Harry Govier Seeley split dinosaurs into two groups, the Order Saurischia ("lizard-hipped" dinosaurs) and the Order Ornithischia ("bird-hipped" dinosaurs). Both orders probably had a common ancestor that lived sometime during the Middle Triassic. Birds belong to the saurischian dinosaur clade.
As in all land animals, there were three bones in each side of the pelvis. The left and right ilia (singular: ilium) firmly gripped the spine in the sacrum. The left and right pubes (singular: pubis) extended down beneath the ilia. The left and right ischia (singular: ischium) extended down and back beneath the ilia and behind the pubes. In some dinosaurs, the pubes extended down and forward, as they do in lizards. This is why Seeley called them saurischian, or "lizard-hipped" dinosaurs. In other dinosaurs, the pubes extended down and back, running beneath and parallel to the ischia, as they do in birds. Seeley called these dinosaurs ornithischian, or "bird-hipped" dinosaurs.
Ornithischian dinosaur pelves (the plural of pelvis) developed from evolutionary changes of the primitive saurischian pelvis. Ornithischians also had other traits that grouped them together: a bone, called the predentary, was at the front of the lower jaw. Also, an "eyelid" bone rimmed the upper part of the eye socket.
Some dinosaurs were neither saurischians nor ornithischians. The earliest, most primitive dinosaurs, such as Staurikosaurus and Herrerasaurus, fit into neither order. They were too specialized to be the direct ancestors of the dinosaurs.
The sauropodomorphs quickly evolved into two major groups, the Prosauropoda and the Sauropoda. Although prosauropods appeared earlier, no known prosauropod could have been the ancestor of the sauropods. The prosauropods were widespread and had at least seven families. They lived until the Early Jurassic. The largest prosauropods, some as long as 40 or more feet, were straight-limbed dinosaurs that resembled the later sauropods in some ways. All prosauropods were plant-eaters.
Family: Thecodontosauridae: The most primitive prosauropod, Thecodontosaurus, was also one of the smallest. It was about six to ten feet long. Like all prosauropods and most sauropods, it had a prominent claw on each front foot and a large claw on each back foot.
Family: Plateosauridae: This is the best-known family of prosauropods, with animals found in Europe, China, and North and South America. They were 25 to 30 feet long with narrow, long snouts, long necks, powerful front and back limbs, and heavy bodies.
The second group of sauropodomorphs, the Sauropoda, probably came from an ancestor much like Thecodontosaurus. This probably happened sometime in the Late Triassic, when sauropods first appeared. All sauropods were giants and four-legged plant-eaters.
Like today's elephants, sauropods had little fear of predators because of their size. Being large also helped them reach food, such as leaves in treetops, that was too high for smaller plant-eaters. Sauropods had many features because of their large size. They lost the grasping function of their front feet, and their legs looked like long, straight columns. Their vertebrae (bones in the spine) had deep hollows to lighten the weight of their backbone. Also, to add strength they had more vertebrae where the pelvis and spine joined.
Sauropod skulls were either blunt (flat) or tapering (came to a point) and the nostrils were back from the tip of the snout. Sauropod heads, which were lightly built and fragile, often broke off after death.
Family: Vulcanodontidae: The earliest true sauropod is Vulcanodon from the Early Jurassic of Zimbabwe. The only skeleton is missing the head, neck, and much of the tail. It had a bulky body and its legs were long and straight. The front limbs were almost as long as the back, and each back foot had five toes.
Family: Barapasauridae: The next most primitive sauropod, Barapasaurus, is known from parts of several skeletons from the Early Jurassic of India. If was up to 60 feet long, with a slender body and a long neck, tail, and limbs.
Family: Euhelopodidae: Most of the sauropods known from the Middle and Late Jurassic of China are now placed in a separate family, the Euhelopodidae. Euhelopodids are one of the more primitive sauropod families, but they include such exotic animals as the extremely long-necked Mamenchisaurus and Omeisaurus.
Family: Cetiosauridae: This family is from the Middle Jurassic, perhaps from an ancestor from the Vulcanodontidae family. The cetiosaurids had expanded and spread into Europe, North and South America, Africa, and Australia by the Middle Jurassic.
