Australia: The Land Where Time Began

A biography of the Australian continent 

Iguanodontids

They were ornithopod dinosaurs, medium to large size, 5-10 m (16.5-33 ft), from the Late Jurassic to the end of the Cretaceous. During the Late Jurassic and Early Cretaceous they coexisted with the hypsilophodontids. When the hadrosaurs, the most advanced ornithopods, appeared they went into decline, apparently surviving only in Western Europe until the dinosaurs died out at the end of the Cretaceous. They were large, heavy animals with broad feet and massive hind limbs and large muscular forelimbs with fingers that that had broad, flattened claws that resembled hoofs, suggesting that their main use may have been for walking, instead of appearing as if they were used for grasping as was the case with the hypsilophodontids. Compared with the hypsilophodontids, which they resembled in overall shape, they are very heavy. Comparison with the largest known hypsilophodontid, Tenontosaurus, it seems their bulkiness may have been mostly a function of size. They had a number of structural features of the skull, teeth, forelimbs and hips that allow them to be assigned to their own family. They retained the basic ornithopod characteristics such as a tail counterbalancing the body as they walked on their hind legs, though were also capable of walking as a quadruped, as is suggested by the presence of  blunt, hoof-like claws on their fingers.

According to the author², in the 19th century Louis Dollo described the discovery of Iguanodon at Bernissart, Belgium, that had been found between 356 and 322 m depth in a coal mine, that were referred to by Dollo as a 'giraffe reptilienne', thogh his reconstruction of the animals has since been found to be almost completely incorrect or at least misleading. As the coal seams being mined were of Palaeozoic age it was unexpected that dinosaurs from the Mesozoic would be found in between coal-bearing strata of the mine. The dinosaur fossils were not actually found in the coal-bearing strata, but rather in pockets cut into the coal beds that were filled with shale of Cretaceous age. Mapping of the mine found that in cross-section the pockets seemed to indicate that they were steep ravines that had been cut into the much older coal-bearing rocks. This led Dollo to believe that the dinosaurs been living in a narrow gorge. Others suggested that they had actually fallen into the gorge when the herd was chased by predators, megalosaurs. This appeared to possibly be supported by the finding, though very rarely, of fragments of bones of large carnivorous dinosaurs in the same beds as the Iguanodons. And there were also chunks of charcoal-like coal present in the rubble between some of the pockets and coal-bearing beds. Before the dinosaurs were discovered it was known that the coal mining area of the Mons Basin where Bernissart was situated had a history of the ground being pock-marked by subterranean pits that formed naturally ('cran'). Each of these cran, of limited extent, was filled with shales. It had been concluded that these were formed when Palaeozoic rocks deep in the rock strata had been dissolved forming large caverns, the roofs of which eventually collapsed under the weight of the overlying rocks, the caverns being filled by the rock layers deposited above them, soft clays and shales. In the Mons area there were earthquake-like rumblings whenever one of cavern roofs collapsed.

According to the author² when the skeletal material from Bernissart was re-examined it was found that the evidence simply didn't conform to the early reconstructions of Iguanodon that had the animal in a posture like a kangaroo, with its tail being used as a prop while it fed. In this reconstruction the tail is curved upward to the hip, whereas all the evidence, documentary and fossil, indicates it held its tail essentially straight, or slightly downwardly curved. The partly prepared skeletons that are on display at the museum in Belgium, as well as the sketches made of them before they were removed from the fossil site, show this virtually horizontal tail. The author² suggests the argument that the straight tail is an artefact of preservation is not plausible with these specimens. On either side of the spine it was 'trussed' by long bony tendons in a trellis-like array that held the spine straight, with the result that the tail that was heavily muscled functioned as an enormous cantilever balancing the weight of the front part of the body on the hips. from the structure of the tail it can be seen that Dollo's reconstruction was physically impossible for the animal. When the old reconstruction was re-examined it was found that the tail had been deliberately broken to assemble it in the form envisioned by Dollo. When the animal is reconstructed with the tail horizontal and balanced on the hips the other parts of the body change position so that the arms are in a position to reach the ground, suggesting a possible explanation for the hoof-like claws on the central 3 fingers on the hand.

Hands or front legs

The first finger (thumb) is comprised of a large bone with a conical claw sticking up at right angles to the hand that had very little movement. The 2nd, 3rd and 4th fingers had 3 long bones, metacarpals, forming the palm of the hand that are tightly bound together by ligaments. The short, stubby fingers are jointed to the ends of the metacarpals, and in end in flattened and blunt hooves. The author² says manipulation of these bones to determine their range of movement found that the fingers splayed outwards, separating from each other showing they were physically incapable of flexing to grasp anything, as would be expected if they were used as a grasping hand, in fact they bore a strong resemblance to the arrangement of their toes, in which the 3 central toes all have a similar arrangement of joints and appear similar to each other, splay apart and have flattened hooves on their ends. The 5th finger is different from all the others and separated from them as is the human thumb, though it was long and apparently more flexible than a thumb, in fact it is believed to have been unusually flexible, having a range of movement at each joint.

