What is a Dinosaur?

A defining feature of dinosaurs was a hole in their hip socket. Other reptiles, such as lizards and crocodiles, have legs that sprawl out to the side, with thigh bones almost parallel to the ground so they walk and run with a side-to-side motion.

[[☃ image 3]]Dinosaurs, on the other hand, stood with their legs positioned directly under their bodies. A hole in their hip socket permitted this upright stance, which allowed dinosaurs to run faster and with greater endurance than other reptiles of the same size.

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Another trait that sets dinosaurs apart is that they walked on their tiptoes all the time. This condition is common in many mammals—some of which can be quite fast, such as cats and dogs—but is unique among reptiles.

A third important trait of dinosaurs is that, whereas primitive reptiles have five toes and fingers, the first dinosaurs used only three toes to stand on the ground (we call this condition a functionally tridactyl foot). The first dinosaurs did have five toes on each foot, but two of them were tiny and did not reach the ground. Something similar happened with their hands—the two outer digits (the ring and pinky fingers) were very small. In some dinosaurs, the two tiny fingers even disappeared completely.

During the Age of Dinosaurs, there were other reptiles living on land and in the seas, including pterosaurs (flying reptiles), and plesiosaurs (ocean-dwelling reptiles). But they did not have a hole in their hip socket and so were not dinosaurs.

Modern birds are classified as a type of dinosaur and share features such as the hole in the hip socket, the three-toed foot, and an s-shaped neck.

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Which came first, the bird or the bird brain?
What makes a dinosaur a dinosaur?

The group called dinosaurs includes all the animals that evolved from the very first dinosaur, the common ancestor. Because the fossil record is incomplete, we have not found fossils of this common ancestor; however, based on the characteristics we see in all dinosaurs, we can determine new and unique characteristics it possessed.

A key evolutionary innovation of dinosaurs is that they walk with a fully erect posture, holding their hind legs vertically under their hips. The top of the thighbone (or femur) has a knob or head that sticks out to the side of the rest of the bone and fits into the hip socket (or acetabulum) in the pelvis. The hip socket has a hole in the center, as well as a rim of bone along the upper margin, which helps support the weight of the body on the leg. Since these features are present in all known dinosaurs, we deduce that they first evolved in the common ancestor.

In popular culture, many other kinds of animals, which don't have this fully developed hip and leg structure, are often incorrectly called dinosaurs. Some examples include Mesozoic marine reptiles, such as plesiosaurs, mosasaurs and ichthyosaurs, as well as a group of Mesozoic flying reptiles called pterosaurs, which includes the pterodactyl.

[[☃ image 2]]Giving any living or extinct organism a scientific name is governed by a set of rules called the International Code of Zoological Nomenclature, and paleontologists who name dinosaurs must follow those rules to make the name valid. Although this process may seem unnecessary, it is extremely important in making sure that all paleontologists understand what kind of dinosaur is being referred to when a name is used.

The code requires that a scientific name be composed to two parts. The first part, called the genus, is always capitalized; the second, called the species is never capitalized. Both names are always italicized, and sometimes the genus name is abbreviated (as in T. rex for Tyrannosaurus rex). The genus name may be used alone to refer to all the species in a particular genus.

Key characteristics

The first person to describe a new kind of dinosaur assigns the animal its name, and these names are often derived from Greek or Latin words. Many names reflect key characteristics of the animal, such as the place the specimen was collected, the person who collected the fossil, or one of the animal's unusual anatomical characteristics.

The new name is based on one particular fossil, which is called the type specimen, and this type specimen must be complete enough to exhibit features that clearly distinguish it from all other species. Any specimens discovered later that are thought to belong to the new species must be found to have no significant differences with the type specimen.

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Dinosaur names

The first fossil of a primitive bird ever discovered was _Archaeopteryx_, a famous Jurassic species known from beautifully preserved skeletons with feather impressions found in Germany. _Archaeopteryx_ was discovered in the late 1800s, and its affinities with both birds and dinosaurs were recognized even at that time. The long, feathered forelimbs showed the presence of wings and flying capabilities, an unquestionable link with birds. And just like modern birds, it also had wishbones, breastbones, a crescent-shaped wrist bone, and feet with three main toes. However, a close inspection of the animal’s skeleton showed features that are common in theropods (a type of carnivorous dinosaur), that modern birds lack, such as a long snout with teeth, three well-developed fingers with claws, and a long tail formed by many vertebrae. Some specimens later showed that _Archaeopteryx_ also had a sickle-shaped claw on its feet, like those of the therapod groups troodontids and dromaeosaurids. For a long time, _Archaeopteryx_ was the only known Jurassic flying avialan (dinosaur in the group that includes birds), but now many species found on different continents show that primitive birds were common and coexisted with other dinosaurs for a long time.

