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The Classification of Dinosaurs

Learn the two orders of dinosaurs, Saurischia and Ornithischia.

By John Ostrom
  • Grades: 3–5, 6–8

Dinosaur (Greek for "terrible lizard") is the term given to various kinds of extinct reptiles of the Mesozoic Era, from 230 to 65 million years ago. During the era these often (but not always) very large reptiles were the dominant land animals on Earth. The term was proposed as a formal zoologic name in 1842 by the British anatomist Sir Richard Owen, in reference to large fossil bones unearthed in southern England, but it is used today only in an informal sense. The animals are classified in two formal categories, the orders Saurischia and Ornithischia, within the reptile subclass Archosauria.

Thousands of dinosaur remains have been found nearly worldwide. The earliest discoveries were for many years attributed to two Englishmen. The first, Gideon Mantell, a doctor, made his find in Sussex, England, in 1822 and called it Iguanodon ("iguana tooth"). The second, the Rev. Willam Buckland, made his find near Oxford, England, and called it Megalosaurus ("great lizard"). In fact, fossil remains had already been discovered in 1818 at Windsor, Conn., by Solomon Ellsworth, Jr. The remains, reported as human by Nathan Smith in the 1820 issue of the American Journal of Sciences and Arts, were only recently recognized as pertaining to the Late Triassic/Early Jurassic prosauropod Anchisaurus.

Dinosaurs varied greatly in form and size and were adapted for diverse habitats and lifestyles. They ranged in weight from 2 to 3 kg (4 to 6 lb), in the case of Compsognathus, up to 73 metric tons (160,000 lb), in the case of Brachiosaurus. Most known dinosaurs were large, weighing more than 500 kg (1,100 lb), and few weighed less than 45 kg (100 lb). Most were herbivores, but saurischians such as the theropods were carnivorous. Most dinosaurs were also four-legged, but some were obligatory bipeds, unable to adopt a four-legged posture. Others were facultative bipeds, able to walk occasionally on their hind legs.

Dinosaurs have always been classified as reptiles, because they all have anatomical features found in living reptiles and not in mammals or amphibians. For that reason they traditionally have been assumed to have been like reptiles in physiology, as well: that is, cold-blooded (ectothermic), or dependent on external heat sources. In recent years, however, different lines of evidence have been interpreted as indicating that dinosaurs may have been warm-blooded, with rates of metabolism comparable to those of mammals and birds. Such evidence includes their upright posture and carriage, mammallike microscopic structure of bones, skeletal features suggestive of high activity, specialized food-processing dentitions, and the low ratios of dinosaurian predators to prey animals, suggesting large food requirements. In addition, what appears to be a fossilized heart was found in the chest cavity of a dinosaur fossil from the late Cretaceous, and computerized imaging of the organ in 2000 suggested that it had a birdlike or even mammalian structure with four chambers. All such evidence requires further study and is not conclusive, however, and all the facts can be alternatively explained. The reasonable possibility remains that some dinosaurs may have been endothermic, maintaining body temperature and heat production by means of internal metabolic processes.

Evidence indicates that dinosaurs, like most modern reptiles, reproduced by laying eggs. Fossil eggs once attributed to one of the horned dinosaurs ( Protoceratops) are now known to belong to the oviraptorids and dromaeosaurids, because of identifiable embryo contents. Fossil eggs attributed to the sauropod genus Hypselosaurus have been discovered in Mongolia and France, and fragments presumed to be of dinosaur eggs have also been found in Brazil, Portugal, Tanzania, and the U.S. states Colorado, Montana, and Utah. Fossils of unhatched dinosaur eggs have been discovered in Montana and Utah, in Alberta (Canada), and in the Gobi Desert. A few scientists suggest that some dinosaurs, such as sauropods, may have given birth to live young, but there is as yet no conclusive evidence. To the contrary, paleontologists in Argentina in 1997 discovered embryo-containing sauropod eggs for the first time. The embryos - the first ones found in the Southern Hemisphere - were also remarkable because their mineralized skins were sufficiently well preserved to show scale patterns.