The author² concludes on the basis of the re-examination that the hand of Iguanodon is the most peculiar such structure of any known animal, extinct or extant. The author² refers to the thumb of this unusual hand as 'without doubt an impressive, stiletto-like weapon of defence'. The 3 central toes were obviously designed as weight-bearing structures, rather than as grasping structures as is the usual function of hands. the 5th finger was long enough and sufficiently flexible to act as a true finger that is prehensile, designed for grasping, a veritable 'Swiss army knife', different fingers being highly adapted for specific functions.

Further work was carried out on the arm to determine whether or not it was actually capable of being used for walking as as well as the other functions. The wrist bones are fused together to form a solid block, the individual bones being firmly joined to each other by bony cement so that there was no possible movement that would allow the hand to swivel against the forearm. Strands of bony ligament also wrapped around these cemented wrist bones to further strengthen the structure. These adaptations that fused the hand to the forearm bones indicate that the arm could be used to bear weight, allowing them to act as feet, while retaining the specialised functions of the thumb and 5th finger.

The remainder of the arm bones were built to be extremely strong, another way in which it was designed to bear weight, instead of the flexibility of normal arms. As a result of the adaptations of the arm when the hand was placed on the ground the fingers pointed outward and the palm inward, an arrangement the author² suggests is an unusual consequence of converting a hand into a foot. Tracks of this animal have been found that clearly show this unusual condition in which the the toes point outwards.

The humerus is described as 'rather pillar-like' and there are indications that it was an anchor for huge arm and shoulder muscles. The arm is also unusually long for an animal that was not a full quadruped being more than 3/4 the length of the leg. In the original construction the arms appeared shorter because they had been folded against the chest.

The large size and powerful build of the shoulder bones make sense if the arm was being used as a leg, though there are also unexpected features of the shoulder. The larger specimens at Bernissart also had an irregular bone that grew between the shoulder joints across the centre of the chest. Of pathological origin, this intersternal ossification grew as a result of strain within the chest when the animal walked as a quadruped.

Following all these observations a reassessment of the natural posture of Iguanodon has the backbone horizontal, the weight being distributed along the spine and largely balanced at the hips and supported by the massive, strong hind legs. Distributed along the spine they had ossified tendons above the chest, hip and tail that the author² says were clearly tensioners that distributed the weight along the spine. In this pose the front limbs could reach the ground where they were used for weight support when the animal was not moving. It is believed that for at least most of the time they probably moved slowly quadrupedally.

Iguanodon skin

Small patches of skin impressions were found at Bernissart that shoe Iguanodon had finely scaled skin similar to that of living lizards.

Teeth

The first fossils to be found of Iguanodon were teeth. It was seen from these teeth that it was herbivorous, with chisel-shaped teeth slice and crush plant material in the mouth prior to swallowing. At the front of the mouth was a horny beak for cropping plant material that was then moved to the sides of the jaws that were lined with chisel-like teeth that form irregular edged blades in an almost parallel array. The working teeth were slotted against the neighbouring teeth in a rank- and-file arrangement. There were replacement crowns in the jaw beneath the working teeth that slot into place when a tooth is worn away. Reptiles in general have this arrangement of continuous replacement of teeth. The feature of Iguanodon teeth that is unusual, even among reptiles, is the way working teeth and replacement teeth are held together in an ever-growing magazine as though they were all part of a single giant, grindstone-like tooth. Throughout the life of the Iguanodon wear between the upper and lower teeth maintained a grinding surface. The author2 suggests this could be called a disposable model relying on the continuous replacement of individually simpler teeth.

The lower teeth have an inner surface that is coated with thick enamel that is very hard, the outer surface being  and the remainder of the tooth was made of dentine that is bone-like material that is not as hard as the enamel. The upper teeth have the thick enamel on the outer surface of the teeth. Opposing blades slide past each other when the mouth closed, the hard enamel leading edge of the lower jaw teeth meeting the enamel side of the upper teeth in a cutting/shearing like scissors. After passing the enamel surface of the opposing teeth the enamel surfaces then cut against the softer dentine in a tearing and grinding action, that was unlike the action of scissors but ideal for crushing tough plants fibres.

The worn surfaces of the upper and lower 'magazines' are oblique, the lower surfaces facing outward and upwards, the worn surfaces of the upper upper teeth face inwards and downwards. The result of this pattern is that, unlike that of conventional reptiles in which closing of the lower jaw occurs by a simple hinge effect, the simultaneous closing of the jaws on either side of the mouth, an isognathic bite, the Iguanodon had a completely different type if bite. The author² suggests there would need to have been some ability of the jaws to move sideways as they closed for angled wear surfaces to have developed.

An anisognathic jaw closure is used by living herbivorous mammals to achieve this type of movement. In the extant herbivorous mammals the lower jaw is narrower than the upper jaw. Controlling very precisely the position of the jaw so that the teeth on one side of the jaw meet, after which the lower teeth are forcibly moves inwards, grinding the teeth against one another. This type of jaw movement is also used by humans, but in such mammals as cattle, sheep and goats the movement of the jaws is far more exaggerated to the extent that the movement is obvious to an observer.

