Here follows a couple of examples of topics for discussion, for fun, or for a school essay, if you like. But, first I wish to distinguish between different ways of analysing a topic (I focus on analysis, since topics for mere description should not be hard to find, in addition to the fact that texts that only describe rarely make good essays, and it is impossible to build a debate on descriptions). To discuss means both to give a detailed account of an issue, but also to consider more than one perspective. Consequently, a topic for discussion must be a topic where there are, or could be, several different, sometimes completely opposite points of view. Moreover, as in any debate or essay, you wish to reach some form of conclusion, so it is essential to actually compare and evaluate the different perspectives, rather than just stating them. To compare and contrast strictly means only to describe (or maybe discuss) similarities and differences, but usually implies assessment of their importance and/or implications (implication in this sense refers to the results or effects of an argument, discovery or revelation). To evaluate is to assess something, usually an argument, a piece of evidence, or a model. When you evaluate, you usually point out strengths and weaknesses, and weigh them against each other to reach a conclusion to whether the argument or evidence is strong or weak overall.
As
you might notice, the distinctions between these forms of analysis are very
fine: a discussion does not exclude comparison or evaluation, and evaluation is
in essence a comparison. However, it can be worth its weight in gold to be
clear about what these terms imply. If you are asked to write a comparative
essay, you are not expected to evaluate evidence; you would probably not
loose points for doing so, but you might well be wasting precious time. Knowing
exactly what kind of answer is expected is, of course, even more crucial for
exam writing, where you are under considerable time pressure and need to give
precise answers. Moreover, not answering the demands of a question does not
impress the examiner, while giving a strictly focused answer shows that you
know what you are doing.
Let
us move on to the more fun part: the topics. In some cases, I provide brief
examples both to show why this is a topic worth looking into, and to aid you in
your search for more information. The idea is for you to dig deeper on your
own, and discover the myriad of different views and explanations, and to
realise why this is or was a topic of controversy.
How/why
did the dinosaurs grow so enormous? Here, you can discuss advantages
and disadvantages of having a large body, and evaluate which you believe would
have been the main advantage(s). (Given that the dinosaurs did grow
enormous suggests that the advantages did outweigh the problems.) There are
several examples of advantages, including invulnerability from smaller animals,
lower rate of heat loss (larger bodies give off heat much slower, so it is
easier to keep warm), and the ability to travel farther. Some disadvantages
would be restricted movement (especially in places like a dense forest; this is
why most large dinosaurs lived in the open plains), higher food requirements,
and several physiological constraints. Moreover, you could spice your
discussion with an investigation into why some dinosaurs did not grow so
large; a handful of dinosaurs weighed less than a kilo (e.g. Compsognathus,
Archaeopteryx, Sinosauropteryx), and a great deal of theropods and some
ornithopods weighed under a tonne (e.g. the dromaeosaurids, or “raptors”, ornithomimids,
most oviraptorids, and the hypsilophodontids). Understanding why these
preferred a more modest size may shed some light on why other dinosaurs
favoured gigantism instead.
Were
the dinosaurs warm-blooded? This is probably the greatest issue in modern dinosaur
paleontology, and there is so much evidence, both for and against, out
of which no one seems to be strong enough to convince a majority of the
experts, that it seems practically impossible to reach a decisive answer to
this debate – which makes it the more interesting! Here is a true mystery to be
solved! Many are just sitting and hoping that one day we will discover a fossil
that reveals everything, but I say that we should look more carefully at the
evidence we already have, and search for aspects we did not think were relevant
before. Analyse and evaluate different pieces of evidence, emphasise their
strengths and weaknesses, use your imagination to uncover more ideas and
formulate new hypotheses, and test them using what material there is available.
This site has a section dedicated to this topic alone (see Warm-blooded?),
where you might find useful information to start with.
Who
was the ancestor of dinosaurs? No fossils of a definite ancestor to the Dinosauria have
been found as yet, and probably never will be. Therefore, perhaps this question
should be rephrased as what was the ancestor of dinosaurs like? which
invites you to speculate about what features this ancestor likely would have
had. Did it walk on two or four legs? Was it carnivorous or herbivorous?
Was it small? intelligent? warm-blooded? Was it closer to crocodiles, or to
pterosaurs, or to any other kind of animals? And, finally, the most important
question: given what features you attribute to this hypothetical ancestor, what
would be the implications, or consequences for our picture of the
dinosaurs and their evolution in general. For example, the paleontologist Paul
Cereno, among others, has suggested that a small, bipedal carnivore like Lagosuchus
would have been the ancestor of the dinosaur lineage; if this is so, one
implication would be that the four-legged dinosaurs would have evolved their
quadrupedality secondarily, or re-evolved it, since the
ancestral land vertebrates were (most certainly) four-legged. Now, this might
not be the most interesting idea, but it poses intriguing follow-up questions:
why did they go back to walking on four legs? was it something that changed in
their anatomy that made it preferable to be quadrupedal (e.g. growing so large
that four legs are needed to carry the body), or could it have been a change in
the environment that caused this evolutionary reversion?
