Stegosaurs are a group of quite large (up to eight
meters long), quadrupedal, ornithischian dinosaurs.
That is, they have a pelvis in which the pubis
points backward, and thus they are allied with all
other herbivorous dinosaurs except the gigantic
sauropods. Their closest relatives are the other armored
dinosaurs, the ankylosaurs, with which they
are grouped as Thyreophora ("shield bearers"),
characterized by rows of plates along the back and
sides of the body. The earliest stegosaurs are represented
by fragmentary remains fromthe Middle Jurassic
of England, but by the Late Jurassic they are
well known from complete skeletons from Africa,
Asia, Europe, and North America. The Late Jurassic
was the most successful time for stegosaurs and
they are particularly well known from articulated
skeletons of Stegosaurus of this age from North
America.Bythe Early Cretaceous their distribution
had contracted to Europe, Africa, and China, and
by the Late Cretaceous they are absent from all
continents except India. India was separate from
all the other continents from the Middle Jurassic
onward as it drifted away from Africa and toward
Asia, and this isolation may have protected the
stegosaurs as they became extinct everywhere else.
Anatomy and Lifestyle
The best known stegosaur is Stegosaurus ("roofed
reptile"), fromthe Late Jurassic of North America,
and thus it is commonly used to typify the entire
group. Stegosaurus had a small head with an unusually
small brain, even for a dinosaur, and a narrow,
horn-covered beak at the tip of the snout. The
sides of the jaws were lined with leaf-shaped serrated
teeth but these were not arranged in batteries
and there is no evidence of the sophisticated
grinding apparatus that was developed in some
other herbivorous dinosaurs. However, Stegosaurus
was a large animal and must have needed
large quantities of plant food, so it is likely that it
was a low-level browser that chopped up vegetation
and then quickly passed it back to a large
stomach, where it would have fermented, perhaps
with the help of gastroliths (stomach stones)
to further break it down.
The most distinctive feature of Stegosaurus is
the rowof vertical plates along the back. There has
been some disagreement as to their relationship to
each other, as they were set in soft tissue and not
attached to the skeleton; however, it is generally
agreed that they formed two parallel rows in
which the plates were staggered. The plates are
not optimally positioned for defense and it has
been suggested that they were devices for thermoregulation,
acting as radiators to gain or lose heat
and help maintain a constant internal body temperature
in animals that may have had an ectothermic metabolism (an internal temperature regulated
by the ambient temperature, as in most
modern reptiles). Experiments have shown that
the plates were ideally shaped and positioned
to do this. The surface of the plates is covered by
fine grooves, indicating the presence of numerous
blood vessels, and the plates themselves are hollow,
implying that they were richly supplied with
blood. Thus the animal could have flushed blood
over the plates to cool itself if its internal temperature
had risen too much, or to warm the blood by
exposure to the sun if its internal temperature was
dropping.
The proportions of the legs in Stegosaurus show
that it was not a fast runner, and thus as a large
and relatively slow-moving herbivore it would
have needed some means of defense against predators.
This would undoubtedly have been the
spike-bearing tail. Two pairs of spines projected
laterally from the tip of the tail and could have inflicted
severe injuries on an attacking carnivorous
dinosaur as the tail was lashed from side to side
while the animal backed toward the attacker.
Classification:
Kingdom: Animalia
Subkingdom: Bilateria
Phylum: Chordata
Subphylum: Vertebrata
Class: Reptilia
Order: Ornithischia
Suborder: Thyreophora
Geographical location: North America, Europe,
China, India, and Africa
Habitat: Terrestrial habitats
Gestational period: Unknown
Life span: Unknown
Special anatomy: Large, quadrupedal dinosaurs
in which the head was small, the back was covered
by a double row of large vertical plates,
and the tail bore large spikes
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