Cetiosaurid skulls were blunt and box-like, with nostrils at the side of the snout. The neck was short, usually with 12 vertebrae. They ranged from small to large for sauropods; most were 35 to 60 feet long. The best-known genus is Shunosaurus from the Middle Jurassic of China. It had a small, bony club at the end of its tail. Cetiosaurids lasted until the Late Jurassic.
Family: Brachiosauridae: The front limbs of the brachiosaurids were as long or longer than the back limbs. This gave the body a backward slope from the neck to the tail. The number of neck vertebrae in this family increased to 13 or more. The nostrils were farther back from the tip of the blunt snout and above the eyes in the skull of Brachiosaurus. Most brachiosaurids were larger than the cetiosaurids, 80 or more feet long even though they had shorter tails. They were among the heaviest land animals known. To reduce weight, their huge vertebrae were almost completely hollow. Known worldwide, brachiosaurids appear in the fossil record during the Middle Jurassic, were most numerous in the Late Jurassic, and almost vanished by the end of the Early Cretaceous.
Family: Camarasauridae: In this family the skull was boxlike. They still had 12 neck vertebrae and the front limbs were slightly shorter than the back limbs. One of the last known camarasaurids was Opisthocoelicaudia from Mongolia, a heavy-bodied sauropod with a short tail that probably helped support it when it stood on its hind limbs to reach food.
Family: Diplodocidae: This family includes some of the most well known sauropods, including Apatosaurus and Diplodocus. Diplodocid skulls were long and tapered to a spoon-shaped snout and had nostrils on top of the skull. Their small, rod-shaped teeth were in the front of their snout. Diplodocids had long necks, with up to 15 vertebrae. Their backs were short compared to the length of their back limbs, and their tails ended in a whiplash that was probably used as a weapon. The long necks and tails made some diplodocids the longest animals that ever lived.
Family: Titanosauridae: Almost all southern-hemisphere sauropods from the Late Cretaceous, and many earlier ones, were titanosaurids. Their limbs were stocky. The vertebrae from the front and middle of the tail were unique and are the best feature that distinguishes the family. Not one complete or nearly complete titanosaurid skull has been found. One of the most interesting titanosaurids was Saltasaurus, which was squat and covered with armor similar to the ankylosaurs.
Most titanosaurids were about 40 to 50 feet long, but a few became gigantic. The titanosaurids lived mainly in the southern hemisphere during the Cretaceous Period, surviving there as the northern-hemisphere sauropods became extinct.
The theropods were all the predatory dinosaurs except the herrerasaurians. From the smallest dinosaurs to the largest meat-eaters, the theropods had the most different kinds of saurischian dinosaurs of all suborders. These two-legged meat-eaters had clawed feet with no more than three functional toes.
The wings and feet of birds are similar to the arms and feet of theropod skeletons. Also like birds, all theropods to some extent had hollow hones. The best ancestral bird is the small, feathered, theropodlike Archaeopteryx from the Late Jurassic.
The theropods evolved into two major groups: the Ceratosauria, with flexible tails; and the Tetanurae, with stiff tails. All the earliest theropods were ceratosaurians. Their fossil record is from the Late Triassic through the Late Jurassic. The tetanurans appeared in the Middle Jurassic, diversified in the Late Jurassic, and were the main northern-hemisphere predators until the Late Cretaceous.
Family: Podokesauridae: The earliest ceratosaurians include Coelophysis from the Late Triassic of western North America. It was small and nimble and had a long, slender skull with many teeth.
Families: Halticosauridae and Ceratosauridae: Dilophosaurus, which lived during the Early Jurassic, had a double crest on its head. Ceratosaurus was from the Late Jurassic and had a horn on its head. Both were from North America and are examples of later members of the ceratosaurians. After the Late Jurassic, ceratosaurians apparently vanished in the northern hemisphere but survived in South America.
Family: Abelisauridae: The abelisaurids are a group of medium to large African and South American theropods characterized by short, tall skulls. Carnotaurus from Argentina and Majungatholus from Madagascar are similar with the exception that Carnotaurus has two large horns on the skull.
