Iguanodon and dietary

The first recognizable fossils of Iguanodon were teeth, whose telltale
features showed that it was a herbivorous animal; they were
chisel-shaped to be able to slice and crush plants in the mouth
before they were swallowed.

The need to cut and crush plant food hints at some important
considerations concerning the diets of extinct creatures and some of
the clues that their skeletons may contain.

Carnivores have a diet largely comprising meat. From a biochemical
and nutritional perspective, a diet of meat is one of the simplest
and most obvious of options for any creature. Most of the other
creatures in the world are made of roughly similar chemicals as the
carnivores that eat them. Their flesh is therefore a ready and rapidly
assimilated source of food, provided the prey can be caught, sliced
into chunks in the mouth using simple knife-like teeth (or even
swallowed whole), and then quickly digested in the stomach.
Iguanodon’s brain
The structure of the brain cavity shows large olfactory
lobes at the front, suggesting that Iguanodon had a welldeveloped
sense of smell. Large optic nerves passed through
the braincase in the direction of the big eye sockets, apparently
confirming that these animals had good vision. The
large cerebral lobes indicate a well coordinated and active
animal. The inner ear cast shows the looped semicircular
canals that provided the animal’s sense of balance, and a
finger-like structure that was part of its hearing system.
Beneath the brain cavity hangs a pod-like structure that
housed the pituitary gland, which was responsible for regulating
its hormone functions. Down either side of the cast
are seen a series of large tubes, which represent the passages
through the original braincase wall (chipped away
here of course) for the twelve cranial nerves. Other smaller
pipes and tubes passing through the braincase wall are also
preserved, and these hint at the distribution of a set of blood
vessels that carried blood into the floor of the brain from the
heart (via the carotid artery) and, of course, drained the
blood away from the brain through the large lateral head
veins that lead back down the neck.

This whole process has the potential to be relatively quick and
biochemically very efficient in that little is likely to be wasted.
Herbivores face a rather more challenging problem. Plants are
neither particularly nutritious nor readily assimilable when
compared to animal flesh. Plants are primarily built from large
quantities of cellulose, a material that gives them strength and
rigidity. The crucial, and extremely awkward, point about this
unique chemical, so far as animals are concerned, is that it is
completely indigestible: there is simply nothing in the armoury of
chemicals in our guts that can actually dissolve cellulose. As a result,
the cellulose portion of plants passes straight through animals’ guts
as what we call roughage. So, how do herbivores survive on what
appears to be such an unpromising diet?
Plant-eaters have successfully adapted to this diet because they
exhibit a number of characteristic features. They have a good set
of teeth with hard-wearing, durable, complex, and rough grinding
surfaces, and powerful jaws and muscles that can be used to grind
up plant tissues between the teeth to release the nutritionally
usable ‘cell sap’ that is enclosed within plant cell walls. Herbivores
eat large quantities of plant food in order to be able to extract
sufficient nutrients from such comparatively nutrient-poor
material. As a result, herbivores tend to have barrel-shaped bodies
that accommodate large and complicated guts, which are
necessary to store the large volumes of plants that they have to eat
and allow sufficient time for digestion to take place. Herbivores’
large guts house dense populations of microbes that live within
special chambers or pouches in the gut wall; our appendix is a
tiny vestige of such a chamber, and hints at herbivory in our
primate ancestry. This symbiosis allows herbivorous animals to
provide a warm, sheltered environment and constant supplies of
food for the microbes; in their turn, the microbes have the ability
to synthesize cellulase, an enzyme that digests cellulose and
converts it into sugars that can then be absorbed by the host
animal.

By most standards, Iguanodon (11 metres long and weighing about
3–4 tonnes) was a large herbivorous animal, and would have
consumed plants in large quantities. Given this background
information, questions about precisely how Iguanodon fed and
assimilated its food can be explored in detail.
One persistent theory concerning its method of feeding was its
suggested use of a long tongue to pull vegetation into the mouth.
This began with Gideon Mantell, who described one of the first,
nearly complete lower jaws of Iguanodon. The new fossil included
some tell-tale teeth, so the ownership could not be doubted, and
it had a toothless, spout-shaped front end. Mantell speculated
that the spout shape allowed a long tongue to slide in and out of
the mouth, rather like a giraffe’s does. Mantell could not have
known that the tip of the newly discovered lower jaw was
incomplete and was capped by a predentary bone that filled in
the ‘spout’.

