The Age of Dinosaurs

In the Middle Triassic, the advanced pseudosuchian thecodonts with the vertical posture of the hind limbs gave rise to the earliest dinosaurs. This stage of the archosaurian evolution is documented by their records in the Middle Triassic of South America. The straight vertical position of the hind limbs and tendency towards bipedal locomotion are among the most important and characteristic features distinguishing both dinosaurs and their thecodont ancestors. It is most likely that bipedal gait was initial in all dinosaurs and quadrupedal locomotion in some of their groups was developed secondarily. More advanced locomotory abilities of dinosaurs as well as a diversity of forms and different modes of living allowed them to win the competition and replace their thecodont ancestors. Approximately at the same time when the first dinosaurs appeared, pseudosuchian thecodonts also gave rise to pterosaurs and crocodiles. First birds, or proto-birds already existed at the end of the Triassic or even earlier. Though the thecodont ancestry of the pterosaurs and crocodiles seems to be well established and is widely accepted, the origin of birds is a much more intricate question, possibly the most disputed one during the last decade in the evolution of higher vertebrates. There are several competing hypotheses of avian origin. The most popular of these relates the birds origin with maniraptoran dinosaurs (see below), or suggests that maniraptoran dinosaurs and birds share a common ancestor. The evolutionary history of crocodiles and birds was quite successful and they survived until present, whereas the pterosaurs shared the fate of dinosaurs and became extinct at the end of the Mesozoic. The dinosaurs are separated into two large divisions on the basis of the pelvis structure: saurischians (lizard-hipped dinosaurs) and ornithischians (bird-hipped dinosaurs). In ornithischians (with exception of armored dinosaurs, the ankylosaurs) the pubis forms relatively long frontal extension, being comparable in size to elongated frontal part of the ilium, while the rear part of the pubis is always directed backward. In saurischians the pubis may be directed forward, downward, or backward but it never possesses such a well-developed frontal part as in ornithischians. Known Middle Triassic remains of potential proto-dinosaurs are insufficient to determine definitely the relationships between them and their more advanced descendants. It seems more probable that the two principal groups of dinosaurs, the saurischians and the ornithischians, originated independently from distinct pseudosuchian ancestors. Moreover, different groups of saurischian dinosaurs might have had their own ancestors. There were already several groups of both saurischians and ornithischians by the end of the Triassic. Although these early dinosaurian groups became extinct by the Middle Jurassic, the diversity of dinosaurs grew quickly in the Jurassic and Cretaceous until its peak in the first half of the Late Cretaceous. The disappearance of dinosaurs at the boundary of the Mesozoic and Cenozoic was not absolutely sudden and as coincidental as one might think. Actually, only about one third of families of the Late Cretaceous dinosaurs survived until the end of the Mesozoic, whereas other two-thirds became extinct many millions of years earlier. That means there was a general tendency toward extinction of dinosaurs as a whole that was caused by gradual ecological changes and environmental factors rather by any catastrophic events. This conclusion is also confirmed by the successful survival of many groups of terrestrial tetrapods, such as amphibians, crocodiles, lizards, turtles, mammals, and birds through the Mesozoic-Cenozoic boundary. Certainly, any catastrophic event could accelerate dramatically the process of dinosaur extinction.

Saurischian Dinosaurs

The saurischian dinosaurs may be divided into several main groups: the bipedal predators Theropoda, aberrant herbivorous Segnosauria, and herbivorous quadrupedal Sauropodomorpha. Both the theropods and sauropodomorphs existed from the Late Triassic until the end of the Late Cretaceous and the segnosaurs are known only from the Late Cretaceous. The theropods greatly diversified in size, structure and mode of living. Two of their groups, the carnosaurs and maniraptorans, are presented in the exhibition. The carnosaurian theropods include the largest terrestrial predators to have ever inhabit the Earth. They began to evolve as early as the Early Jurassic or even Late Triassic but flourished during the Late Cretaceous. At that time the largest carnosaurs of the genus Tyrannosaurus lived in both what is now Central Asia and North America. The Asian species, Tyrannosaurus baatar (which is frequently assigned to its own genus Tarbosaurus), is smaller than its North American relative Tyrannosaurus rex. As the former is somewhat older than the latter, it suggests the genus could initially appear in Asia and then entered the North America through the wide land bridge connecting these continents during the Cretaceous. The tyrannosaurs possessed a huge head with very large cutting serrated teeth, stout and rather long hind limbs, and forelimbs greatly reduced as typical of all carnosaurs. The brain in this tremendous animal was unbelievably tiny in comparison with body size. The maniraptoran theropods are presented at the exhibit by dromaeosaurs and avimimids. These dinosaurs never reached such huge size, like the carnosaurs. All maniraptorans are distinguished from the latter by well-developed, long and powerful forelimbs, although they were definitely bipedal like the carnosaurs, and did not use the forelimbs for support when walking. The dromaeosaurs were medium-sized predators with an extremely large sickle-like talon on the inner toe. The earliest dromaeosaur Deinonychus comes from the Early Cretaceous of North America. In the Late Cretaceous several genera of dromaeosaurs are known from North America and Central Asia (Velociraptor). A single known species of avimimids come from the Late Cretaceous of Mongolia. This peculiar dinosaur shares with birds some characters in the structure of the forelimb and shoulder girdle. The segnosaurs come only from the Late Cretaceous of Mongolia and China and are not well studied because of the insufficient fossil record. They were large or medium-sized bipedal plant eaters with relatively small head, massive body and stout hind limbs. Very long and powerful forelimbs in segnosaurs possessed giant claws, that might be up to 80 centimeters (2,6 feet) long, like in Therezinosaurus from Mongolia.

Sauropods

The evolutionary history of sauropodomorphs was very successful and they had global distribution beginning from the Late Triassic until the end of the Late Cretaceous. Their advanced members, the sauropods, embody the largest terrestrial tetrapods ever to have inhabited the Earth. Some of them were more than 40 meters long and their weight attained 60-70 tons. Besides the skeletal remains the record of this abundant and diverse group of dinosaurs includes well preserved egg clutches, such as the one from the Late Cretaceous of Mongolia displayed at the 'Russian Dinosaur Exposition'.