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.

Genealogy of dinosaurs

Up to this point, our focus has been largely, if not exclusively,
tuned to exploring aspects of the anatomy, biology, and way of
life of the dinosaur Iguanodon. It must be obvious that Iguanodon
was just one dinosaur that fitted into far larger tableaux of life
in the Mesozoic Era. One of the important tasks that falls to
palaeontologists is to try to discover the genealogy, or evolutionary
history, of the species that they study. To put dinosaurs as a whole
into some sort of perspective, it will be necessary to outline the
techniques used to do this, and our current understanding of
dinosaurian evolutionary history.

One feature of the fossil record is that it offers the tantalizing
possibility of tracing the genealogy of organisms not just over a
few human generations (which is the ambit of modern genealogists)
but over thousands, or millions, of generations, across the
immensity of geological time. The primary means by which such
research is carried out at present is the technique known as
phylogenetic systematics. The premise of this technique is really
quite simple. It accepts that organisms are subject to the general
processes of Darwinian evolution. This does not require anything
more profound than the assumption that organisms that are more
closely related, in a genealogical sense, tend to physically resemble
each other more closely than they do more distantly related
creatures. To try to investigate the degree of relatedness of creatures
(in this particular case fossil creatures), palaeosystematists are most
interested in identifying as wide a range of anatomical features as
are preserved in the hard parts of their fossils. Unfortunately, a
great deal of really important biological information has simply
rotted and been lost during the process of fossilization of any
skeleton, so, being pragmatic about things, we simply have to make
the most of what is left. Until quite recently, the reconstruction of
phylogenies had relied on hard-part anatomical features of animals
alone; however, technological innovations have now made it
possible to compile data, based on the biochemical and molecular
structure of living organisms, that can add significant and new
information to the process.

What the dinosaur systematist has to do is compile lengthy lists of
anatomical characteristics, with the intention of identifying those
that are phylogenetically important, or contain an evolutionary
signal. The task is intended to produce a workable hierarchy of
relationship, based on groupings of ever more closely related
animals.

The analysis also identifies features that are unique to a particular
fossil species; these are important because they establish the special
characteristics that, for example, distinguish Iguanodon from all
other dinosaurs. This probably sounds blindingly obvious but,
in truth, fossil creatures are often based on a small number of
bones or teeth. If other partial remains are discovered in rocks
elsewhere from the original, but of very similar age, it can be quite a
challenge to prove convincingly whether the new remains belong to,
say, Iguanodon, or perhaps a new and previously undiscovered
creature.

Beyond the features that identify Iguanodon as unique, there is also
a need to identify anatomical features that it shares with other
equally distinct, but quite closely related animals. You might say
that these were the equivalent of its anatomical ‘family’. The more
general the characters that ‘family’ groups of dinosaurs share, the
more this allows them to be grouped into ever larger and more
inclusive categories of dinosaurs that gradually piece together an
overall pattern of relationships for them all.
The real question is: how is this overall pattern of relationships
achieved? For a very long time, the general method that was used
might be described simply as ‘I know best’. It was quite literally the
view of self-styled experts, who had spent much time studying a
particular group of organisms and then summarized the overall
patterns of similarity for their group; their methods for doing this
might vary considerably, but in the end their preferred pattern of
The case of Baryonyx
The Early Cretaceous rocks of south-east England have been
intensely investigated by fossil hunters (starting with Gideon
Mantell) and geologists (notably William Smith) for well
over 200 years. Iguanodon bones are very common, as are
the remains of a limited range of other dinosaurs, such as
‘Megalosaurus’, Hylaeosaurus, Polacanthus, Pelorosaurus,
Valdosaurus, and Hypsilophodon. Given the intensity of such
work, it would be thought highly unlikely that anything new
would ever be discovered. However, in 1983 the amateur collector
William Walker discovered a large claw bone in a clay
pit in Surrey that led to the excavation of an 8-metre-long
predatory dinosaur that was entirely new to science. It was
named Baryonyx walkeri in honour of its discoverer, and
has pride of place on exhibition at the Natural History
Museum in London.

The moral of this story is that nothing should be taken for
granted; the fossil record is likely to be full of surprises.
relationship was little more than just that: their preference,
rather than a rigorous, scientifically debated solution. While this
method worked reasonably well for restricted groups of organisms,
it proved far more difficult to properly debate the validity of one
interpretation compared with another because the arguments,
when boiled down to their essentials, were circular, relying on one
person’s belief over another’s.

This underlying problem was brought into sharp focus when
groups of organisms were very large in number and varied in
many subtle ways. Good examples are groups of insects, or some
of the bewildering varieties of bony fish. If the general scientific
community was happy to accept the authority of one scientist for a
period of time then all was apparently fine. However, if experts
could not agree, the end result was frustratingly circular debates.
Over the past four decades, a new methodology has gradually been
adopted that has proved far more valuable scientifically. It does not
necessarily give the correct answers, but it is at least open to
scientific scrutiny and real debate. This technique is now widely
known as cladistics (phylogenetic systematics). The name is treated
with a fair degree of trepidation by some, but this is largely because
there have been some very fierce arguments about how cladistics is
done in practice and what the overall significance of the results
might be in an evolutionary context. Fortunately, we do not need to
consider much of this debate because the principles are actually
surprisingly simple and clear-cut.

