How Do Fossils Date Rocks? Species That Lived Briefly and Spread Widely

William Smith, a surveyor working on canals in England from the 1790s, noticed that the same sequence of rock layers appeared in different places and that each layer contained a characteristic assemblage of fossils, always in the same order. That observation, published in his geological map of England and Wales in 1815, established that fossil assemblages succeed one another in a definite and recognisable order, which means a layer can be identified by its fossils regardless of what the rock itself is made of. It was a practical discovery made for practical reasons, since knowing which layer you were in predicted what lay beneath, and it was also a conceptual breakthrough, because it implied that life had changed systematically through time, an implication that predated any theory explaining why. Combined with superposition, the principle that undisturbed layers are younger upward, it allowed a relative timescale to be built for the whole of the fossil record.

Not every fossil is useful for dating, and the requirements are specific and partly contradictory:

• •A short existence as a species, so its presence pins the layer to a narrow interval, which is the most important property
• •Wide geographic distribution, so the same marker can be found on different continents, which favours floating or swimming organisms over those confined to one habitat
• •Abundance, so it is actually found rather than being a rare prize
• •Easy and unambiguous identification, ideally in a small fragment, since drill cores produce chips rather than specimens
• •Preservation potential, meaning hard parts that survive burial, which is why soft-bodied organisms are almost absent from the working toolkit
• •Independence from local environment, since a species restricted to one water depth marks that environment rather than that time
• •Graptolites, ammonites, trilobites, conodonts and foraminifera meet these conditions well and dominate the practice, with microfossils being particularly valuable because a handful of drill cuttings contains thousands

Correlating layers by their fossils across Europe and then across the world produced the geological column, dividing the record into eras, periods and stages named largely in the nineteenth century, frequently after the places where a characteristic sequence was first described, which is why the Cambrian is named after Wales, the Devonian after Devon and the Jurassic after the Jura mountains. The boundaries between units were originally placed where the fossil assemblage changed sharply, which frequently corresponds to a mass extinction, and are now formally defined by designating a specific point in a specific rock outcrop somewhere in the world as the reference, a system that removes ambiguity by making the definition physical. That relative framework was complete and ordered long before anyone could attach numbers to it. Radiometric dating, arriving in the twentieth century, supplied the numerical ages, and the two methods are complementary rather than competing: fossils correlate layers precisely and cheaply across the world, while radiometric methods anchor the sequence to absolute time.

Biostratigraphy remains a working commercial discipline rather than a historical curiosity, and its largest employer has been the oil and gas industry, where identifying which layer a drill has reached, in real time and from cuttings, determines where to drill next and whether a target has been passed. Microfossils dominate that work because they survive being ground up. The same methods date archaeological deposits through pollen and small mammal remains, correlate ocean sediment cores for climate reconstruction, and provide evidence in cases where the age of a deposit is disputed. The limits are real: it cannot date a rock with no fossils, which excludes most igneous and metamorphic rock, it gives relative rather than absolute ages by itself, and it requires the fossils to be in place rather than reworked from older deposits, which is a genuine source of error. Its enduring advantage is speed and cost, since identifying a species under a microscope takes minutes and a radiometric date takes weeks and a laboratory.

William Smith established from canal cuttings that fossil assemblages succeed one another in a fixed order, so a layer can be identified by what it contains rather than by the rock itself. A useful index fossil existed briefly, spread widely, was abundant, is easy to identify and preserved well, which is why ammonites, graptolites and microfossils dominate. That relative timescale was complete before radiometric dating supplied numbers, and biostratigraphy is still used commercially because it is fast and cheap.