Why
were the dinosaurs so successful? Ideas on this topic usually circle around suggestions of
some advantages dinosaurs had over the other land-living animals at that
time. Such could include their erect limb posture, giving them greater speed,
agility and the ability to breathe while running, that they may have been
warm-blooded and so capable of being more active, and so on. But, these
suggestions fail if you take into account that mammals, which also
possess these features, evolved at about the same time; you must be able to
explain why it was dinosaurs, and not mammals, that rose to world domination –
in short, you must point to some character which mammals, and preferably no
other animal back then, possessed, for instance their continuously replaced
teeth. (You may also point to some character that mammals had, which could have
put them at a disadvantage against the dinosaurs, such as their different
reproductive strategies.) On the other hand, you could (in addition) try to
figure out if there could have been anything in the environment, any
special circumstances, that may have favoured dinosaurs above all others, and
why it was in particular the dinosaurs that benefitted from this. One example
could include the fact that the Mesozoic was remarkably stable in terms of
climate, compared to the later, chaotic age of mammals – it may be that mammals
are more adaptable to climate change, and therefore took over only once the
climate became turbulent.
What
was the major cause for the extinction of the dinosaurs? The hypothesis that
a meteor the size of Mount Everest hit the Earth 65 million years ago
and brought about the demise of the dinosaurs has become widely accepted,
thanks to strong evidence, such as the discovery of a huge crater in the
Mexican Gulf (the size of this crater matched the size predicted by a series of
calculations of how large a meteor would be required to cause the mass
extinction), and the identification of a thin layer of iridium, an
element that is very rare on Earth, but common in meteors and other objects
from space, covering most of the Earth right at the time when the dinosaurs
disappeared from the fossil record. Now, the meteor alone did not exterminate
the dinosaurs (they did not all stand in one spot and were mashed together
under the big meteor); rather, it was the effects of the massive impact that
drove life towards its doom. These effects include earthquakes, volcanic
eruptions (ensuing in the release of poisonous and volatile gases that could
cause wide-spread forest fires), tsunamis, and the globe being covered in a thick
cloud of ash and debris that would block the sun for months (thus killing off
most plants, thereby causing the ecological balance to collapse). It is highly
plausible that all these forces acted together to cause this cataclysm, but the
challenge would be to examine which force was the worst – which catastrophe did
the most damage? This is a call for extensive and in-depth comparison and
evaluation.
Moreover,
this mass extinction did not only affect the dinosaurs. It has been estimated
that about 75% of all species living at that time went extinct. Even major
groups, like the pterosaurs, marine reptiles (plesiosaurs, mosasaurs), and
ammonites were exterminated. An intriguing trend among land animals is that
apparently only animals smaller than a large dog survived. Why is that?
What kind of catastrophe would kill only large animals? Here is a truly tricky
mystery, which if solved, could give us the key to understanding what killed
the dinosaurs. Who knows, you might even find out that the meteor had nothing
to do with the mass extinction!
Dinosaur
diets.
A wide range of different diets have been attributed to the dinosaurs, and
there are major trends punctuated by some anomalies. In rough
terms, theropods were carnivorous (meat-eating) and all other dinosaurs, i.e.
sauropodomorphs plus ornithischians, were herbivorous (plant-eating). There are
some exceptions, however: most paleontologists agree that two theropod groups,
the therizinosaurs and the ornithomimids, were actually herbivorous; also, it
has been suggested that prosauropods would have been omnivorous, meaning
that they fed on both plants and meat (see the article on Prosauropoda
for a detailed discussion). Furthermore, one can subdivide carnivorous and
herbivorous diets into more specific classes: carnivores may be insectivorous
(insect-eating) or piscivorous (fish-eating), implying certain adaptations to
such a diet. Small, agile dinosaurs with needle-like teeth are usually regarded
as insectivores, as well as feeding on small lizards, mammals, snails, etc.,
while there is considerable evidence that the spinosaurid family were
predominantly fish-eaters. On the herbivore side, there has been arguments as
to which major groups ate what type of plants: sauropods had poor dentition and
could not chew, yet their long necks seem to be an adaptation to browsing among
the tall trees, where only hard-digested conifers existed; by contrast, the
ornithopods and ceratopsians, with a dentition superior to all other animals,
seemed to prefer soft fern and angiosperm leaves.
There
are several intriguing approaches to a discussion on the topic of dinosaur
diets. What caused these exceptions to change their diet? What could pressure
an animal to eat only fish? Given that meat is far more easily digested than
plant material, why would a carnivore change to feed on plants? Why would
plant-eaters with bad dentition adapt to eat the tougher plants, while those
with more effective oral food processing feast on the softer types? (Were the
former just stupid, or did they have another way of extracting nutrients from
their selected food?) And, perhaps most interestingly, what evidence do
we have for the diets of the different dinosaur groups? There are several cases
where there is evidence for different food preferences for the same group or
species; a typical example would be the oviraptorids, which were once believed
to have fed upon the eggs of other dinosaurs, but later evidence point to them
being either herbivores, carnivores, nut-eaters, or a combination. What type of
evidence is the strongest? What should we believe?
Dinosaur
ecology.
