The Earliest Dinosaurs

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...

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...