A cladogram is a branching tree diagram that links together
all the species that are being investigated at the time. To create
one, the researcher needs to compile a table (data matrix)
containing a column listing the species under consideration
and against this a compilation of the features (anatomical,
biochemical, etc.) that each species exhibits. Each species is
then ‘scored’ in relation to whether it does (1) or does not (0)
possess each character, or in some instances if the decision is
uncertain this can be signified as a (?). The resulting matrix
of data (these can be very large) is then analysed using a number
of proprietary computer programs, whose role is to assess the
distribution of 1s and 0s and generate a set of statistics that
determines the most parsimonious distribution of the data
that are shared by the various species. The resulting cladogram
forms the starting point for a considerable amount of further
investigation that is aimed at determining and understanding
the degree to which there are common patterns or overall
similarities, and the extent to which the data might be misleading
or erroneous.

The cladogram that results from this type of analysis represents
no more than a working hypothesis of the relationships of the
animals that are being investigated. Each of the branches on
the tree mark points at which it is possible to define a group
of species that are all connected by their sharing a number of
characteristic features. And using this information it is possible
to construct what is, in effect, a sort of genealogy or phylogeny
representing a model of the evolutionary history of the group
as a whole. For example, if the known geological times of
occurrence of each of the species are plotted on to this pattern,
it becomes possible to indicate the overall history of the group,
and also the probable time at which various of the species may
have originated. In this way, the cladogram, rather than simply
representing a convenient spatial arrangement of species, begins
to resemble a real genealogy. Obviously, each such phylogeny
created in this way is only as good as the data available, and
the data and how it is scored can change with the discovery
of new, better, or more complete fossils, and also as new
methods of analysis are developed or older ones are
improved upon.

Dinosaurs in perspective

The fossilized remains of dinosaurs (with the notable exception of
their lineal descendants the birds – see Chapter 6) have been found
in rocks identified as belonging to the Mesozoic Era. Mesozoic rocks
range in age from 245 to 65 million years ago (abbreviated to Ma
from now on). In order to put the time during which dinosaurs lived
into context, since such numbers are so large as to be quite literally
unimaginable, it is simpler to refer the reader to the geological
timescale.

During the 19th and a considerable part of the 20th centuries, the
age of the Earth, and the relative ages of the different rocks of which
it is composed, had been the subject of intense scrutiny. During the
early part of the 19th century it was becoming recognized (though
not without dispute) that the rocks of the Earth, and the fossils that
they contained, could be divided into qualitatively different types.
There were rocks that appeared to contain no fossils (often referred
to as igneous, or ‘basement’). Positioned above these apparently
lifeless basement rocks was a sequence of four types of rocks that
signified four ages of the Earth. During much of the 19th century
these were named Primary, Secondary, Tertiary, and Quaternary –
quite literally the first, second, third, and fourth ages. The ones that
contained traces of ancient shelled and simple fish-like creatures
were ‘Primary’ (now more commonly called Palaeozoic, literally
indicative of ‘ancient life’). Above the palaeozoics was a sequence of
10
rocks that contained a combination of shells, fish, and land-living
saurians (or ‘crawlers’, which today would include amphibians
and reptiles); these rocks were designated broadly as ‘Secondary’
(nowadays Mesozoic, ‘middle life’). Above the mesozoics were found
rocks that contain creatures more similar to those living today,
notably because they include mammals and birds; these were
named ‘Tertiary’ (now also called Cenozoic, ‘recent life’). And finally,
there was the ‘Quaternary’ (or Recent) that charted the appearance
of recognizably modern plants and animals and the influence of the
great ice ages.

This general pattern has stood the test of time remarkably well.
All modern geological timescales continue to recognize these
relatively crude, but fundamental, subdivisions: Paleozoic,
Mesozoic, Cenozoic, Recent. However, refinements in the way
the fossil record can be examined for example, through the use
of high-resolution microscopy, the identification of chemical
signatures associated with life, and the more accurate dating of
rocks enabled by radioactive isotope techniques have led to a more
precise timescale of Earth history.

The part of the timescale that we are most concerned with in this
book is the Mesozoic Era, comprising three geological periods:
the Triassic (245–200 Ma), the Jurassic (200–144 Ma), and the
Cretaceous (144–65 Ma). Note that these periods of time are not by
any means equal in duration. Geologists were not able to identify a
metronome-like tick of the clock measuring the passing of Earth
time. The boundaries between the periods were mapped out in the
last two centuries by geologists who were able to define particular
rock types and, very often, their constituent fossils, and this is
usually reflected in the names chosen for the periods. The term
‘Triassic’ originates from a triplet of distinctive rock types
(known as the Lias, Malm, and Dogger); the ‘Jurassic’ hails from
a sequence of rocks identified in the Jura Mountains of France;
while the name ‘Cretaceous’ was chosen to reflect the great
thickness of chalk (known as Kreta in Greek) such as that which forms the White Cliffs of Dover and is found widely across Eurasia
and North America.

The earliest dinosaurs known have been identified in rocks dated
at 225 Ma, from the close of the Triassic (a period known as the
Carnian), in Argentina and Madagascar. Rather disconcertingly,
these earliest remains are not rare, solitary examples of one type of
creature: the common ancestor of all later dinosaurs. To date at
least four, possibly five, different creatures have been identified:
three meat-eaters (Eoraptor, Herrerasaurus, and Staurikosaurus),
a tantalizingly incomplete plant-eater named Pisanosaurus, and an
as-yet-unnamed omnivore. One conclusion is obvious: these are not
the earliest dinosaurs. In the Carnian there was clearly a diversity of
early dinosaurs. This indicates that there must have been dinosaurs
living in the Middle Triassic (Ladinian-Anisian) that had ‘fathered’
the Carnian diversity. So we know for a fact that the story of
dinosaur origins, both the time and the place, is incomplete.