Chew on this

One important quality of ornithischians is that, to a greater or lesser extent, all ornithischians apparently chewed their food. Because humans and many other mammals chew, it can be surprising to learn that most vertebrates don't chew; that teeth are really most commonly used only for biting off chunks of whatever is being eaten. The fundamental act of chewing - of grinding food down to a paste that can be digested relatively efficiently - is, as we shall see in the following chapters, done by other organs in most vertebrates. But ornithischian dinosaurs got into the chewing game, so a look at what chewing's all about is useful for understanding these dinosaurs.

Chewing in mammals

We start by looking at the basics of chewing in a familiar living group: mammals. Among herbivorous mammals, no matter the size, the skull is generally divided into three sections (Figure II.3): at the front is the cropping part, where blade-like teeth (generally incisors) bite off chunks of food. Behind the cropping section is the diastem, a gap that is toothless (or nearly so), likely used for the manipulation of the food by the tongue. Finally, further back in the mouth are the cheek teeth (molars in mammals) - most commonly a block of teeth, relatively tightly fitted against one another, which are used to grind or shear plant material. In mammals the upper and lower cheek teeth occlude - or fit tightly against each other when the jaw is shut - which ensures that, as the chewing takes place, the grinding is efficient.

Ugetiere Gebisstypen

Figure II.3. Selected herbivorous mammal skulls (not drawn to scale). (a) Horse (Equus), (b) llama (Lama), (c) rabbit (Lepus), and (d) rat (Rattus). Divisions of skulls indicate: anterior cropping section (dark blue), diastem (grey), block ofgrinding cheek teeth (light blue), and coronoid process (black). Despite the range of sizes and herbivorous behaviors, all skulls show the same basic organization.

Two other features are associated with chewing in mammals. Firstly, toward the back of the lower jaw is a large expansion of bone, the coronoid process, which serves as an attachment site for strong jaw-closing muscles (Figure II.3). Secondly, the tooth row is deeply inset toward the midline of the skull. This makes room for cheeks, muscular tissues that play the obviously essential role of keeping food in the mouth while it is being chewed.

In mammals, then, chewing leaves a recognizable imprint on the design of the skull, the lower jaw, and the teeth. It's striking that in almost all ornithischian dinosaurs, many of these same adaptations for chewing can be found.

Chewing in dinosaurs

The primitive tetrapod condition is that in which the jaw joint is right at the same level as the tooth row (Figure II.4a). The jaw thus functions a bit like a scissors: the bite slices sequentially, from the back of the jaw forward. This means that the teeth in the upper jaw move past those in the mandible as the jaw closes.

By contrast, when the jaw joint is below the level of the tooth row as it is in all orni-thischians, the jaw functions a bit like a water-pump pliers, in which the jaws close simultaneously along their entire length (Figure II.4b). The blocks of cheek teeth grind against each other simultaneously instead of sheering past each other sequentially. As it turns out, even

Figure II.4. Positions ofjaw joints. When the jaw joint is at the same level as the tooth rows, it functions a bit like a pair of scissors, slicing sequentially from the back ofthe jaw forward (a). By contrast, when the jaw joint is below the level ofthe tooth rows, as it is in all ornithischians, the blocks of cheek teeth grind against each other simultaneously, much like the water-pump pliers (b).

Figure II.4. Positions ofjaw joints. When the jaw joint is at the same level as the tooth rows, it functions a bit like a pair of scissors, slicing sequentially from the back ofthe jaw forward (a). By contrast, when the jaw joint is below the level ofthe tooth rows, as it is in all ornithischians, the blocks of cheek teeth grind against each other simultaneously, much like the water-pump pliers (b).

the most primitive ornithischians had the required goods for serious chewing. And the many ornithischians that followed never kicked the habit.

In many dinosaurs, including all ornithischians, the cropping function of the mouth was carried out not by teeth, but by a beak, or rhamphotheca. Rhamphothecae were made of keratin, a protein-based substance that makes up horns, nails, hooves, and claws. It is also a key ingredient of hair, although you'll never find that in a dinosaur!

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