How Index Fossils Help Reconstruct Earth’s Ancient and Mysterious Past

In the late 1700s, while surveying a coal mine in southwest England, a young surveyor named William Smith noticed a surprising trend: from one location to another, rock layers (or “strata”) were arranged in a predictable pattern, always in the same positions relative to each other.

Additionally, the fossils in each layer were also consistent: you could identify any given stratum simply by the preserved remains of plants and animals it contained.

It wasn’t a quirk on my part either. Smith and others quickly discovered that sedimentary rocks everywhere exhibited reliable ordering of fossils, a phenomenon Smith called “faunal succession” (another half century would pass before Charles Darwin explained this changing of the biological guard in evolutionary terms).

Geology was just beginning to take shape as a scientific discipline, and this was one of its first crucial discoveries. This gave the field an indispensable tool: the fossil index.

Strata-specific fossils

The epiphany came when geologists realized that certain fossils were unique to specific strata and did not appear anywhere else in the rock record. This meant that every time they found the same fossil in a new location, it served as a clue telling them precisely where that layer belonged in the scheme of things – even if the rock was unfamiliar to them, they could relate it to other known strata, connecting distant regions of the world (with their long-dead inhabitants) to create a unified timeline.

More than two centuries later, scientists are still using index fossils to refine our understanding of the past. One of them is Spencer Lucas, a paleontologist and stratigrapher – that is, he studies rock strata – at the New Mexico Museum of Natural History and Science.

“We’re trying to reconstruct Earth’s history,” he says, “as accurately as possible.”

Learn more: New evidence debunks theory that dinosaurs were in decline before the asteroid

How Index Fossils Are Used

Although the saga of our planet dates back 4.5 billion years, stratigraphers have managed to impose order on deep time by dividing it into eons, eras, periods, epochs and, finally, ages – so, so many ages. This daunting system is called the geologic time scale. Each division captures the defining characteristics of this moment: the types of rock formation and living organisms that proliferate in various regions of the world.

Much of the geologic time scale is built on index fossils. Because each is characteristic of a specific period of geologic time, they provide a convenient way of delineating all these periods.

For example, according to a study conducted in Encyclopedia of geology, strata from the Cambrian period (the time when most major groups of animals first appeared) are teeming with all kinds of weird and wonderful trilobites, a long-extinct class of marine arthropods; This distinct set of species allowed 19th-century geologists to connect the Cambrian rocks of England with other rocks of continental Europe, North America, and beyond. Similar advances occurred in other periods – the Ordovician, the Silurian, the Cretaceous – and a global story of Earth’s history began to unfold.

Determining the age of rocks

At the time, however, the best geologists and paleontologists could do was place rock layers and fossils relative to each other; they had no idea how old the materials were Really were. Then, at the turn of the 20th century, New Zealand physicist Ernest Rutherford discovered a way to determine the age of rocks: radiometric dating.

Many rocks contain unstable atoms called radioactive isotopes, which decay at a fixed rate into stable isotopes. If you know this rate – and we know it for many isotopes – you can date rocks simply by measuring the proportion of decayed isotopes in a sample. Thanks to this numerical precision, modern stratigraphers generally consider radiometric dating as the reference. Although some still prefer to use fossils, Lucas jokes that “we’re going extinct.”

Still, he sees a place for old-school methods, at least in the near future. For one thing, rocks can’t always be dated using radioactive isotopes, and even when possible, the process isn’t foolproof. Lucas argues that the best policy is to use both approaches – index fossils and radiometric dating – so that each can fact-check the other. “There’s a huge amount of effort that goes into trying to integrate them,” he says.

A fossil record in flux

Fossil index status is not accessible to just any organism; the best are geographically widespread and easy to identify, and belong to species that have existed for a relatively short period of time. But as long as a candidate fossil meets these criteria, stratigraphers aren’t picky: coiled-shell ammonites, eel-like conodonts, turtles, dinosaurs, and mammals have all had the honor of serving as our guides to geologic time. “People like me use them day in and day out,” says Lucas, “usually with pretty good reliability.”

But the problem with index fossils is that sometimes even the most promising ones deteriorate. This last criterion – species that existed for a relatively short period of time – is always subject to revision as paleontologists discover new fossils. Indeed, so-called clue fossils sometimes appear where no one expected them: in rock layers that are too old or too young. When this happens, their title must be revoked.

An ongoing controversy centers on the king of all fossils: Tyrannosaurus rex. Inimitable and ranging from Montana to Texas, “T. rex It looked like a pretty good clue fossil,” Lucas says. But, according to a study published in Scientific Reports, while experts once thought it only lived in the last 2 million years of the Cretaceous period, until the asteroid impact, the recent discovery of a much older tyrannosaurus – which may or may not represent a different species – muddies the waters.

If T. rex loses its status as an index fossil, it would be the last in a long series of such victims; In fact, Lucas is betting that the majority of index fossils ever proposed have turned out to be impostors. In reality, there are no definitive index fossils – only hypotheses waiting to be disproved in future excavations.

Like any science, stratigraphy advances one discovery at a time, with new evidence overturning old theories. And despite everything we have learned so far, the history of the Earth’s strata is far from complete. As Lucas said: “There are a few centuries of work left to do, that’s easy. »

Learn more: Eggshells fill 30-million-year fossil gap for dinosaur migration

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