More on Iguanodon

A ‘twist’ in the tail
Re-examining the skeletal evidence from first principles, the
anatomy of the skeletons from Bernissart reveal some disconcerting
features. One of the most obvious concerns the massive tail of
Iguanodon. The well-known reconstruction shows the animal
(Figure 12) propped, in true kangaroo style, using its tail and hind
legs tripod-like. To adopt this posture, the tail curves upward to the
hip. In sharp contrast, all the documentary and fossil evidence
points to this animal normally holding its tail essentially straight or
somewhat downwardly curved. This is clearly seen in the specimens
arranged on banks of plaster in the museum, and in the wonderful
pencil sketches made of their skeletons before they were exhibited
(Figure 20). It could of course be argued that this shape was simply
an artefact of preservation, but this explanation is definitely not
plausible here. The backbone was in effect ‘trussed’ on either side by
a trellis-like arrangement of long bony tendons that held the
backbone quite deliberately straight; these can be seen in Figure 20.
As a result, the heavy, muscled tail served as an enormous cantilever
to balance the weight of the front part of the body at the hips. The
truth is that the upward sweep of the tail seen in Dollo’s
reconstructions would have been physically impossible for these
animals in life. Careful examination of the skeleton revealed that
the tail was deliberately broken in several places to achieve the
upward bend – a case perhaps of Louis Dollo making the skeleton
fit his personal ideas a little over-zealously.
This discovery disturbs the pose of the remainder of the skeleton. If
the tail is straightened so that it can adopt a more ‘natural’ shape,
then the tilt of the body changes dramatically, with the backbone
becoming more horizontal and balanced at the hip. As a result the
chest is lower, bringing the arms and hands closer to the ground
and raising questions about their likely function.
Hands or feet?

The hand of Iguanodon has become part of dinosaurian folklore
for one obvious reason. The conical thumb-spike was originally
identified as a rhinoceros-like horn on the nose of the Iguanodon
(Figure 9) and was immortalized in the giant concrete models
erected at London’s Crystal Palace (Figure 2, Chapter 1). It was not
until Dollo provided the first definitive reconstruction of Iguanodon
in 1882 that it was proved to everyone’s satisfaction that this bone
was indeed a part of the dinosaur’s hand. However, the hand (and
the entire forelimb) of this dinosaur held a few more surprises.
The thumb, or first finger, comprises a large, conical, claw-bearing
bone that sticks up at right angles to the rest of the hand and can be
moved very little (Figure 21A). The second, third, and fourth fingers
are very differently arranged: three long bones (metacarpals) form
the palm of the hand and are bound tightly together by strong
ligaments; the fingers are jointed to the ends of these metacarpals
and are short, stubby, and end in flattened and blunt hooves. When
these bones were manipulated, to see what their true range of
movement was likely to be, it was found that the fingers splayed
outwards (away from each other) and certainly could not flex to
form a fist and perform simple grasping functions, as might have
been expected. This distinctive arrangement looks similar to that
seen in the feet of this animal: the three central toes of each foot are
similarly shaped and jointed, splay apart, and end in flattened
hooves. The fifth finger is different from all the others: it is quite
separate from the previous four and set at a wide angle from the
remainder of the hand; it is also long and has a wide range of
movement at each joint, and was presumably unusually flexible.
This re-examination led me to dramatically revise earlier ideas and
conclude that the hand is one of the most peculiar seen in the entire
animal kingdom. The thumb was without doubt an impressive,
stiletto-like weapon of defence (Figure 21B); the three central
fingers were clearly adapted to bear weight (rather than for
grasping things as hands usually do); and the fifth finger was
sufficiently long and flexible to act as a truly finger-like grasping
(prehensile) organ (Figure 21A).

The idea that the hand could act as a foot for walking upon, or at
least supporting some of the body weight, was revolutionary – but
was it true? This prompted further research on the arm and
shoulder for additional evidence that might confirm such a radical
reinterpretation.

First of all, the wrist proved to be interesting. The bones of the wrist
are welded together to form a bony block, instead of being a row of
smooth, rounded bones that could slide past one another in order to
allow that hand to swivel against the forearm. All the individual
wrist bones have been welded together by bony cement, and are
further strengthened around the outside by strands of bony
ligament. These features obviously combined to lock the wrist
firmly against the hand and forearm bones and resist the forces
acting through them during weight-bearing, as would be necessary
if the hands were truly acting as feet.