Ecology includes a multitude of different aspects that can be discussed.
Disagreement about the population sizes of dinosaurs comes
readily to mind. It is not that the experts argue about the actual number of
individuals in a population – that can never be known, so there is no point in
even trying to figure it out – rather, the case is that many just feel that the
dinosaurs were too large to have been able to survive in the ecological sense:
the larger each individual animal is, the smaller is the number of individuals
a given environment can sustain; the problem is that if a population has too
few individuals, any random catastrophe (e.g. disease, forest fire, severe
drought or cold) could wipe out such a large proportion of the population that
there would not be enough individuals to reproduce effectively, which will
eventually lead to extinction. Many experts simply cannot imagine that animals
weighing above 100 tons could possibly have had populations large enough to be
viable (able to survive in the long run). This is, however, more a question of
gut feeling and lack of imagination, because there is no way we could even
estimate the population size of any extinct species precisely enough.
Other
topics include predator-prey interactions (who ate who?) and/or
hypothetical food webs (model of predatory-prey interactions for several
organisms in the same habitat or ecosystem). There is unambiguous evidence for
specific dinosaur predator-prey relations: the classical example would be the
“fighting dinosaurs” specimen – a Velociraptor and a Protoceratops
fossilised during actual combat. Moreover, bite and claw marks, predator teeth
found together with the carcass of the prey, or simply temporal and geographic
correlation (i.e. they lived in the same place at the same time) constitute
more or less strong evidence for some dinosaurs preying on others. Of course,
here it is also important to assess the evidence, especially if there is room
for contradiction.
Dinosaur
sociality.
Were the dinosaurs more, less or just as social as reptiles? Or maybe they
should be compared to birds, or even mammals instead? How can we tell?
Fossilised footprints reveal that some dinosaurs, especially herbivorous ones,
lived in herds of varying sizes, while several predatory dinosaurs have
been suggested to engage in group hunting (these could be cooperation
within a family, or more sporadic gatherings to take down large prey). Ornaments
were diverse and wide-spread within the Dinosauria, and are reasonably
interpreted as strictly social tools (while some structures, such as the
ceratopsian neck frill or the back sail on Spinosaurus, are debated
about whether they indeed had social, or, say, defensive functions). Other
dinosaurs, most notably the lambeosaurines with their hollow head crests, are
thought to have communicated using sound, which is superior to visual
signals in several ways, especially as warning signals. Some dinosaurs,
including the oviraptorids, show evidence of parental care,
something only seen in mammals and birds today, suggesting that dinosaurs may
have been closer to these animals regarding their social behaviour. On the
other hand, brain scans of dinosaurs such as Allosaurus reveal brains
that more resemble those of reptiles. Perhaps some dinosaur types had social
structures like birds and mammals, while others were only as advanced as other
reptiles in this aspect. Once again, evaluation of different sources of
evidence is essential.
Use
of some special structures. Why did the ceratopsians have a neck frill? Some propose
that they were for defence, either as a physical shield, or as a visual trick
to make the animal look much bigger and scarier, others hypothesise that the
large area could have had enormous jaw muscles attached to it, giving the
ceratopsians and incredibly powerful bite; finally, many argue that they were
used to transmit visual signals to other members of the herd. There is a myriad
of other special structures found in different dinosaur groups whose function
is unresolved. What was the purpose of the stegosaur back plates, the domed
skull of the pachycephalosaurs, the sauropod tail, the back sail on Oranosaurus,
the huge thumb claw of the sauropodomorphs, the short arms of Tyrannosaurus
rex, the horn of Ceratosaurus, etc. etc.
Coevolution between dinosaurs
and plants.
Coevolution is a phenomenon where two species or groups mutually affect the
evolution of the other; they evolve together, so to speak. There is a more or
less clear correlation between the emergence of the angiosperms, or
flowering plants, in the early Cretaceous, and the increased success of the ornithischian
herbivores (especially the ornithopods and ceratopsians). Previously,
slow-growing and hard-digested conifers dominated the land flora, and the
colossal sauropods were the most prominent herbivores. With the arrival of the
angiosperms, which grow considerably faster and have more succulent and
nutrient-rich leaves, the sauropods declined in importance, especially in North
America, and the ornithischians took over. There have been several attempts to
explain these events, but there is no consensus over which was “first” – did
the emergence of the angiosperms allow the rise of the ornithischians, or was
it the ornithischians who enabled the angiosperms to take over? There are
reasons to suspect either scenario, but every explanation has shortcomings
(usually, they do not fit precisely with the apparent succession in the fossil
record, which is a bit messy in this part). One can look at several aspects
here: the different methods of digesting plant material by the sauropods and
the ornithischians; the levels at which the two herbivore groups graze; the
different features of conifers and angiosperms; and the effect of heavy grazing
on plants (the paleontologist Robert T. Bakker proposed that the
low-grazing ornithischians may have caused the decline of conifers by eating up
their shoots, so few new conifer plants would grow, while the sauropods eat all
the full-grown trees; the conifers would effectively disappear, and the
sauropods would starve to death, giving the ornithischians and the fast-growing
angiosperms room to spread and take over).
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