The two orders of dinosaurs are distinguished by numerous features, the most diagnostic being the arrangement of the three bones of the pelvis. In saurischians these bones were arranged in a triradiate pattern similar to that of modern crocodilians and lizards — hence the term Saurischia, which means "lizard hip." The ornithischian pelvis was usually rectangular or tetraradiate in outline, with the pubic bone projecting down and back — hence the term, which means "bird hip."

Saurischia

The order Saurischia includes two different kinds of dinosaurs: the carnivores, suborder Theropoda; and the huge herbivores and their ancestors, suborder Sauropodomorpha. Aside from the saurischian pelvic arrangement, the two groups had few anatomical features in common.

Theropods. The theropods traditionally have been subdivided into two categories, the small, lightly built coelurosaurs and the large, heavily built carnosaurs. Some evidence indicates that this division may not accurately represent evolutionary relationships among the theropods, and some scientists prefer to divide them into primitive (infraorder Ceratosauria) and advanced (Tetanurae) categories. The former, including small and medium to large animals such as Coelophysis, Syntarsus, and Ceratosaurus, ranged from Late Triassic to Late Jurassic time. The Tetanurae included all other theropods. Ranging from very small (Compsognathus) to very large ( Tyrannosaurus), they were dispersed throughout Mesozoic time. Some coelurosaurian types with more birdlike features, such as Ornitholestes, are placed together in the subcategory Maniraptora. (It is from a form of maniraptoran that a number of paleontologists - though not all - believe evidence indicates that birds evolved. Some paleontologists would even say that birds can be classified directly as dinosaurs, not just as dinosaur descendants. In any case, the relationship between dinosaurs and birds is complex.)

Theropods were obligatory bipeds, unable to assume a four-legged stance. The hind legs were strong and bore birdlike feet. In some the hind-limb proportions indicate high running speed. The forelimbs bore sharp, curved claws for seizing and holding prey, and the long tails functioned as dynamic stabilizers or counterbalances. The head was relatively to disproportionately large, and the jaws usually contained many bladelike teeth with serrated edges front and back - clear evidence of flesh-eating habits. The "ostrich" dinosaur (Struthiomimus) had toothless jaws that probably were covered with a horny birdlike beak. A few scientists have interpreted this as indicating that these theropods may not have been carnivorous, but this is an unlikely thesis.

The smallest known theropod is Compsognathus (Late Jurassic, Europe), and the largest is Tyrannosaurus (Late Cretaceous, North America), which grew to a weight of 5 or 6 metric tons (about 12,000 lb) - several thousand times heavier than Compsognathus - a height of 5 m (16 ft), and a length of 15 m (49 ft). Others include Deinonychus, its cousin Velociraptor, and Baryonyx. Theropods first appeared in mid-Triassic times and thrived until the end of the Cretaceous, having apparently dispersed worldwide.

Sauropods. The suborder Sauropodomorpha comprises two subcategories: the Sauropoda, nearly all of which resembled the best-known member of their group, Apatosaurus, and the Prosauropoda, their more primitive and less well-known earlier relatives.

Prosauropods, common during the Late Triassic and Early Jurassic, have been found on all continents except Antarctica and Australia. Except for certain resemblances in skull and teeth, they were quite different from the sauropods and much smaller, ranging from a few hundred to perhaps 1,500 kg (3,300 lb). All were quadrupedal, but many were capable of bipedal posture and gait. They all appear to have been herbivores, although carnivorelike teeth have been found associated with skeletal remains of some kinds. Best known of the prosauropods is Plateosaurus (Late Triassic, Europe).

Once considered semiaquatic or amphibious, the sauropods include the largest of the dinosaurs. The hypothesis that these animals must have been swamp or lake dwellers came from the belief that their limb bones were not strong enough to support their great weight without the buoying effect of water. Their long and flexible necks were supposed to have enabled their heads to reach the water surface for air. However, sauropod tracks - first recognized as such in Texas in the 1930s - amply demonstrate that the animals did travel on land and were not confined to water-covered environments. Indeed, water pressure even at shallow depths is greater than the atmospheric pressure that would have enabled their lungs to expand. The long necks instead probably enabled them to browse on high trees.

