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The Dinosauria

Diplodocus family by Michael Skrepnick
Diplodocus carnegii
Dinosaurs, one of the most successful groups of animals (in terms of longevity) that have ever lived, evolved into many diverse sizes and shapes, with many equally diverse modes of living. The term "Dinosauria" was invented by Sir Richard Owen in 1842 to describe these "fearfully great reptiles," specifically Megalosaurus, Iguanodon, and Hylaeosaurus, the only three dinosaurs known at the time. The creatures that we normally think of as dinosaurs lived during the Mesozoic Era, from late in the Triassic period (about 225 million years ago) until the end of the Cretaceous (about 65 million years ago). But we now know that they actually live on today as the birds.
Some things to keep in mind about dinosaurs

Not everything big and dead is a dinosaur
All too often, books written (or movies made) for a popular audience include animals such as mammoths, mastodons, pterosaurs, plesiosaurs, ichthyosaurs, and the sail-backed Dimetrodon. Dinosaurs are a specific subgroup of the archosaurs, a group that also includes crocodiles, pterosaurs, and birds. although pterosaurs are close relations, they are not true dinosaurs. Even more distantly related to dinosaurs are the marine reptiles, which include the plesiosaurs and ichthyosaurs. Mammoths and mastodons are mammals and did not appear until many millions of years after the close of the Cretaceous period. Dimetrodon is neither a reptile nor a mammal, but a basal synapsid, i.e., an early relative of the ancestors of mammals.

Not all dinosaurs lived at the same time
Different dinosaurs lived at different times. Despite the portrayals in movies like King Kong and Jurassic Park, no Stegosaurus ever saw a Tyrannosaurus, because Tyrannosaurus didn't appear on the scene until 80 or so million years following the extinction of stegosaurs. The same goes for Apatosaurus ("Brontosaurus") — it's bones were already well-fossilized by the time T. rex came along.

Dinosaurs are not extinct
Technically. Based on features of the skeleton, most people studying dinosaurs consider birds to be dinosaurs. This shocking realization makes even the smallest hummingbird a legitimate dinosaur. So rather than refer to "dinosaurs" and birds as discrete, separate groups, it is best to refer to the traditional, extinct animals as "non-avian dinosaurs" and birds as, well, birds, or "avian dinosaurs." It is incorrect to say that dinosaurs are extinct, because they have left living descendants in the form of cockatoos, cassowaries, and their pals — just like modern vertebrates are still vertebrates even though their Cambrian ancestors are long extinct.

Find answers to some commonly asked questions about dinosaurs

A worker at Dinosaur National Monument excavates bones
A worker at Dinosaur National Monument in Utah pauses during an excavation of one of the many dinosaur bones preserved there.

Visit these special exhibits on some popular dinosaurs

Fossil record
Dinosaur fossils had been known for centuries as "dragon bones" or the remains of giants, but it wasn't until Dean William Buckland of Oxford, England described the carnivorous "lizard" Megalosaurus (in 1824) that they were formally studied as an extinct group of giant reptiles. The English country doctor Gideon Mantell described Iguanodon in 1825, and today there are more than 800 known types of non-avian dinosaurs (plus some 10,000-plus birds).

The term "Dinosauria" was invented by Sir Richard Owen in 1842 to describe these "fearfully great reptiles." The irony of Owen's invention of Dinosauria is that he devised the taxonomic group as an argument against progressive evolution, but actually had presented evidence supporting evolution.

Deinonychus head
An artist's reconstruction of the head of Deinonychus. In this interpretation, the head is featherless.
Although dinosaur remains had been found earlier elsewhere, it was the discoveries of dinosaurs in North America in the second half of the 1800s that provided the first real glimpse of what these animals were like, and gave paleontologists some clues about the past diversity of life on Earth. The late 1800s were the "golden age" of dinosaur paleontology, when many animals that you might be familiar with were discovered and named. Today we seem to be in another "dinosaur renaissance," with new information accumulating rapidly.

The first dinosaurs, in the Late Triassic period (about 230 million years ago), apparently were not major components of the fauna. However, by the Early Jurassic (about 30 million years later), after many other Triassic vertebrates had gone extinct, dinosaurs were diversifying rapidly. By then they had become dominant occupants of many major terrestrial adaptive zones, judging from their frequently large size and considerable morphological and taxonomic diversity. The Late Jurassic (about 145 million years ago) through the Late Cretaceous (about 65 million years ago) was the heyday of the dinosaurs. It was also the Late Jurassic that saw the bird lineage diverge from its flightless theropod ancestors, and birds enjoyed an explosion in diversity in the Cretaceous period and beyond.