The mammalian type of jaw movement is dependent on a very complex arrangement of jaw muscles, as well as a complex nervous control system and a set of skull bones that are specially constructed to cope with the stresses that result from this type of chewing. Iguanodon, as well as more conventional reptiles don't have an anisognathic jaw arrangement, and lack the complex muscular arrangement that allows for the very precise lower jaw positioning, the presence of the complex nervous system is said by the author² to be irrelevant, and there is no specific reinforcing of the skulls to withstand the lateral forces that act on the bones of the skull.

Iguanodon does not fit any of the models, either the anatomy was wrong or it was doing something different.

In Iguanodon the lower jaw bones bones were strong and complex, with a predentary bone clamping the front of the 2 halves of the lower jaw together. The arrangement of the teeth are essentially parallel to the length of the jaw, and there was a coronoid process, a tall prong of bone, at the rear as an attachment for powerful jaw muscles, and a lever that increased the closing force that could be exerted on the teeth. A group of tightly clustered bones supporting the hinge-like jaw joint are behind the coronoid process. In the act of biting the upper jaw would have been subjected to vertical forces resulting from the upward closing of the lower jaws and teeth against the upper, as well as sideways forces generated by the lower teeth wedging themselves between the upper teeth as the biting force increased.

Sideways forces were the forces the Iguanodon skull was least able to withstand. The long snout, the area in front of the eyes, had a U-shaped cross section. Without the bracing of bony 'joists' connecting the upper jaws that developed in mammals it needed some way of resisting the sideways forces acting on the teeth. If it had no such bracing the head could be expected to be vulnerable to splitting along the midline as a result of the great leverage created by the depth of the cheek bones against the roof of the snout by the forces acting on the teeth.

This potentially catastrophic midline breakage was avoided by Iguanodon by diagonally arranged hinges down either side of the skull  allowing the sides of the skull to flex outwards simultaneously as the lower teeth were forced between the upper teeth. The amount of movement possible along this hinge so the the upper jaws didn't flop around loosely was controlled by structures within the skull. The author² named this system pleurokinesis ('slide movement'). A grinding motion between opposing sets of teeth is allowed by the pleurokinetic mechanism.

The author² suggests this chewing mechanism could be linked to another important observation in Iguanodon as well as other dinosaurs of this kind, the teeth being recessed, set inwards, from the side of the face, that creates a depression that could possibly have been covered by a fleshy cheek, a feature that is very un-reptilian. The author² suggests that given that the upper teeth slide past the lower teeth in the cutting up of food it is logical that with every bite at least half of the food would be lost from the sides of the mouth, unless it was caught and recycled in the mouth by some sort of fleshy cheek.

The author² concludes that as well as having a surprisingly sophisticated method of chewing food they also had cheeks that were mammal-like, they also would have had a large muscular tongue, and strong certaobranchials - the tongue muscle bones.

Following the identification of this chewing mechanism, the author² recognised pleurokinesis in other ornithopod dinosaurs, finding it was actually widespread among ornithopods, the group to which Iguanodon belonged. According to the Author2 ornithopods became increasingly diverse and abundant over time during the Mesozoic. By the latest Cretaceous the ornithopods reached their greatest expression in the ecosystems of the time, apparently being the most numerous of all known land animal fossils. Ornithopods were represented by hadrosaurs, duck-billed dinosaurs, that were very abundant and diverse. Evidence has been found in North America in the latest Cretaceous of hadrosaurs herds that may have numbered in the many tens of thousands. The dental grinders, each with as many as 1000 teeth at any one time, of hadrosaurs were the most sophisticated as well as having a well-developed pleurokinetic system.

The author² suggests that the abundance and diversity of these dinosaurs was due to a large extent because they were efficient at eating plant food by means of the pleurokinetic system .He also suggests that it was the pleurokinetic system, first identified in Iguanodon, that led to their evolutionary success.

Brain of Iguanodon

At the front were large olfactory lobes, suggesting a sense of small that was well-developed. Large optic nerves passed through the braincase towards the large eye sockets, indicating that they had good vision. A well-coordinated and active animal is suggested by the large cerebral lobes. It had a looped semicircular canals on the cast of the inner ear, and a finger-like structure that was part of its hearing system. There is a pod-like structure housing the pituitary gland responsible for regulating its hormone functions. There are a series of large tubes down either side of the cast representing the passages trough the original braincase wall. There are also other smaller pipes and tubes that pass through the braincase wall that have been preserved hinting at the distribution of a set of blood vessels that carried blood from the heart to the floor of the brain, via the carotid artery and drained the blood from the brain through the large lateral head veins leading back down the neck.

Sources & Further reading

1. Long, John A, 1998, Dinosaurs of Australia and New Zealand, University of New South Wales Press.

2. Norman, David, 2005, Dinosaurs: A Very Short Introduction, Oxford University Press