The smallest varieties of sauropods were larger than elephants, and the largest - Brachiosaurus (Late Jurassic, East Africa) and several new, unnamed even larger discoveries - are estimated to have weighed 73 metric tons (about 160,000 pounds) or more. Best known are Apatosaurus and Diplodocus from the Late Jurassic of North America. All sauropods had the same basic structure: large, barrellike body; sturdy, columnar legs; long, heavy tails that probably were carried well off the ground; and a relatively small head at the end of a long, flexible neck. The sauropods were dominant herbivores during the Jurassic but less dominant in the Cretaceous.

Ornithischia

The Ornithischia are divided into four or five suborders, or infraorders, placed within two major categories: the Cerapoda (Ornithopoda; Pachycephalosauria, included within the ornithopod suborder by some authorities; and Ceratopsia), and Thyreophora (Stegosauria and Ankylosauria). In addition to the birdlike pelvis common to all ornithischians, all except a few very primitive kinds had a toothless front part of the mouth, which was covered by a horny beak or bill.

The ornithopods were the most successful and diverse of the ornithischians, having a worldwide distribution throughout their Mesozoic occurrence. The earliest yet known are Middle and Late Triassic in age (Pisanosaurus, South America; Heterodontosaurus, Africa). At the close of the Cretaceous, 140 million years later, ornithopods were the most abundant of the herbivores, replacing the sauropods as the dominant plant eaters among the dinosaurs. All were capable of a bipedal stance and gait, and some apparently were quite adept bipeds. All were also able to assume a four-legged stance, which was probably their preferred feeding posture. The most important feature of the ornithopods was the specialized dentition many had for crushing and grinding up their plant food, whatever that may have been.

A number of ornithopods, however, instead had a highly effective method of tooth replacement to offset their rapid rate of tooth wear. This dental specialization was most highly developed in the Late Cretaceous duckbilled dinosaurs, among which were Corythosaurus and Anatosaurus. It was this special dental design of the duckbills (and also the ceratopsians) that first led some paleontologists to speculate about dinosaurian food requirements and possible metabolism. Their grinding dental batteries, consisting of hundreds of tightly compacted teeth, are evidence of a high-volume intake of food, probably related to a high metabolism. Aside from Camptosaurus (Late Jurassic, North America) and Iguanodon (Early Cretaceous, Europe), the duckbills of Asia and North America are best known.

The horned dinosaurs (Ceratopsia) were the last of the major kinds of dinosaurs to appear. Ceratopsians also featured specialized dental batteries for processing large quantities of plant food. The teeth were arranged for slicing rather than for grinding the food, which was perhaps highly fibrous. The earliest-known ceratopsian, Protoceratops, a low-slung, calfsize animal, is known only from the Upper Cretaceous of the Gobi Desert, Mongolia. It probably descended from an earlier Mongolian ornithopod dinosaur resembling the bipedal Psittacosaurus. Although all other horned dinosaurs arose from that Asiatic beginning, they have been found only in North America. Animals with large head and neck shields and horns evolved there, culminating in Triceratops and Torosaurus - two of the last surviving dinosaurs.

The plated dinosaurs (Stegosauria, of the suborder Thyreophora) were never as important as the cerapods. The few fossils found have been from Jurassic rocks, mostly from North America, East Africa, and Europe. Like all other ornithischians, stegosaurs were herbivores, but with a small head and few small teeth that seem strangely designed to feed the animal's large body. There is no evidence of the dental specialization seen in other ornithischian kinds. The distinctive feature of Stegosaurus was the row of alternating upright bony plates that developed along the back. Long thought to have been protection in some way against predators, these plates were recently shown to have been highly vascularized and probably heavily suffused with blood during life. This has led to the suggestion that they may have been cooling fins, functioning as temperature regulators.

The other thyreophorans, the Ankylosauria, succeeded the stegosaurs in the Cretaceous, apparently filling that vacated niche. To varying degrees all ankylosaurs were encased in a mosaic-work shield of dermal bony plates and spikes that covered the back and flanks. All were relatively low, broad animals with short, pillarlike legs. The large body indicates a herbivorous diet, but the small, weakly developed teeth seem inconsistent with the necessarily large volume of food intake. Palaeoscincus and Ankylosaurus (Late Cretaceous, North America) are the best-known examples.

  • Subjects:
    Archaeology, Dinosaurs, Paleontology and Fossils
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