Dinosaurs ruled the world during the Mesozoic Era, which is divided into three Periods. During the Triassic Period, dinosaur ancestors were evolving. In the Late Triassic, the world saw the first true dinosaurs. In the Jurassic Period, the number of dinosaurs grew. By the Cretaceous Period, many different types of dinosaurs had evolved.
Paleontologists study the different kinds of dinosaurs to find their relationships to other dinosaurs and to find the ancestors of the dinosaurs. This can tell scientists much about evolution. It can also tell them some things about the world the dinosaurs lived in. Dinosaurs that could migrate were similar, but those dinosaurs that were isolated evolved differently. Dinosaurs and other organisms have historically been placed into hierarchical categories, using a system of classification called the Linnaean system. Today, research on dinosaur relationships uses an approach called cladistics, which uses the presence of shared morphological features to recreate the branching tree of dinosaur evolution.
Because scientists have incomplete information for dinosaurs, groupings may change when new information is found. So each new dinosaur fossil that is discovered could be a key that unlocks some of the information about dinosaur evolution and ancestry.
The Linnaean System of Classification
In the 1750s, Swedish botanist Carl von Linné (who is known by the Latin form of his name, Linnaeus) developed a system to classify all living things. Each living thing has two scientific names, a genus and a species name. The scientist who first describes a new organism (any living thing) names it. Since Linnaeus began using this system, over a million species have been named.
A scientific name is given in the Linnaean System because living organisms are called different things in different languages. The house cat, for example, is die Katze in German; le chat in French; but English, French, and German biologists call it Felis catus.
In choosing a name, a scientist may highlight an interesting feature of the organism or may name it in honor of a person or the place it was found. So, the enormous meat-eating dinosaur Tyrannosaurus rex is the "tyrant lizard king" that reigned over other dinosaurs.
In the Linnaean System, similar species are grouped into a genus, similar genera into a family, similar families into an order, similar orders into a class, similar classes into a phylum, and similar phyla into a kingdom.
Dogs, coyotes, and wolves are in the genus Canis. Vulpes, the fox genus, and Canis are both in the dog family Canidae. Canidae and Ursidae, the bear family, are part of the order Carnivora (meat-eating animals). Carnivores and people are in the class Mammalia (all mammals). Mammals and fishes are in the phylum Chordata (animals with backbones, or chordates). Chordates and corals are members of the kingdom Animalia (animals). These categories are known as taxa (singular: taxon), and the study of these classifications is called taxonomy.
Using Cladistics to Analyze Evolutionary Relationships
One of the most active areas of dinosaur research in the last two decades has been a thorough reconsideration of their relationships using a new approach called cladistics, which is also known as phylogenetic systematics. Unlike the Linnean system, which puts organisms into hierarchical categories, cladistics attempts to determine the many speciation events that resulted in the separation by branching of all organisms, living and extinct. In simpler terms, cladistics is a method of analyzing the evolutionary relationships between groups to construct their family tree.
Cladistics groups organisms on the basis of shared derived characters (synapomorphies) and uses a philosophical concept called parsimony, which holds that the simplest branching pattern (the one with the fewest steps) is most likely close to the true one. Scientists using cladistics do not place organisms into nested categories like the Linnean system, because they assume that each branch occurs by the same simple process of speciation. However, the various Linnean categories are still widely used for placement of groups into categories, as is done in this book.
Classification and Evolution
After Charles Darwin published his theory of evolution, biologists began to understand why organisms fall into natural groupings. For example, species in the dog genus Canis look like one another because they all had a common ancestor. Foxes (genus Vulpes) and dogs (genus Canis) do not look as much alike because their common ancestor was farther back in time. The farther back a common ancestor lived, the longer its descendants have had to evolve and change.
It is almost impossible to prove that two species share a common ancestor. But by making an extensive list of characteristics, scientists can show how likely it is that two species are related. The more traits two species share, the more likely they are closely related and got those traits from a shared ancestor.
For example, both sparrows and bats have arms and hands that are wings, but sparrow wings and bat wings are much different. Sparrow wings and bat wings evolved separately, and not because of a common ancestor. This is called convergent evolution. On the other hand, the wings of sparrows, eagles, ostriches, and all other birds are alike. This shows that today's bird species are closely related and came from a common ancestor.
Dinosaurs as Archosaurs
Only a few dinosaurs are known from complete or nearly complete skeletons; almost half of the known species are based only on teeth or bone fragments. The shapes of bones are used for dinosaur classification. Only the hundred or so dinosaurs for which good remains are known can be studied for relationships.
Bones are rarely fossilized. Living things usually decay and vanish after death. It is difficult for paleontologists to describe an incomplete fossil skeleton and to decide what the animal looked and acted like from just a few fossilized remains. The discovery of a new dinosaur-or new fossils of a poorly known dinosaur-may change the family tree. We will never know all the different dinosaur groups that lived, so their family tree will always be incomplete.
Dinosaurs are classified as reptiles, but all reptiles do not form a single clade (a group that includes a common ancestral species and all the species that descended from it). There are two reptilian clades. One clade includes all living reptiles, dinosaurs, ichthyosaurs, plesiosaurs, and birds (the Sauropsida). The other clade is the mammals and the extinct mammallike reptiles (the Theropsida). Crocodilians and birds are more closely related to each other than either is to lizards and snakes. They are part of a smaller sauropsid clade, the Archosauria. Lizards and snakes are in the clade Lepidosauria. Archosaurs had a large opening in the front of each eye. As the many groups of archosaurs evolved, this antorbital fenestra ("window in front of the eye") sometimes closed (in crocodilians and the later plant-eating dinosaurs) or merged with the nostril (in pterosaurs). It was the largest opening in the skull of the large predatory dinosaurs, such as Allosaurus and Tyrannosaurus,
The earliest archosaurs are found in Permian rocks, formed before the Mesozoic Era began. In the beginning of the Mesozoic, when animal life was recovering from the worst mass extinction in the world's history, the archosaurs expanded and quickly spread. Most of those first archosaurs were extinct by the end of the Triassic Period, but the Pterosauria, Saurischia, and Ornithischia survived to the end of the Mesozoic, and the Crocodilia survived to the present. Birds have not been found in the Triassic, although some puzzling Triassic birdlike animals have recently been discovered in Asia, Europe, and Texas.
Two important evolutionary changes took place among the archosaurs. They changed from-sprawling, lizardlike animals to animals that walked with their legs held directly under their bodies. The other change was from a cold-blooded, lizardlike metabolism (the way a body uses energy) to a warm-blooded, birdlike metabolism. These changes did not take place in all archosaurs, but they happened in the dinosaurs. Crocodilians are the only surviving example in which those changes did not occur; birds are the only surviving group in which they did occur.
Warm-bloodedness may have appeared early in the dinosaur-bird clade, so that almost all dinosaurs were warm-blooded. This trait may have been inherited from a common ancestor dinosaurs shared with birds. Or, these changes may have happened after dinosaurs and birds separated, so that only a few advanced predatory dinosaurs were warm-blooded.
The ancestors of dinosaurs developed a stronger ankle. This kind of ankle occurs in pterosaurs, dinosaurs, and birds. Lagosuchus, Lagerpeton, and Pseudolagosuchus were small, bipedal (they walked on two legs) archosaurs with advanced ankles and other features that suggest they were closely related to dinosaurs.