Life history & ecology
If the diverse and numerous dinosaurs (except birds) are extinct, how can we better understand how they lived? Even though the great dinosaurs of the Mesozoic are gone, they have left us many clues. Dinosaur fossils are not limited to bones, but include skin, eggs, nests, footprints, and other special kinds of fossils that give us clues about their lifestyles.

Dinosaur nests
The news has recently carried several stories of wonderful finds of nesting dinosaurs, and it is true that an explosion of data on dinosaurian nesting and social behavior has been uncovered in the past 20 years. Some of the most well known and compelling evidence comes from Jack Horner's (Museum of the Rockies) work at the "Egg Mountain" site in Montana, where he has documented evidence of a large nesting area used by hadrosaurian (duckbilled) dinosaurs. These dinosaurs were named Maiasaura, "good mother reptile," referring to the closely packed nests that contain fossilized eggs, embryos, and juveniles (such as the one pictured below right). This is one case where we can be fairly confident that parental care was involved in these dinosaurs' lifestyle. Actually, this is not a surprising assertion, because both crocodilians (their closest living relatives) and birds (their living descendants), both show some degree of parental care and extensive nest building.

Other dramatic finds of dinosaur nests include theropod dinosaurs (Oviraptor and Troodon) that apparently died while brooding their nests, and abundant nests of the early ceratopsian dinosaur Protoceratops. An interesting story about Oviraptor: the so-called "egg stealer" was so named because it was found atop a clutch of eggs that were assumed to belong to Protoceratops. This idea held for some 70 years until a find in the 1990s showed an Oviraptor embryo inside one of those eggs … "egg stealer" exonerated!

Dinosaur footprint and baby Maiasaura

Dinosaur footprints
We know of literally thousands of non-avian dinosaur footprints scattered around the globe, from Late Triassic to Late Cretaceous age. You might not think that a footprint or a sequence of footprints (called a trackway) could tell us much, but actually it can tell us some general things about the biology of dinosaurs.

From trackway data, we can tell that:

  1. Some non-avian dinosaurs travelled in large groups;
  2. Non-avian dinosaurs moved with their feet held underneath their body (as birds and mammals do); and
  3. Some non-avian dinosaurs moved rather quickly, but some plodded along at a more leisurely pace — see our section on dinosaur speeds for more info.

Dinosaur diet
Dinosaurs, living and extinct, have varied diets. We have some strong evidence of exactly what the diets of some of the extinct dinosaurs was, and we can observe birds directly to learn about their diets. Dentition (tooth structure) is one of the most abundant lines of evidence useful for determining dinosaur diets. Most ornithischian and sauropodomorph dinosaurs had rather simple, short stubby crenellated teeth, which are similar to those of living herbivores, and clearly not too good for eating much meat.

Sauropods feeding

Theropod teeth, on the other hand, retain the primitive archosaurian characteristic of being recurved, serrated, laterally-compressed, and knife-like. There is some variation in tooth structure among extinct theropods, but most are fairly similar and obviously related to a carnivorous diet.

Stomach contents are another line of evidence, somewhat more direct but also a bit trickier to interpret accurately. Well-preserved dinosaur skeletons sometimes have traces of apparent food items preserved in their abdominal cavity, where it's safe to assume that they had a stomach. This includes pine cones and/or needles in some herbivores' guts, and traces of some vertebrates in some theropods' guts. So this independent line of inquiry substantiates the data from tooth morphology. Also, some sauropodomorph stomachs contain well-rounded stones, called gastroliths, that were probably used to grind food in a muscular crop or gizzard, like some birds (and crocodilians) do.

The general hypothesis that most ornithischians and sauropodomorphs were largely, if not completely herbivorous, and that theropods (at least before the origin of birds) were mostly carnivorous, thus holds. More specific hypotheses have been proposed and supported by data, while others have fallen by the wayside. It is likely that new discoveries will illuminate more about dinosaur diets as the global "dinosaur renaissance" continues.

You can learn more about the diets of sauropods from our page on that subject.

Move deeper into the systematics of dinosaur groups by selecting one of the links below.

