic, replacing sauropods, and were common until replaced in turn by another kind of ornithopod, the hadrosaur.


Hadrosaurs, a very successful dinosaur typified by this Hadrosaurus, are often called duck-billed dinosaurs because they had a broad, flattened snout with a toothless beak used to crop plants. Thirty feet long, but less massive than iguanodonts, hadrosaurs had long hind legs---they walked on four legs, but were able to rise up and run on their hind legs like an ostrich in times of danger.

Many had bony crests on the tops of their heads, which may have been used as trumpets. In the Cretaceous, hadrosaurs were the most abundant dinosaurs in North America.


Mouth Design. Both iguanodonts and hadrosaurs had beaks in the front of the mouth for cropping plants, with a sharp, horny edge---like a turtle. There were large batteries of hundreds of grinding teeth on each side of the mouth---more teeth than any other dinosaur. At any moment, rows of young teeth are growing into the mouth, replacing worn-out teeth. The grinding surface is perhaps the most effective that has ever evolved, far superior to today's mammal herbivores. Unlike theropods or sauropods, ornithopods had cheeks! This was a necessary adaptation for retaining plant material in the mouth while chewing it.

Backbone Design. The bipedal posture of ornithopods presents exactly the same requirements for strengthening the backbone as we encountered in designing a theropod, but here the solution is quite different. Theropods have large muscles on either side of the spine to strengthen and stiffen the back, but in ornithopods these muscles became modified into long, bony ossified tendons, a latticework of long rods which provide more support for much less weight. Running from mid-back to mid-tail, this bony supporting sheath acts like a stiff girdle. The backbone of many ornithopods bent down sharply at the shoulder, locating the head low-to-the-ground when on all fours.

Stomach Design. The belly of plant eaters needs to be large, as plant material takes a considerable time to digest. In ornithopods, the bone that in lizard-hipped sauropods and therapods faces forward (the pubis bone) is rotated backward parallel to the bone at the back. This leaves a great deal more space for the belly, which is now free to extend back between the legs. Not only can the belly be larger, this shift also repositions the weight of the stomach back between the legs, allowing an upright posture. Without it, such a large-bellied animal would be obliged to walk on all fours.

Hand Design. In iguanodonts, hadrosaurs, and other large bipedal ornithopods, the toes on the front feet end in small rounded hooves, permitting them to walk on all fours occasionally. The thumb was modified as a spike for defense, while the little finger was able to bend inward---a grasping finger!

Armored Dinosaurs

Plant-eating dinosaurs of the Cretaceous, these bird-hipped dinosaurs were designed like battle tanks. Individuals weighed up to 3 tons, their bodies encased in heavy armor. The armor consisted of bands of horn-covered plates embedded in the skin from neck to tip of tail, with sharp spikes sticking out from each side of the body. Armored dinosaurs were among the most diverse of all dinosaurs. There were two basic kinds:


From 6 to over 25 feet long, these dinosaurs had hundreds of "nodes" of bone studding their bony plates like thorns (hence their name). Their small heads were long and slender. Nodosaurs were large, such as this Nodosaurus, 18 ft long. Sauropelta was 25 feet long and estimated to have weighed over three tons. Weighing this much, they could not have moved fast---their shoulders and hips were powerfully developed to lumber along under their great weight. Most nodosaurs cropped plants with a turtle-like beak in front, and had weak teeth on the side of the jaw.


Replacing nodosaurs in the late Cretaceous, these armored herbivores had a unique defensive weapon at the tip of their tails---a large ball of fused bone that was swung from side to side as a club. Like a bowling ball on a chain (only even heavier), it could smash a theropod lethally. Bigger than nodosaurs, ankylosaurs were shorter and stockier---typified by this Ankylosaurus, which was fully 33 feet long and weighed 4 tons, its broad squat body supported by strong stumpy legs set directly beneath the body. Ankylosaurs had hips fused to the backbone, forming a super-strong anchor for the hind legs.


Stegosaurs were large plant-eating dinosaurs of the Jurassic. They had small heads and a row of great bony plates that extended down the backbone, projecting alternately to one side, then the other (many pictures erroneously portray them as having two rows of plates!). The bony plates may have been useful in defense, but appear to have functioned primarily as a heating and cooling system---blood flowed through the thin epithelium covering the plates and was either cooled by breezes or warmed by the sun. Defense was provided by three-foot-long sharp spikes on the tail, used to whack any threatening predators. Unlike sauropods and theropods, stegosaurs were bird-hipped.


All 24 kinds of stegosaurs are basically similar, typified by Stegosaurus, the largest: Stegosaurus had four spikes on the tip of its tail, while other stegosaurs had 8 or more. On average, Stegosaurus was 20 ft long and weighed 2 tons. Much too heavy to stand upright, stegosaurs walked on four legs. The massive hind legs were twice the length of the forelegs, which meant that the body sloped steeply forward from the hips. The skull was only 16 inches long, containing a brain the size of a walnut. It had a toothless beak in front for cropping cycads, and tiny non-abrading teeth on the sides---like sauropods, it crushed plant material with stones in its stomach.

