remainder of the hand; it is also long and has a wide range of §

movement at each joint, and was presumably unusually flexible. o

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.

21A. Iguanodon's hand, showing a range of uses

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.

21B. The stiletto-like thumb of Iguanodon in action

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.

The upper arm (humerus) is massive, rather pillar-like, and shows evidence that it anchored huge arm and shoulder muscles. This is

22. New reconstruction of Iguanodon

also unusually long, over three-quarters the length of the hind limb. The true size of the arms is somewhat masked in the original skeletal reconstructions, because they were folded against the chest and always seemed shorter than they really are.

Finally, the shoulder bones are large and powerfully built, which makes perfect sense if the arms are functioning as legs. But the shoulders show another unexpected feature. In the centre of the chest of the larger skeletons at Bernissart there is an irregular bone that grew in the soft tissues across the centre of the chest between the shoulder joints. This bone is pathological in origin - it formed as a response to strain within the chest created while the animal was walking on all fours (and is called an intersternal ossification).

Reassessing the posture of Iguanodon in the light of these observations, it seems clear that a more natural pose of the |

backbone was horizontal, with the body weight distributed ;

along the backbone and largely balanced at the hips and o supported by the massive and strong hind legs. The ossified §

tendons distributed along the spine, above the chest, hip, and tail, § clearly acted as tensioners to distribute body weight along the o backbone. This pose allowed the front limbs to reach the ground, and these were used for weight support while these animals were stationary. Iguanodon probably moved slowly on all fours at least part of the time (Figure 22).

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