The Archosaurian Diapsids

The diapsids are divided into two primary groups, the Lepidosauro-morpha and the Archosauromorpha. The Lepidosauria, a subgroup of the Lepidosauromorphs, includes lizards and snakes and their extinct ancestors. During the Mesozoic, this group of highly diverse but small animals lived in the shadows of the dinosaurs. The Archosauria, or ruling reptiles, was a subgroup of the Archosauromorpha and represented a commanding lineage of am-niotes that includes pterosaurs, dinosaurs, crocodylians, and...

Reshaping Of The Continents And Oceans

The Earth underwent dramatic geologic changes during the 180 million-year span of the Mesozoic Era. During most of the Trias-sic Period, the continents that are known today were still joined together as the supercontinent Pangaea. Pangaea was an expansive landmass that filled most of the Earth's Western Hemisphere. Because of Pangaea's size, distinct interior climate zones and new habitats for plants and animals developed. The formation of Pangaea also marked a low ebb in ocean depths this was...

Theropod Teeth and Feeding Styles

Dinosaur teeth, particularly those of theropods, are among the most common type of fossil found with the remains of these extinct creatures. The hard, enameled construction of teeth improved their chances of becoming fossilized, and some specimens still retain their original, enamel outer surface. Sometimes the only clue to the presence of a theropod is a tooth that it left behind. Fossil teeth have been found singly, without any other associated body parts, embedded in the bones of fossilized...

Ceratosaurs of the Late Triassic and Early Jurassic

The clade Ceratosauria includes two groups, the Coelophysoidea and the Neoceratosauria. The first of these is made up of theropods from the early radiation of the dinosaurs discussed in Dawn of the Dinosaur Age. The group Neoceratosauria includes later theropods that span the Late Jurassic to Late Cretaceous Epoch and that retained some of the primitive features associated with ceratosaurs. The Coelophysoidea (hollow forms) were small to medium-sized carnivores that measured from 3 to 10 feet...

Sauropods of the Late Triassic and Early Jurassic

Fossil evidence for the earliest sauropods is often incomplete and fragmentary. Listed here are some of the best known taxa. Their geographic range was widespread, like that of the prosauropods. Evidence for basal sauropods has been found on the continents of Africa, Asia, Europe, and possibly North America, South America, and Australia. Antetonitrus (Late Triassic, South Africa). One of the earliest known sauropods is Antetonitrus, from Late Triassic rocks dating from 220 million to 215...

Prosauropod Traits and Lifestyle

Prosauropods were medium- to large-sized herbivores that measured from 8 to 33 feet (2.5 to 10 m) long. The prosauropod body was stout, with a long neck, a small head, and a long tail. The hind legs of most prosauropods were longer and stronger than the fore-limbs this indicates that these animals could walk on two legs at least some of the time. The teeth of prosauropods were small and spatula-shaped, with rough serrations on the marginal edges. This kind of tooth could not be used to chew...

The Crurotarsi Crocodiles and Their Relatives

True crocodylians first appeared in the Early Jurassic, but the line of crurotarsan archosaurs leading to the crocodylians included several diverse groups that lived during the Triassic Period. Of these, only basal crocodylomorphs including a line of small to large, swiftly moving proto-crocodylians known as the sphenosuchians survived the two mass-extinction events that closed the Triassic Period. Crurotarsans arose during the Early Triassic by the Middle Triassic, they dominated terrestrial...

The Earliest Dinosaurs

Herrerasaurus Pelvis

Dinosaurs made their first appearance in the fossil record in the earliest part of the Late Triassic Epoch. While a few of these early dinosaurs are known from excellent fossils, many are only understood on the basis of fragmentary remains. The complete story of early dinosaurs is woefully incomplete in most cases. Even the partial record of early dinosaurs shows that they existed at about the same time in several widely separated locations around the world, however these locations include what...

Basal Archosaurs Before the Dinosaurs

Basal archosaurs were the stock from which all later archosaurs arose. These early archosaurs were once known by the scientific term Thecodontia, a name given to them by British paleontologist Richard Owen (1804-1892) in 1859. This group originally included any and all archosaurs that dated from the Triassic Period but were not clearly dinosaurs or crocodiles. Although it was assumed that thecodonts had a common tetrapod ancestor, the evolutionary relationships among the descendants of this...

Early Sauropod Traits

The sauropods were the more advanced members of the Sauropodomorpha and shared a common ancestor with the prosauropods. The prosauropods maintained a basic body plan that grew to no more than about 35 feet (10.5 m) in the largest species. They had a jaw joint that was lower than the tooth line of the upper jaw, had three sacral vertebrae, and had longer hind limbs that permitted them to move about on two legs with relative ease. It was on these basal traits that sauropods evolved adaptations...

Theropod Courtship Reproduction and Parenting

The reproductive behavior of dinosaurs will never be fully understood because their fossil remains can only hint at the kinds of behavior that might have been natural for these extinct animals. There are no clear-cut anatomical clues for determining a dinosaur's gender that can be applied to every kind of dinosaur skeleton. Yet paleontologists remain fascinated with the family life of dinosaurs because of provocative fossil evidence about nesting behavior, parental care, and the growth of...

Ornithodira Dinosaurs and Their Relatives

The ornithodirans are defined as the group containing the common ancestor of the pterosaurs and dinosaurs and all of its descendants. By the Early Jurassic Epoch, the ornithodirans were well on the way to becoming the dominant terrestrial vertebrates of the Mesozoic. Without the dinosaurs, the so-called Age of Reptiles may have become something entirely different perhaps even an age of mammals. As the natural history of the Mesozoic Era unfolded, however, mammals continued to develop in the...

Theropod Traits And Lifestyle

About 40 percent of all known dinosaur taxa are theropods. The most basal members of the clade were among the first dinosaurs to walk the Earth. Nonavian theropods were also among the last dinosaurs to perish, their success spanning the entire age of dinosaurs. They are survived today by avian theropods, the birds. Theropod remains have been found on every continent. These animals ranged in size from the tiniest of dinosaurs, the chicken-sized Compsognathus, to hulking monsters such as...

Ornithischians Of The Late Triassic And Early Jurassic

The clade Ornithischia is well established and defined, but its earliest members remain largely a mystery. Some are known only from teeth. The best fossil clues to their anatomy are fragmentary at best. Of about 15 specimens tentatively assigned to this group, paleontologists are most confident in the ornithischian nature of the following three taxa. Lesothosaurus Early Jurassic, Lesotho . This dinosaur from Africa is the best understood of the basal ornithischians. Paleontologist Peter Galton...

Predatory Saurischian Dinosaurs The Theropods

The best known of the earliest dinosaurs were small to medium- sized, bipedal predators that lived 228 million years ago, at the beginning of the Late Triassic Epoch. Before the end of the Triassic, these creatures were joined by the prosauropods, a widespread group of l arge-bodied herbivores that represented a prototype for the giant, long-necked plant eaters to follow in the Jurassic Period. Both of these lines of dinosaurs were saurischians and had the characteristic lizard hip pelvis...

Ceratosauria The Early Radiation Of The Theropods

The Ceratosauria horned lizards is a group that includes many of the earliest theropods but that also includes several later theropods that retain several primitive features. The earliest ceratosaurs lived during the Late Triassic and Early Jurassic Epochs. Several other prominent lines of ceratosaurs persisted, particularly in regions of the Southern Hemisphere now South America, India, and Madagascar , well into the Late Cretaceous. The name Ceratosauria is a nod to Ceratosaurus Late...