How Can a Tiny Fossil Tooth Tell Us the Age and Heating History of Rock?

1. What are conodonts, and why were they a mystery for so long?

In 1856 a scientist named Christian Pander, who was studying Silurian fish fossils from eastern Europe, described these tiny tooth-shaped objects and gave them the name "conodont," which roughly means "cone tooth" [1]. They are made of a phosphate mineral (a calcium-and-phosphorus compound in the same mineral family as the hard parts of our own teeth and bones) [1]. That tough material is why they survive in rock while soft parts rot away.

They turn up in marine rock, from the Cambrian period until they disappeared in the late Triassic [1]. But the animal that carried them was not described until 1983, about 127 years after Pander's report (more on that in section 5) [8]. So geologists spent more than a century using fossils without knowing what animal they came from.

2. Why can a fossil with an unknown owner still work as a clock?

To date a rock layer, you do not need to know what the animal looked like. You need two things from the fossil. First, its shape must change as time passes, so that each shape belongs to a limited slice of history. Second, it must be common and widespread, so you can find it again in rocks far apart.

Think of old photographs. If you see a certain style of sneaker or hairstyle, you can guess the decade without knowing the person. Conodont teeth work like that, except that the "styles" changed through evolution rather than fashion. Researchers noticed this usefulness early: Ulrich and Bassler described many North American species in 1926 and were the first to recognize their value for dating rock, and from the 1960s conodonts became one of the most important dating tools for Paleozoic and Triassic rock [1].

Geologists call a fossil used this way an index fossil. Matching up layers in different places by the fossils they contain is part of stratigraphy, the study of rock layers. Conodonts are especially handy because they are small, so a single piece of rock can hold many of them, and they occur in marine rock around the world [1].

There is one limit: this method only works for rocks from the time conodonts lived. Sources also differ slightly on exactly when conodonts ended, but all agree it was in the Triassic [1]. For younger rock, other fossils are needed.

3. How did one conodont species mark the border between two geological periods?

Earth's history is divided into named periods, and the border between two of them has to be pinned to a particular spot in a particular rock. Such an internationally agreed reference spot is called a GSSP, a "Global Stratotype Section and Point." You can think of it as the official place on Earth where one period is said to begin.

The border between the Permian and Triassic periods, about 252 million years ago, is one of the most important. It sits just above rock that records the biggest mass extinction known, when more than 81% of marine species (earlier estimates said about 96%) disappeared [3]. The official spot is the Meishan D section in Changxing County, Zhejiang Province, China, set in a 2001 study [3].

What marks the line is a conodont. The base of the Triassic is placed where a species called Hindeodus parvus first appears, at the bottom of bed 27c at Meishan [4]. That one tiny tooth appearing in the rock is what tells scientists "the Triassic starts here."

A rule like this is only useful if people can recognize the marker in other places, and that is where it gets complicated. A study of rock in South China found that at another site, called Shangsi, the first H. parvus showed up about 4.5 metres above the level that matches the event boundary. The authors suggested a small gap in the rock record at the official spot as a likely explanation [4]. So a conodont border is a careful agreement that scientists keep testing, not a magic line.

4. How can the color of a fossil tooth record how hot the rock got?

Conodonts also carry a second record. When the rock around them is buried and heated, the teeth change color. In 1977 Anita Epstein and colleagues at the US Geological Survey published a report on this, titled "Conodont color alteration; an index to organic metamorphism" [5]. Their idea became the Conodont Alteration Index (CAI): compare a tooth's color with a reference scale, and you get a rough estimate of how much heat the rock went through. The link between color and temperature is not a simple one-way darkening, so geologists rely on the reference scale rather than guessing by eye.

A color does not match one exact temperature, because how long the rock was heated also matters, so this is an estimate with a range.

The estimate has real uses. Oil and natural gas come from certain rock layers that were heated just enough. A US Geological Survey team used conodont color, along with another measure, to map how thoroughly heated Ordovician and Devonian rocks in Pennsylvania were, because those rocks hold the source of much of the region's oil and gas [6]. The color tells you about heat history; it does not by itself tell you how much oil or gas is there.

5. Whose teeth were they? How was the conodont animal finally found?

The clue came from a rock layer near Edinburgh, Scotland, called the Granton Shrimp Bed, which dates to the Early Carboniferous, roughly 359 to 323 million years ago. It preserved soft-bodied animals unusually well [8]. In 1983 Derek Briggs, Euan Clarkson and Richard Aldridge described a fossil from there showing a set of conodont teeth together with the imprint of a soft body: the first good evidence of what the owner looked like [8][2].

By 1993 ten specimens from Granton were known. The fossil animals were long and slim, 21 to 55 millimetres long, with a short head, a body with V-shaped muscle blocks, and a fin on the tail. The head had two rounded structures that the researchers interpreted as the supports of large eyes, and the teeth lay below and behind the eyes [2]. In other words, a small eel-like swimmer, with its feeding teeth near its mouth. Teeth fossilize easily, while soft bodies rarely do, and Granton is an exceptional place where a soft body was preserved too. That is thought to be why the animal stayed unknown for so long [8].

Does this mean conodonts are our teeth's ancestors? Not exactly. In 2013 a University of Bristol-led team in Nature used X-rays to look inside conodont teeth and concluded that their tooth-like structures evolved within the conodont line, independently of the teeth of other vertebrates (animals with backbones, like us) [7]. Conodonts are probably distant relatives, not the source of human teeth.

6. What can you try yourself, and where can you read more?

You can picture the idea of "heat changes color" with toast. With an adult's help, toast several slices of bread for slightly different times, then line them up from lightest to darkest. You can tell which slice had the most heat just from color. Treat this only as a mental picture: toast and fossil teeth do not change color in the same way, and the toaster and hot bread can burn you, so keep an adult involved.

If this made you curious, try these questions. What do you think a conodont's large eyes say about how it lived? And why might one species, like H. parvus, be easy to use as a border marker while another is not? The sources below, especially the Bristol press release and the Meishan description from the International Union of Geological Sciences, are good places to start.

Sources