Dinosauria
Saurischia
Saurischians are 'lizard hipped' dinosaurs. Their hip
bones, like all Tetrapods, had pelves (hips) composed
of three elements: the ilium, ischium, and pubis.
However, what distinguishes Saurischians is that the
pubis points downwards and forward at an angle to
the ischium, similar to other reptiles. Saurischians were
also characterized by an asymmetrical hand with an
enlarged thumb, and the vertebrae in the back also had
extra joints.
It hasn’t always been easy to identify the unique characteristics of
dinosaurs. However, today palaeontologists accept that dinosaurs
are a single group of animals that share a unique set of features.
For example, all dinosaurs have:
zz Jaw muscles that extend onto the roof of the skull
zz Well-developed processes at the ankle, hip and knee
zz A large muscle ridge on the humerus
The oldest known dinosaurs, from the middle Triassic
of South America, were saurischians. For reasons
that are not well understood, the fossil record of
saurischians in the Late Triassic is much better than
that of the ornithischians, though both groups were
simultaneously beginning to diversify.
These shared characteristics can be used to diagnose dinosaurs as
they are always present and only present in dinosaurs. These sorts of
derived, shared characteristics are called ‘synapomorphies’ and they
are extremely important for classifying organisms because they
help to identify unique events in evolutionary history.
Dinosauria is a diverse group, containing several smaller
groups, each marked by the evolution of a new set of
features. All groups retain the original ancestral features of
dinosaurs but have also added something unique.
The saurischians form two major groups, the Sauropoda
and the Theropoda. The Sauropoda were mainly
quadrupedal (walked on four legs) large herbivores
such as Apatosaurus and Diplodocus, although their
first members were small and bipedal (walked on two
legs) . The Theropoda were all bipedal carnivores (meat
eaters), such as the Velociraptor and Tyrannosaurus,
from which birds evolved at least 144 million years ago
(Late or Middle Jurassic Period). Oddly, birds are derived
from these "lizard-hipped" dinosaurs and not from the
"bird-hipped" ornithiscian dinosaurs!
ilium
The word
‘Tetrapoda’ means
‘four legs’ in Greek.
Tetrapods include
all land-living
vertebrates.
The most basic division of dinosaurs is into two main groups –
Saurischia and Ornithischia. These two groups first appeared in the Late
Triassic and survived until the end of the Late Cretaceous.
pubis
Evolutionary tree showing major groups of dinosaurs
MESOZOIC
Triassic
Jurassic
FRONT
ischium
CENOZOIC
Cretaceous
Birds
Saurischians
Theropods
Prosauropods
Sauropods
Sauropodomorphs
Dinosaurs
Thyreophorans
Carcharodontosaurus
Stegosaurs
Ankylosaurs
Ornithischians
Ornithopods
Pachycephalosaurs
Marginocephalians
8 | FUSE
Ceratopsians
9 | FUSE