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 M 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 | ofjaw mechanism, especially when eating tough foods, but it is far ; more exaggerated in some classically herbivorous mammals such o as cows, sheep, and goats, where the swing of the jaw is very §

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