LIFE × EVOLUTION

Conodonts: the First Teeth

~515 million years ago

Phanerozoic · Paleozoic · Cambrian

Conodonts were eel-shaped animals, known mostly from the millimetre-scale phosphatic elements, hard pieces of a phosphate mineral, that lined their throats and worked as teeth about 515 million years ago.

Conodonts were eel-shaped animals. Their only common fossils are the millimetre-scale phosphatic elements that lined their throats, and together these elements are called the conodont apparatus. The elements functioned as teeth. They consist of crystals of apatite, a calcium phosphate mineral. Through conodont evolution, these crystals align ever more precisely at right angles to the feeding surfaces, the surfaces that met the food. This orientation is the one that best resists crushing, so the crystals are arranged as the work of a tooth requires. The elements are the oldest mineralised tissue, that is, tissue hardened by a mineral, in the vertebrate lineage, the line of animals with backbones.

Why it matters

Researchers long took the conodont elements as evidence that teeth preceded jaws. Murdock and colleagues (2013) contest that reading. They examined the ancestors of conodonts by X-ray tomography, a method that images the inside of a fossil without cutting it. They conclude that the hard tissue of the elements is not vertebrate enamel, the hard outer layer of a tooth, but an innovation that arose separately and resembles enamel. A resemblance acquired separately is called a convergence. If the convergence holds, two lineages that were already separate each produced an enamel-like tissue.

Dating & uncertaintyDisputed

Sansom and colleagues (1992) reported bone-like tissue in conodont elements. Their finding moved the first vertebrate hard tissue back from the Middle Ordovician to the Late Cambrian, an older part of geological time, around 515 million years ago. Whether the elements are teeth in the strict sense is contested. Murdock and colleagues (2013), from tomography of the ancestors of conodonts, find that the resemblance to enamel is a convergence. That would imply that jaws came first and teeth followed.

Earth history as one day

21:16:392 h 43 min before midnight

if Earth’s 4.54 billion years were compressed into a single day