The Handy Dinosaur Answer Book. Patricia Barnes-Svarney

      The earliest carnivorous dinosaurs, or meat-eaters, came in many different shapes and sizes. It is thought that the 20-foot-(6-meter-) long, Early Jurassic Dilophosaurus was a typical carnivore: This dinosaur had strong hind legs, but short, weak forelimbs. It also had thin parallel ridges on its forehead, which could have acted as radiators to control temperature, or as decoration, possibly for territorial or mating displays. The earliest herbivorous dinosaurs, or plant-eaters, also came in many different shapes and sizes. One typical herbivore was the Early Jurassic Heterodontosaurus, a small, turkey-sized dinosaur that had sharp incisors, canine-like tusks, and grinding teeth for chewing plants.

      How are dinosaurs classified?

      All animals (and plants) fall into a modern classification—a system first developed by Swedish naturalist Carolus Linnaeus (Carl von Linné, 1707–1778). In the standard Linnean system of animal classification, the hierarchy to dinosaurs is as follows: Animalia (kingdom); Chordata (phylum); Reptilia (class); and Dinosauria (infraclass).

      There are even more divisions, and the list seems to get longer and longer every time another dinosaur fossil is found. But in general, they are often divided into two main groups, based historically on their hipbone structure. Those with hips that had the two lower bones pointing in opposite directions, with the pubis bone pointing forward are called saurischian, or lizard-hipped dinosaurs. Those with hips that had the two lower bones lying together behind the back legs, and the pubis bone pointing backward are called ornithischian, or bird-hipped dinosaurs. Tyrannosaurus rex is an example of a lizard-hipped dinosaur; while the Iguanodon is an example of a bird-hipped dinosaur.

      How has the definition of dinosaur changed over the years?

      In 1842, Sir Richard Owen (1804–1892) assumed dinosaurs all descended from a common ancestor (in paleontological terms, they were a monophyletic group). This meant all dinosaurs shared some common characteristics. And when finally determined, the characteristics could be used to differentiate true dinosaurs from other organisms.

      In 1887, Harry Seeley (1839–1909) discovered there were two major dinosaur groups: the saurischians and the ornithischians. This led scientists to assume there was more than one common ancestor. To further complicate matters, discoveries of diverse dinosaur species pointed to many ancestors. As a result of this, dinosaurs were seen as a group of reptiles with few characteristics in common. Because of these earlier scientific interpretations, dinosaurs no longer shared one set of common characteristics; thus, they were classified as a polyphyletic group, arising from many sources among the archosaurs. They became a polyphyletic group, arising from many sources among the archosaurs. Interpreted this way, there was no longer one set of common characteristics that dinosaurs shared.

      How many species of dinosaurs are currently known?

      The English zoologist Richard Owen coined the term “dinosaur” back in 1842 (iStock).

      Not all dinosaur species that have been found have been named. Currently, there are between 600 and 700 known dinosaur species. But there is a major caveat: only about half of these specimens are complete skeletons and usually only complete (or nearly complete) skeletons allow scientists to confidently say the bones represent unique and separate species. Amazingly, many scientists speculate there may be between 700 and 900 more dinosaur genera that have yet to be discovered.

      How many species of dinosaurs have been named since the first skeletons were uncovered?

      The number of species named depends on the text—from 250 to more than 1,000 have been mentioned. Suffice it to say that hundreds of dinosaurs have been named since the first skeletons were dug up in the nineteenth century. Between the lack of consensus in terms of species and new fossils found every year, unraveling dinosaur patterns of evolution has been a major obstacle in dinosaur research.

      How do the number of known dinosaur species compare to some modern species?

      Even if there are truly 700 valid dinosaur species, the number is still less than one-tenth the number of currently known bird species; less than one-fifth the number of known mammal species; and less than one-third the number of known spider species.

      How is a dinosaur currently defined using cladistic analysis?

      Cladistics is a method of classifying all organisms by a common ancestry, and it is based on the branching of the organism’s evolutionary family tree. Those that share common ancestors—and thus have similar features—fall into taxonomic groups called clades. Thanks to cladistic analysis, all dinosaurs were found to have many unique characteristics in common. In fact, these reptiles are defined as a monophyletic group descending from a common ancestor. And with the development of modern cladistic “testing,” true dinosaurs can be distinguished from their closely related, but non-dinosaur, contemporaries.

      Using cladistic analysis, a reptile is a dinosaur if it has several specific characteristics in its fossilized skeleton, including some of the following: an elongated deltopectoral crest on the humerus; three or fewer phalanges in the fourth finger of the hand; the absence of a postfrontal bone; a crest on the tibia; three or more sacral vertebrae; a fully open hip socket; a ball-like head on the femur; and a well-developed ascending process on the astragalus, fitting on the front face of the tibia. In other words, identifying a dinosaur has a great deal to do with its skeletal anatomy. Although these characteristics are very technical, the main point is that scientists now have a clear test to determine if a fossil skeleton is truly that of a dinosaur.

      How are dinosaurs named?

      Dinosaur names come from a number of places, but in general, they are named after a characteristic body feature (for example, the Hypsilophodon, or high-crowned tooth); after the place in which the first bones were found (for example, the Muttaburrasaurus); or after the person(s) involved in the discovery (for example, the Leaellynasaura).

      In most cases, the names include two Greek or Latin words, or even combinations of the words. For example, Tyrannosaurus rex is a combination of Greek and Latin translated as “king of the tyrant lizards.” Overall, the two names, known as the genus and species names, are used by biologists to describe all organisms on Earth, such as humans (Homo sapiens sapiens), domestic dogs (Canis familiaris), or rattlesnakes (Crotalus horridus).

      How did dinosaurs evolve?

      It’s hard to determine which charts are right or wrong when it comes to dinosaur evolution, mainly because, since 1980, over 150 evolutionary trees of dinosaurs have been published, most of them looking at small groups of species.

      How are dinosaurs classified?

      The following table explains how dinosaur species are organized by order, suborder, infraorder, and family.

      *This is only one representation of how dinosaurs may be classified. There are actually many other dinosaur classifications, depending on the scientific study, and there will continue to be more as fossils are discovered and interpreted.

      Is there a continent where dinosaur fossils have never been found?

      No. It was once thought that Antarctica was the only continent that did not have any dinosaur fossils. But in December 2003, researchers working in separate sites thousands of miles apart in Antarctica found what they