The Morrison Formation - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 140

The Morrison
Formation

B ACKGROUND : TERRESTRIAL LIFE
IN THE MID -M ESOZOIC
From the end of the Triassic to the close of
the Cretaceous, life on land was, of course,
dominated by the hugely successful
dinosaurs. Since different periods of
the Mesozoic were characterized by dif-
ferent assemblages of dinosaurs, it is useful
to understand something of their classi-
fication. Dinosaurs are classified into two
main groups, the reptile-hipped saur-
ischians and the bird-hipped ornithischians.
In saurischians, the hip bones are
arranged similarly to those of most other
reptiles. The blade-like upper bone, the
ilium, is connected to the backbone by a
row of strong ribs and its lower edge forms
the upper part of the hip socket. Beneath
the ilium is the pubis, which points
downward and slightly forward, and behind
this is the backwardly extending ischium.
The saurischians are further divided into
two groups. Theropods include all
of the carnivorous (meat-eating) dinosaurs;
most have powerful hind limbs ending
in sharply-clawed, bird-like feet, lightly-
built arms, a long, muscular tail,
and dagger-like teeth. Examples include
the giant Tyrannosaurus ('tyrant lizard')
and Albertosaurus , the small and agile
Velociraptor ('fast-thief'), and some tooth-
less types, such as Oviraptor ('egg-thief')
and Struthiomimus ('ostrich-mimic'), all
from the late Cretaceous (Chapter 10). The
group also includes the classically

huge predators of the Jurassic, such as
Allosaurus and Megalosaurus and the tiny
Compsognathus from the same period.
The second group of saurischians, the
sauropodomorphs, were all herbivorous
(plant-eating) dinosaurs. They ranged
in size from diminutive forms (the pro-
sauropods, such as Massospondylus ) from the
late Triassic and early Jurassic, to the
gigantic true sauropods of the late Jurassic,
such as Diplodocus , Apatosaurus (pre-
viously known as Brontosaurus ),
Brachiosaurus , and Camarasaurus . They
tend to have long, slender bodies, whip-like
tails, long, shallow faces, and thin, pencil-
shaped teeth.
In the bird-hipped ornithischians the
arrangement of the hip bones is similar to
that of living birds (although confusingly
the ornithischians did not give rise to
birds). While the ilium and ischium are
arranged in a similar manner to the
saurischians, the pubis is a narrow, rod-
shaped bone which lies alongside the
ischium. In addition, all ornithischians
seem to possess a small, horn-covered beak
perched on the tip of the lower jaw.
Ornithischians were entirely
herbivorous and are classified into five
major groups: the ornithopods (medium-
sized animals such as the early Cretaceous
Iguanodon and Tenontosaurus , and the
hadrosaurs, or duck-billed dinosaurs, such
as the late Cretaceous Edmontosaurus );

the ceratopsians (horned and frilled