[[☃ image 2]]_Archaeopteryx_ is the most primitive known bird and therefore most primitive known avian dinosaur.

Moving up the genealogic tree from _Archaeopteryx_ to modern birds, the next common bird is _Confuciusornis_. There are nearly a thousand specimens of this early bird, which unlike _Archaeopteryx_, lacks a long tail and has no teeth. Nevertheless, it still has three separate bones in its hand. Some _Confuciusornis_ specimens preserve long tail feathers, like those of a bird of paradise. These feathers may represent the breeding plumage of males.

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Birds continued their evolution and eventually acquired the features we see in modern birds: Teeth disappeared and beaks developed; the tail was reduced to a single small bone we call a pygostyle; and the primitive three-fingered hand was modified so that the claws disappeared, one of the digits was reduced, and the two other fused together (check the bones in your chicken wings!). One thing became crystal clear over the last few decades: All modern birds descend from dinosaur ancestors. In the same way that humans descend from mammals and more specifically from primates, modern birds descend from dinosaurs and more specifically from avialans. Humans are distinct and unique primates, but they still are primates, and they still are mammals. The same logic applies to birds—they are distinct dinosaurs.

[[☃ image 2]]Although most of the news stories one reads about dinosaurs involve spectacular, if not speculative, aspects of dinosaur history, such as new theories about their behavior, extinction, or ecology, less attention is paid to studies analyzing their evolutionary relationships. This is unfortunate because understanding questions of dinosaur behavior and biology depends on a firm understanding of their evolutionary relationships.

Cladogram

To reconstruct the evolutionary relationships between different groups of dinosaurs, one must carefully study and analyze the anatomy, essentially searching for characteristics that are shared among different kinds of dinosaurs, which are then interpreted to have been present in the common ancestor, or first member of a group, and inherited by its descendants. This analysis produces a branching diagram, called a cladogram, on which different kinds of dinosaurs are shown at the endpoints of the branches. The branching points connecting the different branches represent the common ancestor for the group whose branches come out of that branching point, and based on the characteristics shared by dinosaurs on those branches, we can infer that those shared characteristics were present in the common ancestor.

The result is a pattern of smaller groups of dinosaurs contained within larger groups of dinosaurs. For example, T. rex is a member of a small group (Tyrannosauridae) that also contains Albertosaurus. Both those species are members of a larger group, called theropods, which contains other meat-eating dinosaurs such as Allosaurus, Velociraptor and birds, called Theropoda. Then, all theropods belong to a larger group that also contains the giant sauropods, called Saurischia.

A good way to think about this is to compare it to your evolutionary relationships as a human. Humans, along with apes, belong to a larger group of mammals called hominoids, and hominoids along with monkeys, lemurs and tarsiers all belong to the larger group called primates. Or to think about this process in reverse, by following the sequence of characteristics inherited from an earlier common ancestor to a more recent one, one can trace the anatomical evolution of a particular kind of animal.

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Beyond t-rex, a cladogram (American Museum of Natural History)
Understanding cladistics

Grab your notebook or computer and write out your answer to each of the following questions. Then compare your answer to the suggested answer found in the “Answers to exploration questions” article. Have fun! (Hint: the information needed to answer these questions can be found in the video and the articles.)

1. All dinosaurs evolved from the very first dinosaur, the common ancestor. What do scientists know about this common ancestor?

2\. _Archaeopteryx_ was the most primitive known bird. How was _Archaeopteryx_ similar to modern birds? How was it different from modern birds, and more like advanced non-avian dinosaurs?

3\. Explain how a cladogram works.

If you are wondering where the suggested answers came from, you can review the videos and articles in this tutorial.

1. All dinosaurs evolved from the very first dinosaur, the common ancestor. What do scientists know about this common ancestor?

**Answer:** Scientists haven’t found fossils of the common ancestor. However, they can determine its characteristics based on traits that are shared by all dinosaurs that were new and unique to this group of animals.

2. _Archaeopteryx_ was the most primitive known bird. How was _Archaeopteryx_ similar to modern birds? How was it different from modern birds, and more like advanced non-avian dinosaurs?

**Answer:** Like modern birds, _Archaeopteryx_ had feathers, wishbones, breastbones, a crescent-shaped wrist bone, and feet with three main toes. But it also shared some features with advanced therapods that are unlike modern birds, such as a long bony tail and teeth.

3\. Explain how a cladogram works.

**Answer:** A cladogram is a branching diagram that shows evolutionary relationships between different groups of organisms. Organisms with like features are grouped together. Every branching point represents a physical feature that’s unique to that group.