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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.
