The whole mammalian type ofjaw 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.
Iguanodon appears to present us with a conundrum: it does not fit any of the expected models. Is the anatomy wrong, or was this dinosaur doing something unexpected?
The lower jaws of Iguanodon are strong, and quite complex, bones. At the front end each lower jaw is clamped to its neighbour by the predentary bone. The teeth are arranged essentially parallel to the length of the jaw, and at the rear there is a tall prong (coronoid process) of bone which acts as an attachment area for powerful jaw-closing muscles, and as a lever to improve the force of closure that can be exerted on the teeth. Behind the coronoid process are a group of tightly clustered bones that support the hinge-like jaw joint. The upper jaws, during biting, would have been subjected not only to vertical forces, created by the upward closure of the lower jaws and teeth against the uppers, but also to sideways forces generated by the lower teeth wedging themselves between the upper teeth as the bite force increased.
Of all forces acting on the skull of Iguanodon, the ones that it is least well-equipped to deal with are sideways forces acting on its teeth. The long snout (the area in front of the eye sockets) has a deep inverted 'U' shape in cross-section. To resist sideways forces acting on the teeth, the skull would need to be braced by bony 'joists' connecting the upper jaws; this is the arrangement found in living mammals. Without such bracing, the skull of Iguanodon is very vulnerable to splitting along its midline simply because the depth of the cheek bones creates great leverage against the roof of the snout from forces acting on the teeth. Midline breakage of the skull was avoided by the provision of hinges that are arranged diagonally down either side of the skull; these allow the sides of the skull to flex outward simultaneously as the lower teeth force their | way between the upper ones. Other features deeper within the skull ; helped to provide control over the amount of movement that was o possible along this hinge (so that the upper jaws did not simply flop §
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