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In the Middle Triassic, the advanced pseudosuchian thecodonts with
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 hindlimbs and the tendency toward
bipedal locomotion are among the most important and characteristic features
distinguishing both dinosaurs and their thecodontian 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 life allowed them to win the competition
and replace their thecodontian ancestors.
Approximately at the same time when the first dinosaurs have 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 thecodontian 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 connects the appearance of birds with maniraptoran dinosaurs
(see below), or suggests maniraptoran dinosaurs and birds share a common ancestor.
The evolutionary history of crocodiles and birds was quite successful and they have
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 armourd dinosaurs, the ankylosaurians) the pubis forms relatively long frontal extension, being comparable in size with 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 sach 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
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 ancient 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 the dinosaurs on 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 million years before the boundary.
It means there was a general tendency toward extinction of dinosaurs
as a whole that had been 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
terrestial 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.