Teeth And Bonesthe Hard Stuff

It almost goes without saying that what makes teeth special among organs is their hardness. Teeth have to be harder than the bits of food they break down; imagine trying to cut a steak with a sponge. In many ways, teeth are as hard as rocks, and the reason is that they contain a crystal molecule on the inside. That molecule, known as hydroxyapatite, impregnates the molecular and cellular infrastructure of both teeth and bones, making them resistant to bending, compression, and other stresses. Teeth are extra hard because their outer layer, enamel, is far richer in hydroxyapatite than any other structure in the body, including bone. Enamel gives teeth their white sheen. Of course, enamel is only one of the layers that make up our teeth. The inner layers, such as the pulp and dentine, are also filled with hydroxyapatite.

There are lots of creatures with hard tissues—clams and lobsters, for example. But they do not use hydroxyapatite; lobsters and clams use other materials, such as calcium carbonate or chitin. Also, unlike us, these animals have an exoskeleton covering the body. Our hardness lies within.

Our particular brand of hardness, with teeth inside our mouths and bones inside our bodies, is an essential part of who we are. We can eat, move about, breathe, even metabolize certain minerals because of our hydroxyapatite-containing tissues. For these capabilities, we can thank the common ancestor we share with all fish. Every fish, amphibian, reptile, bird, and mammal on the planet is like us. All of them have hydroxyapatite-containing structures. But where did this all come from?

There is an important intellectual issue at stake here. By knowing where, when, and how hard bones and teeth came about, we will be in a position to understand why. Why did our kind of hard tissues arise? Did they come about to protect animals from their environment? Did they come about to help them move? Answers to these questions lie in the fossil record, in rocks approximately 500 million years old.

Some of the most common fossils in ancient oceans, 500 million to 250 million years old, are conodonts. Conodonts were discovered in the 1830s by the Russian biologist Christian Pander, who will reappear in a few chapters. They are small shelly organisms with a series of spikes projecting out of them. Since Pander's time, conodonts have been discovered on every continent; there are places where you cannot crack a rock without finding vast numbers of them. Hundreds of kinds of conodonts are known.

For a long time, conodonts were enigmas: scientists disagreed over whether they were animal, vegetable, or mineral. Everybody seemed to have a pet theory. Conodonts were claimed to be pieces of clams, sponges, vertebrates, even worms. The speculation ended when whole animals started to show up in the fossil record.

The first specimen that made sense of everything was found by a professor of paleontology rummaging through the basement at the University of Edinburgh: there was a slab of rock with what looked like a lamprey in it. You might recall lampreys from biology class—these are very primitive fish that have no jaws. They make their living by attaching to other fish and feeding on their bodily fluids. Embedded in the front of the lamprey impression were small fossils that looked strangely familiar. Conodonts. Other lamprey-like fossils started to come out of rocks in South Africa and later the western United States. These creatures all had an exceptional trait: they had whole assemblages of conodonts in their mouths. The conclusion became abundantly clear: conodonts were teeth. And not just any teeth. Conodonts were the teeth of an ancient jawless fish.

We had the earliest teeth in the fossil record for over 150 years before we realized what they were. The reason comes down to how fossils are preserved. The hard bits, for example teeth, tend to get preserved easily. Soft parts, such as muscle, skin, and guts, usually decay without fossilizing. We have museum cabinets full of fossil skeletons, shells, and teeth, but precious few guts and brains. On the rare occasions when we find evidence of soft tissues, they are typically preserved only as impressions or casts. Our fossil record is loaded with conodont teeth, but it took us 150 years to find the bodies. There is something else remarkable about the bodies to which conodonts belonged. They have no hard bones. These were soft-bodied animals with hard teeth.

For years, paleontologists have argued about why hard skeletons, those containing hydroxyapatite, arose in the first place. For those who believed that skeletons began with jaws, backbones, or body armor, conodonts provide an "inconvenient tooth," if you will. The first hard hydroxyapatite-containing body parts were teeth. Hard bones arose not to protect animals, but to eat them. With this, the fish-eat-fish world really began in earnest. First, big fish ate little fish; then, an arms race began. Little fish developed armor, big fish obtained bigger jaws to crack the armor, and so on. Teeth and bones really changed the competitive landscape.

Things get more interesting still as we look at some of the first animals with bony heads. As we move up in time from the earliest conodont animals, we see what the first bony-head skeletons looked like. They belonged to fish called ostracoderms, are about 500 million years old, and are found in rocks all over the world, from the Arctic to Bolivia. These fish look like hamburgers with fleshy tails.

The head region of an ostracoderm is a big disk covered by a shield of bone, looking almost like armor. If I were to open a museum drawer and show you one, you would immediately notice something odd: the head skeleton is really shiny, much like our teeth or the scales of a fish.

Ostracoderm

A conodont (left) and an ostracoderm (right). Conodonts were originally found isolated. Then, as whole animals became known, we learned that many of them functioned together as a tooth row in the mouths of these soft-bodied jawless fish. Ostracoderms have heads covered with a bony shield. The microscopic layers of that shield look like they are composed of little tooth-like structures. Conodont tooth row reconstruction courtesy of Dr. Mark Purnell, University of Leicester, and Dr. Philip Donoghue, University of Bristol.

One of the joys of being a scientist is that the natural world has the power to amaze and surprise. Here, in ostracoderms, an obscure group of ancient jawless fish, lies a prime example. Ostracoderms are among the earliest creatures with bony heads. Cut the bone of the skull open, embed it in plastic, pop it under the microscope, and you do not find just any old tissue structure; rather, you find virtually the same structure as in our teeth. There is a layer of enamel and even a layer of pulp. The whole shield is made up of thousands of small teeth fused together. This bony skull—one of the earliest in the fossil record—is made entirely of little teeth. Teeth originally arose to bite creatures; later, a version of teeth was used in a new way to protect them.

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