The tetanurans, the most advanced theropods, included several groups where the relationships are not well understood. Crests and other decorations on the head were usually not present. Their hands had three or fewer fingers, and the "thumb" usually had the largest claw.
Family: Compsognathidae: The most primitive tetanuran was Compsognathus from the Late Jurassic of Europe. It was the smallest theropod, about three feet long and lightly built.
Family: Coeluridae: Ornitholestes and Coelurus, which lived during the Late Jurassic in western North America, were fast-running, lightly built theropods that were two to three feet tall at the hips and from six to ten feet long.
Family: Carcharodontosauridae: This group of giant theropods from Gondwana includes enormous predatory dinosaurs, Giganotosaurus from Argentina and Car-charodontosaurus from North Africa.
Family: Therizinosauridae: The therizinosaurids were apparently herbivorous or omnivorous theropods known from the Late Cretaceous of Asia and North America. The unusual, birdlike pelves and almost prosauropod-like skulls of therizinosaurids have resulted in uncertainty about their evolutionary position, but they have recently been shown to be theropods closely related to the Ornithomimidae.
Family: Spinosauridae: Spinosaurids are a distinctive group of theropods with long, crocodile-like snouts and elongated vertebral spines that may have formed sail-like structures on their backs. Spinosaurids are restricted to the Cretaceous but are known from Africa, South America, and Europe.
Family: Oviraptorisauridae: Another curious theropod from the Cretaceous of Asia is Oviraptor, which has a tall, highly pneumatic skull with a turtle-like beak. Oviraptor got its name ("egg predator") because specimens were found in Mongolia with what were originally thought to be nests of ceratopsian eggs.
Family: Allosauridae: This family is typical of the larger Jurassic and Early Cretaceous theropods that were from 15 to 35 feet long or longer. The biggest allosaurid may have been more than 40 feet long. Allosaurids were slender but dangerous predators.
Family: Tyrannosauridae: Most dramatic of all the theropods were the tyrannosaurids, which probably came from allosauridlike ancestors at the beginning of the Late Cretaceous. Unlike other tetanurans, they had massive bodies, unusually shaped heads, and small two-fingered hands.
Some tyrannosaurids were nearly 50 feet long and became the largest meat-eating land animals known. The smallest, such as Nanotyrannus, were about 18 feet long. The medium-size tyrannosaurids, such as Albertosaurus and Daspletosaurus, were about 30 feet long
Family: Ornithomimidae: Many kinds of small theropods also arose in the Cretaceous. Their skeletons were birdlike. Most had changes in their front legs and hands for a powerful striking action. The "quick-strike" motion of the front limbs may have been the beginning of the power stroke of birds' wings.
The "ostrich dinosaurs" or Ornithomimidae are known best from the Late Cretaceous of eastern Asia and western North America. They had small heads and they usually had no teeth. They had long necks and short, stiff backs. Their front limbs were long, and their powerful rear legs were built for speed. They are thought to have been the fastest dinosaurs.
Family: Dromaeosauridae: Deinonychus and other "sickle claw" theropods are among the best-studied dinosaurs. The discovery of Deinonychus supported the idea of a bird-dinosaur relationship and started the debate about dinosaurs being warm-blooded. Each foot had a large, sickle-shaped claw on the second toe. The end of the tail had vertebrae that locked together to make it stiff.
Family: Troodontidae: This group of "sickle claw" theropods had large brains and large eyes that faced forward. Some, such as Troodon and Saurornithoides, may have been almost as smart as some mammals.
The earliest ornithischian dinosaur was Pisanosaurus, a three-foot-long, two-legged (bipedal) plant-eater from the late Middle Triassic of Argentina. All ornithischians were plant-eaters.
Later ornithischians split into three advanced groups: heavy, armored plant-eaters that walked on all fours; specialized dome-headed dinosaurs and horned dinosaurs; and two-legged plant-eaters that included Iguanodon and duckbilled dinosaurs.
Family: Fabrosauridae: This family is found in Late Triassic through Late Jurassic rocks on several continents. Other primitive ornithischians are usually classified in this family. It was named after Fabrosaurus, from the Early Jurassic of South Africa. All fabrosaurids were small bipedal plant-eaters. The best-known fabrosaurid is Lesothosaurus, also from the Early Jurassic of southern Africa. The pelvis of Lesothosaurus shows some features that put it at the bottom of ornithischians. Scutellosaurus from the Early Jurassic of western North America was protected by small bony plates resembling the larger plates of later armored ornithischians.
