The Handy Dinosaur Answer Book. Patricia Barnes-Svarney

out). Of the modern amphibians, the newts and salamanders are probably the most similar to the early amphibians, although the modern-day versions are much smaller.

      The vertebrate class Amphibia today includes about 3,500 species in three orders: frogs and toads (order Anura), salamanders and newts (order Caudata), and caecilians (order Gymnophiona). There is, however, a much larger number of extinct species of amphibians: this ancient group of animals were the first vertebrates to begin exploiting terrestrial environments, where they became prey for other species.

      Why did the reptiles dominate during the Mesozoic but not the amphibians?

      Besides the ability to not depend on water as much as amphibians, there are probably two main reasons why reptiles became dominant in the Mesozoic. First, reptiles developed adaptations in their skeletal structure, allowing them to move much quicker than amphibians. Second, during the Permian period the climate became hotter and drier, and many water sources disappeared. The reptiles’ new adaptations—from the development of scales to retain water to eggs that could survive without staying in water—allowed them to thrive where amphibians could not.

      Did some reptiles return to the oceans?

      Yes, as the reptiles spread out over the land, some of them returned to the water. Over a period of time, they evolved and adapted to the water again. Their legs gradually evolved back into fins and flippers; eyes adapted to seeing underwater; and bodies became streamlined for better speed in the water. In addition, they could no longer lay their eggs on land. Thus, they evolved a way of producing living young within their bodies, a process called ovoviviparous. The Ichthyosaurs, or “fish lizards,” were the most fishlike true reptiles.

      Though it looks much like modern fish do today, the Ichthyosaur was still a reptile, and one of the first true reptiles to live in the water exclusively (iStock).

      How are reptiles grouped?

      During the 100 million years after the first reptiles appeared, various reptile lines continued to evolve. Today, it is difficult to find agreement about reptile classification. In most cases, they are divided into four living orders (the others have died out over time):

      Crocodilia—Crocodiles, alligators, gharials, and caimans, comprising 23 known species.

      Squamata—Lizards, snakes, and the worm lizards, or amphisbaenids, which make up about 7,900 species.

      Testudines—Turtles and tortoises, which includes about 300 species.

      Sphenodontia—The endangered tuatara, which can only be found in New Zealand and consists of two species.

      There is also another older method of grouping reptiles: subclasses according to the positioning of the temporal fenestrae, or the openings in the sides of the skull behind the eyes: the anapsids, synapsids, diapsids, and euryapsids. The anapsids had no openings in the skull and eventually evolved into today’s turtles and tortoises. The synapsids, or “same hole,” had a low skull opening, and were once thought to be the ancestors of modern mammals (and are now not considered to be true reptiles). It was the animals of the diapsid line, or “two skull openings,” that eventually gave rise to the dinosaurs. Another debatable line is the euryapsids, characterized by a single opening on the side of the skull, which are now usually included with the diapsids.

DINOSAURS FIRST APPEAR

      How did the reptiles give rise to the dinosaurs?

      It was the diapsid group of the reptiles that eventually produced the dinosaurs. The jaw muscles in these reptiles were attached to the two openings on each side of the skull, giving their jaws better leverage and strength. Sometime in the Permian period, the diapsid line branched into two groups: lepidosaurs and archosaurs. The lepidosaur group evolved into today’s lizards and snakes; the archosaurs eventually gave rise to the dinosaurs.

      What were early archosaurs like?

      One of the first archosaurs—and probably typical of many archosaurs—was the big-headed Shansisuchus, a Middle Triassic period creature that lived in what is now China about 220 million years ago. It was about 7 feet (2.2 meters) long and had long back legs and short front legs.

      What were other archosaurs like?

      Archosaurs varied in many ways. Some, like the 1-foot-(30-centimeter-) long Gracilisuchus of the Middle Triassic could run on their hind legs over short distances; others, like the 12-foot-(4-meter-) long Chasmatosaurus, were heavy carnivores that walked on all fours. The 2-foot-(0.7-meter-) long Lagerpeton had very peculiar hind limbs: its feet had an elongated fourth toe that may have been used for perching.

      What are thecodonts?

      Thecodonts (“socket teeth”) were once thought to be a group living from the Late Permian to the end of the Triassic period—a division within the archosaurs that eventually gave rise to the dinosaurs, and perhaps crocodiles, birds, and pterosaurs. It is now considered an obsolete term—and there are definite reasons why.

      Just like many fields of science, the study of early dinosaurs constantly changes—not to mention that differences in opinion abound. By the mid-1980s, many scientists proposed that there was not a group called the thecodonts that evolved from the archosaurs. In fact, most scientists now believe Thecodontia as a group does not exist. Scientists cite that some of the animals listed in this group are more closely related to the crocodiles, some to dinosaurs, and some to the entire group of archosaurs. Other scientists believe that while thecodonts may not be a true group, the term was a handy way to describe certain creatures with socketed teeth within the archosaurs.

      What was the next stage in the evolution towards true dinosaurs?

      As time went on, another phase of dinosaur evolution took place. The animals’ skeletal structure changed, especially the hips, which gave many of the dinosaurs the ability to run on two legs. Euparkeria was a small lizard-like reptile that lived on land and walked on all fours, but it could run on two legs when in a hurry. Further along in time was Ornithosuchus, or “bird crocodile,” which was a two-legged predator. Its front limbs were too small to use for walking on all fours, and its thighs were nearly vertical. From this evidence, it appears that Ornithosuchus walked only on its hind legs.

       How were the dinosaurs unique among the reptiles?

      Dinosaurs, unlike other reptiles, had their legs tucked in underneath their bodies. This gave them the ability to run and walk very efficiently, eventually leading to some species becoming totally bipedal (two-footed). They also had a keener sense of smell, sight, and hearing, unlike the amphibians and primitive reptiles.

      What are some of the earliest known primitive dinosaurs?

      Two of the earliest known primitive dinosaurs were both fast-running carnivores: the Eoraptor, or “dawn hunter,” a small, three-foot-(one-meter-) long dinosaur that weighed about 22 pounds (10 kilograms); and the Herrerasaurus, measuring from 9 to 18 feet (3 to 6 meters) long. Both lived approximately 230 million years ago in the area known today as Argentina. Still another earlier dinosaur was the Staurikosaurus, a Late Triassic (around 225 million years ago) carnivore found in Brazil. It was about 6.5 feet (2 meters) long, with a fully upright gait that allowed for speed. After the evolution of these early specimens, other dinosaurs evolved quickly, becoming more and more diverse, and reachng out into all ecological niches.

      What were early carnivorous and herbivorous dinosaurs