Horned Dinosaurs

The ceratopians or "horned dinosaurs" were the last group of bird-hipped dinosaurs to evolve, late in the Cretaceous, only 20 million years before all dinosaurs became extinct. In this short time they spread all over the world, a very successful group. Unlike the armored dinosaurs, the defensive armor of horned dinosaurs was confined to their heads, which sported a bony cape and long sharp horns. Like all bird-hipped dinosaurs, horned dinosaurs were plant-eaters, with sharp beaks for cropping vegetation and particularly effective batteries of grinding teeth (the individual teeth had multiple roots like your molars do, allowing a very strong bite). Typified by this Chamasaurus and by Triceratops, horned dinosaurs were built like bulldozers, powerful and low to the ground---and they were BIG:

A typical Triceratops individual was over 30 feet long and weighed as much as 11 tons. Its neck frill is a solid sheet of bone, bearing needle-sharp horns over 3 feet long. With stocky but powerful legs, it could run quite fast---imagine those horns bearing down on you like a freight train!

Chapter Summary

45.5 The dinosaurs were the dominant terrestrial vertebrates until they abruptly disappeared 65 million years ago.

45.6 Dinosaurs had a variety of unique skeletal and physiological characteristics.

45.7 Dinosaurs can be categorized into six basic body designs.

Key Concepts

  1. Thecodonts. Early reptiles called thecodonts were crocodile-like carnivorous reptiles that lived during the Triassic Period. Dinosaurs, birds, and crocodiles evolved from them.
  2. Endothermy and ectothermy. Endothermic animals are able to control their body temperature using their own metabolic processes, while ectothermic animals must use the environment to control their body temperature.
  3. Saurischians and ornithischians. Dinosaurs are divided into two main groups based on their hip design. In saurischians, the three hip bones have a three-pronged design and extend out in different directions. In ornithischians, two of the bones extend in the same plane parallel to each other.

Review Questions

  1. How long did dinosaurs live, how did they originate, and do they have living descendants?
  2. Which group of early reptiles gave rise to dinosaurs, birds, and crocodiles?
  3. What key dinosaurian characteristics does Eoraptor exhibit?
  4. How did the way stegosaurs ate differ from the way iguanodonts ate?
  5. What kind of evidence supports the hypothesis that the impact of a meteorite caused the extinction of the dinosaurs?
  6. Why are crocodiles not considered dinosaurs?
  7. Describe the fossil evidence indicating that some dinosaurs may have exhibited parental care similar to that seen among birds.
  8. What special backbone and neck design did sauropods have that allowed them to support their massive bodies?
  9. Which group of dinosaurs is characterized by terrestrial flesh-eaters?
  10. How did the ways ornithopods achieved bipedalism differ from the way theropods achieved it?
  11. What might have been the function of the bony plates that extended along the backbone of stegosaurs?

Thought Questions

  1. Flying reptiles were a very diverse group during the age of dinosaurs. However, although the present age might be considered the age of mammals, bats (the only flying mammals) exhibit far less diversity than did flying reptiles. Can you suggest as explanation for this?
  2. If you were presented with a complete skeleton of a small reptile from the late Triassic, what one feature could you use to establish if the fossil is a dinosaur or a thecodont?

Internet Links

An outstanding site featuring a complete listing of dinosaur clades, dinosaur record breakers, excellent diagrams illustrating key points of anatomy, information on "non-dinosaurs," and many other rich resources.

Detailed information and discussion of many current topics concerning dinosaurs, such as dinosaur-bird relationships, warm blooded dinosaurs, dinosaur extinction, and many other topics.

Teaching About Dinosaurs
A broad array of lesson plans and activities about dinosaurs, both online and offline, with many interesting links.

For Further Reading

Bakker, R. T.: The Dinosaur Heresies, Kensington Publishing Company, New York, 1986.

Benton, M.: The Dinosaur Encyclopedia, Simon & Schuster, New York, 1984.

Cobb, V.: The Monster Who Died, Coward-McCann, Inc., New York, 1983.

Czerkas, S. J. and E. C. Olson, eds: Dinosaurs Past and Present Volume I, The University of Washington Press, Seattle, 1987.

Dixon, D., B. Cox, R. J. G. Savage, and B. Gardiner: The Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals, Macmillan, New York, 1988.

Fastovsky, D. E. and D. B. Weishampel: The Evolution and Extinction of the Dinosaurs, Cambridge University Press, New York, 1996.

Gore, R.: "Dinosaurs," National Geographic, January 1993, vol. 183(1), pages 2-53.

Horner, J. R.: Digging Dinosaurs, Harper & Row Publishers, New York, 1988

Horner, J. R. and Lessem, D: The Complete T. rex, Simon &Schuster, New York, 1993.

Lambert, D.: A Field Guide to Dinosaurs, Avon Books, New York, 1983.

Lambert, D.: The Field Guide to Prehistoric Life, Facts on File, Inc., New York, 1985.

Lessem, D.: Kings of Creation, Simon &Schuster, New York, 1992.

Norman, D.: Dinosaur!, Prentice Hall, New York, 1991.

Norman, D: Prehistoric Life: The Rise of the Vertebrates, Macmillan, New York, 1994.

Psihoyos, L.: Hunting Dinosaurs, Random House, New York, 1994.

Raup, D. M: Extinction: Bad Genes or Bad Luck? W. W. Norton & Company, New York, 1991.

Raup, D. M.: The Nemesis Affair, W. W. Norton & Company, New York, 1986.

Russell, D. A.: An Odyssey in Time: The Dinosaurs of North America, University of Toronto Press and National Museum of Natural Sciences, Canada, 1989.

Spalding, D. A. E.: Dinosaur Hunters, Prima Publishing, Rocklin, CA, 1993.

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