Stegosaurs were the main armored dinosaurs of the Jurassic Period; ankylosaurs remained in the background. But ankylosaurs replaced stegosaurs in the Cretaceous Period. Scelidosaurus was very much like both groups. It was from the Early Jurassic of England and it walked on all four limbs (quadrupedal). It had stegosaurlike teeth, an ankylosaurlike pattern of armor plates and spines, and a pelvis like Scutellosaurus.
Family: Stegosauridae: These armored dinosaurs probably evolved in China during the Early Jurassic. By the Late Jurassic, they were in Europe (Dacentrurus), North America (Stegosaurus), and Africa (Kentrosaurus).
The armor was a double row of large bony plates that ran along the back from behind the head to the tail. Sharp spines on the end of the tail were used as a weapon against predators.
During the Early Cretaceous, ankylosaurs replaced stegosaurs everywhere except in India. Ankylosaurs were different from stegosaurs because they had flexible body armor rather than a double row of tall plates. They were also closer to the ground, with only a slight arch, if any, to their backs. The heads of stegosaurs were long and narrow, but ankylosaurs had short, board skulls. Instead of tail spines, ankylosaurs had shoulder spines or tail clubs for defense.
Family: Nodosauridae: The more primitive ankylosaurians, including all Jurassic and southern-hemisphere genera, belong in this family. Some nodosaurids had large, cone-shaped spines along the neck and shoulders for protection.
Family: Ankylosauridae: This family may have arisen during the Early Cretaceous from a nodosaurid ancestor. Ankylosaurid skulls had horns projecting from the back, giving them a triangular shape when viewed from above. All ankylosaurids had massive, bony tail clubs for defense.
Yaverlandia, from the Early Cretaceous of England, is the oldest known pachycephalosaur. But the only known specimen is a small, thick, skull-cap with two bony lumps that shows little about what the animal may have looked like or its relationships. The dome-headed dinosaurs were scarce, small to medium-size animals of the Cretaceous Period. Most lived in the northern hemisphere but one genus, Majungatholus, is known from the Late Cretaceous in Madagascar.
The back of the head of dome-headed dinosaurs was broadened into a shelf that often had bony lumps or short spikes. One family, the Homalocephalidae, had bones on the top of the skull that were flat and thick (as in Homalocephale and Goyocephale from Mongolia).
In the other family, the Pachycephalosauridae, the bones were raised into a very thick, high dome that was the main feature of the animal's appearance and even grew over the shelf (as in Stegoceras). Pachycephalosaur skeletons are rare, but their skull-domes, since they were solid bone, often were fossilized. They are quite common in some places.
Pachycephalosaurs had broad, chubby bodies. They were bipedal plant-eaters. As an animal grew, its dome got larger. The thick domes were used to compete for mates in head-butting contests between males or to fight predators by butting them in the side.
The Ceratopsia were the horned dinosaurs and their relatives. They were different from the pachycephalosaurs-and all other dinosaurs-because they had a special bone, the rostral, which formed part of a large, parrotlike beak.
Family: Psittacosauridae: The oldest and most primitive ceratopsians belong in this family of small, bipedal runners such as Psittacosaurus from the Early Cretaceous of China and Mongolia.
Family: Protoceratopsidae: In this family, the back of the skull was expanded into a wide frill over the back of the neck. Microceratops was a bipedal animal, like its possible psittacosaurid ancestors. The other protoceratopsids walked on all fours, making their larger heads easier to support.
Family: Ceratopsidae: The Ceratopsidae had the shortest range of any dinosaur family. They arose during the Late Cretaceous in western North America. They quickly evolved into many unusual forms and lived until the end of the Mesozoic Era. From cow to elephant size, the quadrupedal (four-legged) ceratopsids had horns and frills on their heads.
They had powerful jaws with hundreds of teeth for slicing tough plants. Triceratops had the most powerful jaw muscles of any land animal. The horns were used as weapons. The frills may have protected the neck and may have been brightly colored for mating season. The frills may also have helped keep its body temperature even.
