What Is a Conodont? Tiny Teeth Whose Owner Was Missing for a Century

The elements were described in 1856 from Russian rocks and were immediately useful and immediately puzzling. They are tiny, generally under a millimetre, made of calcium phosphate, and shaped variously as cones, blades and comb-like structures, and they occur in marine rocks from the Cambrian to the end of the Triassic in vast quantities. Every attempt to assign them failed. They were proposed as the teeth of fish, of molluscs, of worms, of arthropods and of animals with no living relatives, and each proposal foundered because no host animal was ever found with them in place. Meanwhile their usefulness grew, since they evolve rapidly, occur worldwide and are recoverable in quantity by dissolving limestone in acid, which made them a primary tool for dating and correlating Palaeozoic marine rocks and gave them substantial commercial importance in petroleum exploration. The field of conodont studies therefore ran for over a century as a productive discipline whose central subject was an animal nobody had seen.

The resolution came from exceptional preservation sites and settled a surprising number of questions at once:

• •A specimen from Carboniferous rocks in Scotland, described in 1983, preserved a soft-bodied eel-like animal with the elements arranged in its head end
• •Further specimens from South Africa and elsewhere confirmed the anatomy and showed large eyes and V-shaped muscle blocks along the body
• •Those muscle blocks and a notochord place the animal among the chordates, meaning it is a relative of vertebrates rather than a worm or a mollusc
• •The elements sit in an apparatus in the throat region and functioned as a feeding structure rather than as jaws, since the animal had no jaw
• •Microscopic structure shows growth in layers comparable to vertebrate tooth tissue, which made these among the earliest mineralised skeletal structures in the vertebrate lineage
• •Wear patterns on the elements demonstrate that they occluded against each other and were used to process food, settling a long argument about whether they were teeth at all

The practical value rests on the same properties that make any fossil a good index. They evolved rapidly, producing distinct and recognisable forms that persisted for short intervals. They were widespread in marine environments worldwide, so the same forms correlate rocks between continents. They are abundant, with a modest sample of limestone yielding hundreds of elements. And they are extracted easily, since dissolving carbonate rock in dilute acid releases the phosphate elements undamaged, which means a geologist can process a sample in a laboratory without any delicate excavation. A further property is uniquely useful: the elements change colour permanently when heated, passing through a predictable sequence from pale amber through brown to black and eventually white as temperature rises, so their colour records the maximum temperature the rock has experienced. That index is used directly in petroleum exploration to determine whether a rock has been heated enough to generate oil or gas, or heated too much, which is information that is difficult to obtain any other way.

The group survived several mass extinctions, including the severe end-Permian event, and disappeared at the end of the Triassic, which is the last of the great extinctions to eliminate a major group entirely. Why they vanished then, having survived worse, is not settled, and proposed factors include competition from radiating fish groups, ocean acidification associated with the volcanic activity of the period, and changes in the plankton communities they presumably fed on. Their disappearance is itself a useful marker, since the top of the last conodont zone defines the end of the Triassic in marine sequences. The broader significance is evolutionary, since the elements represent one of the earliest experiments in building hard structures from calcium phosphate within the lineage leading to vertebrates, which is the same material that later became bone and teeth, and the long argument about whether these count as true teeth turns on definitions rather than on facts, which is a familiar position in palaeontology.

The elements were described in 1856, used for dating for over a century and belonged to an animal nobody could identify until soft-bodied specimens from Scotland in 1983 showed an eel-like chordate with the elements in its throat. They date Palaeozoic marine rock because they evolved fast, occur worldwide and dissolve free of limestone in acid. Their colour records the maximum temperature a rock has reached, which petroleum geologists use directly.