Ornithischian dinosaurs

All ornithischians are thought to have been herbivorous and, rather
like modern-day mammals, they seem to be far more diverse, and
numerous, than their potential predators.
Thyreophorans (Figure 28) are a major group of ornithischians that are characterized by bearing bony plates in their body wall, clubs or
spikes adorning their tails, and for having an almost exclusively
quadrupedal method of locomotion. These types of dinosaur
include the stegosaurs, named after the iconic Stegosaurus (well
known for its tiny head, the rows of large bony plates on its back,
and its spiky tail (Figure 31)); and the heavily armoured
ankylosaurs including such creatures as Euoplocephalus. The latter
was a huge tank-like animal that was so heavily armour-plated that
even its eyelids were reinforced by bony shutters and its tail was
terminated in a huge, bony club that it presumably used to skittle
potential predators.

Cerapodans (Figure 28) were very different to thyreophorans. These
were typically lightly built, unarmoured bipeds, although a few did
revert to quadrupedal methods of locomotion. Ornithopods were
one major group of cerapodans. Many of these dinosaurs were
medium-sized (2–5 metres long) and quite abundant (probably
filling the ecological niches occupied by antelopes, deer, sheep, and
goats today). These animals, such as Hypsilophodon, were balanced
at the hip ( just like theropods), had slender legs for fast running,
grasping hands, and, most importantly, teeth, jaws, and cheeks
adapted for a diet of plants. Throughout the reign of the dinosaurs,
small to medium-sized ornithopods were quite abundant, but
through the Mesozoic a significant number of larger types evolved;
these are known as iguanodontians (because they include animals
such as Iguanodon). Most important of all the iguanodontians were
the extraordinarily numerous duck-billed, or hadrosaurian,
dinosaurs of the Late Cretaceous of North America and Asia. Some
(but not all) of these dinosaurs did indeed have rather duck-shaped
snouts, and others had a wide range of quite extravagant, hollowcrested
headgear (see Chapter 7); this headgear may well have been
used for social signalling, and more particularly for making loud,
honking sounds. Marginocephalians were the other major
cerapodan group and appeared in Cretaceous times. These included
the extraordinary pachycephalosaurs (‘thick-headed dinosaurs’);
they had bodies that were very similar in general appearance to the
ornithopods, but their heads were very odd-looking. The majority
had a high dome of bone on the top, which looked vaguely similar to
the headgear of hadrosaurians, except for the fact that
pachycephalosaur headgear was made of solid bone. It has been
suggested that these creatures were the ‘headbangers’ of the
Cretaceous world – perhaps using head clashing in similar fashion
to that seen among some cloven-hooved animals today.
Finally, there were the ceratopians, a group of dinosaurs that
included the fabled Protoceratops referred to in the Introduction,
as well as the well-known Triceratops (‘three-horned face’). All
had a singular narrow beak at the tip of the jaws and tended to
have a ruff-like collar of bone at the back edge of the skull. While
some of these dinosaurs, particularly the early ones, maintained
a bipedal way of life, a considerable number grew greatly in
body size, with an enlarged head, which was adorned with a
huge frill-like collar and large eyebrow and nose horns. Their
great bulk and heavy head led them to adopt a four-footed
stance, and their similarity to modern-day rhinoceros has
not gone unnoticed. Clearly, as this all too brief survey shows,
dinosaurs were many and varied, judging by the discoveries
made over the past 200 years. But even though to date
about 900 genera of dinosaurs are known, this is only a tiny
fraction of the dinosaurs that lived during the 160 million
years of their reign during the Mesozoic Era. Many of these
will, unfortunately, never be known: their fossils were never
preserved. Others will be discovered by intrepid dinosaur hunters
in years to come.