Except an increase in size and the evolution of the most remarkable chewing arrangement of all dinosaurs, different ornithopod families varied from one another in minor details. All were bipedal plant-eaters.
Family: Heterodontosauridae: These small, nimble bipedal plant-eaters have been found mainly in Early Jurassic rocks of southern Africa.
Their teeth were sharp and tusklike in front, but the teeth at the sides of the jaw were built for chewing and slicing plants. They had large caninelike teeth (cone-shaped, pointed teeth) at the corners of the upper and lower jaws.
Family: Hypsilophodontidae: This was the most widespread and longest-lived ornithopod family. It flourished almost worldwide from the Middle Jurassic until the end of the Cretaceous.
Hypsilophodontids were small but had relatively large heads. Their feet were primitive with four functional toes. The first toe was smaller than the others; the fifth toe was only a splint. Hypsilophodontids had small front limbs with tiny hands. Bony tendons strengthened the back and stiffened the tail. Most were small dinosaurs about six to ten feet long, but some Tenontosaurus species were as long as 22 feet. The Late Cretaceous Thescelosaurus grew to about 18 feet long.
Family: Dryosauridae: This short-lived family arose about the same time as the Hypsilophodontidae. The earliest dryosaurid was Dryosaurus from the Late Jurassic of western North America and eastern Africa. Valdosaurus from the Early Cretaceous of Europe and northern Africa and Kangnasaurus from Africa are the other two genera in this family. Dryosaurids, with small front limbs and heads, were larger and more powerful than hypsilophodontids. They lacked teeth at the front of the snout and instead had a well-developed beak that may have had a horny covering.
Family: Camptosauridae: The Late Jurassic genus Camptosaurus from western North America was a chubby, medium-size ornithopod about 15 feet long. It had specialized feet and skull.
Family: Iguanodontidae: Iguanodon is one of the best-known dinosaurs. This bulky, 35-foot-long ornithopod had a deep, narrow skull; a strong, well-developed pelvis; rows of bony tendons running along its back; a hand in which the thumb had become a sharp spike; and three broad toes plus an inner toe, which was reduced to a splint. The teeth were thick and were always being replaced. It ate tough plants, Iguanodon walked on two legs, but it could also use its hands to walk on all fours.
Iguanodontids have been found in Early and Late Cretaceous formations in North America, northern Africa, Europe, and Asia. The most advanced were Probactrosaurus from central Asia and Ouranosaurus from northern Africa. The teeth of Probactrosaurus had a more complicated pattern of replacement than those of Iguanodon. The vertebrae of Ouranosaurus had very long spines, creating a sail. With the rise of duckbilled dinosaurs, iguanodontids faded away.
Family: Hadrosauridae: This group consists of two groups, the Hadrosaurinae and the Lambeosaurinae. They were both duckbilled dinosaurs that were closely related. They were large to very large plant-eaters of the Late Cretaceous in North and South America and Eurasia. The largest were Shantungosaurus from China and Lambeosaurus laticaudus from Mexico, both of which may have grown over 50 feet long and weighed over 20 tons. They may have been the largest known animals able to walk on two feet. Most duckbilled dinosaurs were about 30 feet long.
The teeth were in thirty to forty vertical rows like steps on a moving escalator. As each tooth wore away, it was replaced by the tooth directly below it. This process started when (or even before) the animal hatched and continued as long as it lived. The skulls of hadrosaurines were generally longer and not as deep as those of lambeosaurines, and their ducklike beaks were flatter and broader.
Some hadrosaurines lacked cranial crests (Edmontosaurus); others had arched nasal bones (Kritosaurus); others had solid cranial crests (Prosaurolophus and Maiasaura). But all lambeosaurines in which the skulls are well known (Hypacrosaurus, Corythosaurus, Lambeosaurus, and Parasaurolophus) had hollow crests with looping nasal passages that may have been used to make sounds.
The duckbilled dinosaurs were perhaps the most advanced of all the dinosaurs. They had excellent hearing, eyesight, voice, and sense of smell. They lived in huge herds and may have migrated seasonally, returning to the same place each year to mate and lay their eggs.