Ornithischia Saurischia Heterodontosauridae Thyreophora Ornithopoda Marginocephalia Herrerasaurus Eoraptor Sauropoda Theropoda

With the discovery of many new species since the 1840s, the Dinosauria now contains two major groups of dinosaurs: the Ornithischia, or "bird-hipped" dinosaurs, and the Saurischia, or "lizard-hipped" dinosaurs. The division between the two groups was made by H.G. Seeley in 1888. The etymology behind the two names ("bird-hipped" vs. "lizard-hipped") is not very accurate, since some saurischians had bird-like hips, and ornithischians' hips were somewhat birdlike due to convergent evolution, not due to direct ancestry. In fact, birds are saurischians!

Ornithischia contains several groups of herbivorous dinosaurs, including several basal groups, but primarily three large ones:

Saurischia contains three main groups:

Herrerasaurus is a basal saurischian whose relationships remain controversial. It was discovered in a wonderful middle-late Triassic period fossil locality, the famous Ischigualasto Formation of Argentina, in the 1970s. Another herrerasaur-like dinosaur is Staurikosaurus, which has been known since the 1960s from remains found in Brazil. In 1993, another early saurischian was found in the same general area and named Eoraptor, or "dawn thief." It appears to be closely related to the herrerasaurids, but smaller in size and slightly older. Both Eoraptor and the herrerasaurids seem to have been small to medium-sized carnivores. These curious animals have some basic theropod characteristics, but lack others; in fact, they lack some dinosaurian characteristics as well. Their exact relationships continue to be controversial.

More on morphology
What is the scientific diagnosis of what is a dinosaur, and what is just another archosaur? Several skeletal characteristics are currently used as diagnostic dinosaurian features. You may also view a large-screen picture of a dinosaur skeleton for a lesson in anatomy.

Some basic dinosaurian modifications to the ancestral archosaurian skeleton: Reduced fourth and fifth digits on the manus (hand); pes (foot) reduced to three main toes; three or more vertebrae composing the sacrum (region of the vertebral column which attaches to the pelvis); and an open acetabulum (hip socket; see below). Some of these features were modified during the evolution of later groups, but these features are considered to be synapomorphies, or shared derived features, for the Dinosauria; the first dinosaurs had these features, and passed them on to their descendants.

The dinosaur hip
One important dinosaurian synapomorphy is the perforate acetabulum, simply a "hip bone" (actually three connected bones, together called the pelvis) with a hole in the center where the head of the femur ("thigh bone") sits. This construction of the hip joint makes an erect stance (hindlimbs located directly beneath the body) necessary — like most mammals, but unlike other reptiles which have a less erect and more sprawling posture. Dinosaurs are unique among all tetrapods in having this perforate acetabulum.

Dinosaur hips

A more in-depth classification of dinosaurs uses modifications to this basic hip structure, and many other characters, to help differentiate between separate groups of dinosaurs, and hence reveal to us dinosaurian diversity.

The Dinosauria contains two major groups of dinosaurs: the Ornithischia, or "bird-hipped" dinosaurs, and the Saurischia, or "lizard-hipped" dinosaurs. The most prominent visible difference between the two types of hip is the orientation of the pubis, shown in white in the picture above. In saurischian dinosaurs, this bone points toward the front of the animal, and flares into a keel at the forward end. Ornithischians have a reversed pubis, which points toward the tail and lies alongside and parallel to the ischium. Additionally, some ornithischians have a projection at the forward end of the pubis.

The first recognition of the two groups was made by H.G. Seeley in 1888. The etymology behind the two names ("bird-hipped" vs. "lizard-hipped") is rather confusing, since some saurischians had bird-like hips, and ornithischians' hips were somewhat birdlike due to convergent evolution, not due to shared ancestry. Birds are apparently descended from saurischian dinosaurs, but have a reversed pubis like ornithischians do. Some close relatives of birds within saurischians have this same feature, too, so the ornithischian-saurischian dichotomy is not so simple. The names "Ornithischia" and "Saurischia" are used to refer to the common ancestry of their respective members. The names don't necessarily have to mean anything. They are just names, that's all.

Sources and suggested further reading:

Original page created by Tony Fiorillo prior to 11/94, with later modifications by Allen Collins, Rob Guralnick, Ben Waggoner, John Hutchinson, and Brian Speer; modifications since 2005 by David Smith; systematics update by Randy Irmis, May 2008. Diplodocus carnegii © 1998 Michael Skrepnick. Deinonychus head by Jan Pethick; photo by Dave Smith, UCMP. Baby Maiasaura and dinosaur track © UCMP.