The remainder of the arm bones are extremely stoutly built, again
primarily for strength during weight support, rather than for
allowing flexibility as is more normal with genuine arms. The
stiffness of the forearm has important consequences for the way in
which the hand would have been placed on the ground – the fingers
would have pointed outward and the palms inward – an unusual
consequence of converting a hand into a foot. The pose of the hand,
in this rather awkward manner, has been confirmed by examination
of the shape of forefoot prints left by this dinosaur.

Size and sex
The Bernissart discoveries are notable for comprising two
types of Iguanodon. One (Iguanodon bernissartensis – quite
literally ‘the Iguanodon that lived in Bernissart’) is large and
robustly built, and represented by more than 35 skeletons; the other
(Iguanodon atherfieldensis, formerly called I. mantelli – literally
‘Mantell’s Iguanodon’) is smaller and more delicately built
(approximately 6 metres in length) and represented by only
two skeletons.

These specimens were regarded as distinct species until they
were reassessed in the 1920s, by Francis Baron Nopcsa, a nobleman
from Transylvania and a palaeontologist. The discovery of two
quite similar types of dinosaur that evidently lived in the same
place, at the same time, prompted him to ask the simple and yet
obvious question: are they males and females of the same species?
Nopcsa attempted to determine sexual differences in a number of
fossil species. In the case of the Iguanodon from Bernissart he
concluded that the smaller and rarer species was the male and the
larger and more numerous species was the female. He observed,
perfectly reasonably, that it is often the case that female reptiles
are larger than males. The biological reason for this is that
females often have to grow large numbers of thick-shelled
eggs; these drain considerable resources from the body before
they are laid.

While this seems quite a reasonable supposition, it is in fact
very difficult to prove scientifically. Apart from size, which is
surprisingly variable among reptiles as a whole and not nearly
as consistent a feature as Nopcsa would have had us believe,
the features used to distinguish the sexes among living reptiles
are most commonly found in the soft anatomy of the sex organs
themselves, coloration of the skin, or behaviour. This is particularly
unfortunate because only very rarely do fossils ever preserve such
features.

The most valuable evidence would be the discovery of soft
anatomical fossils of the sexual organs of Iguanodon – unfortunately,
this is an extremely unlikely event. And, since we can never know
their true biology and behaviour, we have to be a little cautious
and also realistic. For the present, it is safer to record the differences
(we may have our own suspicions, perhaps), but simply leave it
at that.

A careful study of the more abundant large Iguanodon from
Bernissart revealed that a few were smaller than the average.
Measuring the proportions of each of these skeletons revealed
an unexpected growth change. Smaller, presumably immature
specimens had shorter arms than would have been expected.
The comparatively short-armed juveniles may well have
been more adept bipedal runners, but as large adult size and
stature was achieved they may have become progressively
more accustomed to moving around on all fours. This also
fits with the observation of an intersternal ossification in
only larger, presumably adult, individuals, which spent more
of their time on all fours compared to smaller, younger
individuals.

Soft tissues
Soft tissues of fossil creatures are preserved only rarely, and under
exceptional preservational conditions, so palaeontologists have
developed techniques to decipher clues concerning this type of
biology of dinosaurs both directly and indirectly.
Louis Dollo reported small patches of skin impression on parts
of the skeletons of Iguanodon. A number of the skeletons from
Bernissart are shown in a classic ‘death pose’ with the powerful neck
muscles contracted, during rigor mortis, pulling the neck into a
sharp curve and turning the head upward and backward. That this
pose has been maintained during the time between death and
eventual burial implies that the carcass of the animal had stiffened
and dried out. Under such conditions, its tough, parchment-like
skin would have formed a rigid surface against which the finegrained
muds would have moulded themselves during burial.
Provided that the entombing sediment compacted sufficiently to
retain their shape, prior to the inevitable rotting and disappearance
of the dinosaur’s organic tissues, then (as with simple clay moulds)
an impression of the texture of the skin surface would have been
preserved in the sediment.