Careful re-examination of the lower jaws of a number of Iguanodon
skulls from Bernissart failed to reveal Dollo’s predentary tunnel.
The predentary has a sharp upper edge that supported a turtle-like
horny beak. The predentary, and its beak, bit against the similarly
toothless beak-covered premaxillae at the tip of the upper jaw, and
this arrangement allowed these dinosaurs to very effectively crop
the plants upon which they were feeding. The advantage of the
horny beak was that it would have grown continuously (unlike
teeth, which gradually wear away) no matter how tough and
abrasive the plants that were being cropped. The ceratobranchial
bones still require some explanation. In this instance, they would
have been used to anchor the muscles that moved the tongue
around the mouth to reposition the food as it was being chewed
and for pushing the food back into the throat when it was
ready to be swallowed. This is exactly the same role that is
performed by the ceratobranchial bones in the floor of the
human mouth.

How Iguanodon chewed its food
Apart from the horny beak that was able to nip off plants at the
front end of the mouth, the sides of the jaws are lined with a
formidable, nearly parallel array of chisel-like teeth that form
irregularly edged blades (Figure 26). Each working tooth slots
neatly against its neighbours in a rank-and-file arrangement, and
beneath the working teeth are replacement crowns that will slot
into place as the working teeth are worn away, forming what is in
effect a ‘magazine’, or battery, of teeth. This continuous replacement
pattern is normal for reptiles in general. What is unusual, even by
reptile standards, is that the working and replacement teeth are
held together in an ever-growing magazine as if they were all
contributing to one giant, grindstone-like tooth. Wear between
opposing (upper and lower) magazines maintains a grinding
surface throughout the life of the dinosaur. Rather than having
permanent, hard-wearing grinders (as we do), this could be
described as a disposable model that relies on constant replacement
of individually simpler teeth.

Opposing edges of each cutting blade of teeth have characteristics
that ensure efficiency in their cutting action. The inner surfaces
of the lower teeth are coated in a thick layer of extremely hard
enamel, while the remainder of the tooth is made of softer,
bone-like dentine. In contrast, the upper teeth have the reverse
arrangement: the outer edge being coated in thick enamel and
the remainder of the tooth is composed of dentine. When the
jaws are closed, these opposing blades slide past each other: the
hard, enamelled leading edge of the lower jaw magazine meets
the enamelled cutting edge of the upper teeth in a cutting/
shearing action rather like the blades of a pair of scissors (Figure
27). Once the enamelled edges have passed one another, the
enamel edges (unlike scissor blades) then cut against the less
resistant dentine parts of opposing magazines in a tearing
and grinding action, which is ideal for crushing up tough
plant fibres.

The geometry of the grinding surfaces of the upper and lower
‘magazines’ is particularly interesting. The worn surfaces are
oblique, the lower surfaces face outward and upward, while the
upper teeth have worn surfaces that face inward and downward.
This pattern has interesting consequences. In conventional reptiles,
the closure of the lower jaw is brought about by a simple hinge
effect, with the jaws on either side of the mouth closing
simultaneously in what is called an isognathic bite. If this type of
bite is proposed for Iguanodon, then it is immediately obvious that
the two sets of teeth on either side of the mouth would simply
become permanently wedged together: the lower jaws jamming
inside the upper ones. This means it is impossible to imagine
how the angled wear surfaces could ever have developed in the
first place.

For the angled wear surfaces to have developed, there would
have had to be some ability of the jaws to move sideways as they
closed. This type of movement is achieved in living herbivorous
mammals through the development of an anisognathic jaw closure
mechanism. This relies on the fact that the lower jaws are naturally
narrower than the upper jaws. Special muscles, arranged in a sling
on either side of each jaw bone, are capable to controlling the
position of the jaw very precisely so that the teeth on one side meet
one another and then the lower set is forcibly slid inwards so that
the teeth grind against one another. We humans employ this type
of jaw mechanism, especially when eating tough foods, but it is far
more exaggerated in some classically herbivorous mammals such
as cows, sheep, and goats, where the swing of the jaw is very
obvious.

The whole mammalian type of jaw mechanism is dependent upon
very complex jaw muscles, a complex nervous control system, and a
specially constructed set of skull bones to withstand the stresses
associated with this chewing method. By contrast, more
conventional reptiles. of which Iguanodon was one, do not have
an anisognathic jaw arrangement, lack the complex muscular
arrangements that allow the lower jaw to be very precisely
positioned (whether they had the nervous system to control
such movements is largely irrelevant), and their skulls are not
specially reinforced to withstand the lateral forces acting on the
skull bones.