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Mountain Slopes and Microscopes: Calibrating Geologic Time in the Guadalupes

rendering by Philippe Janvier, 1997, for the Tree of Life Web Project. Conodonts were tiny invertebrates that plied the world's oceans for more than 300 million years. Fossils of their distinctive, and microscopic, teeth are preserved in ancient rocks, and, for geologists, those fossils have been key in developing the scale of deep time. Conodont fossils and limestone rocks in the Guadalupe Mountains are a global reference point for the Permian Period, when a warm inland sea covered much of West Texas.

In the 1830s, a group of Scottish and English naturalists were pursuing a new and unpopular idea: that the Earth's features could be explained by natural processes, acting over vast stretches of time. Studying the rocks of the British Isles, their attention was drawn to a particular variety of fossilized corals. From lower to higher rocks, the corals grew more complex. Anticipating Darwin, they inferred that they were observing the development of a life form. And they inferred something else: the fossil record could help interpret the rock record.

Two limestones may be similar in composition. But fossil remains are distinct. Wherever they're found, rocks that contain the same distinctive “index fossils” must be of the same age.

It was a turning point in modern geology. During the next 50 years, scientists used “fossil assemblages” to develop the scale of geologic time.

That work continues. And the Guadalupe Mountains in West Texas are part of the undertaking.

In May 2017, a team of geologists – from China, Canada, MIT and Texas – gathered at Guadalupe Mountains National Park. With help from the park's staff, and its hardy mules, they collected hundreds of pounds of rocks, from three locations. Those rocks will be studied intensively. And particular attention will be paid to an ancient sea creature.

Jonena Hearst is the Guadalupes' park geologist.

“There is a very small animal – a very small animal, maybe a couple centimeters long,” Hearst said. “These are called conodonts. And they look like little tiny eels almost.”

Cruising, feeding on plankton, these tiny invertebrates swam the world's seas for more than 300 million years. They became extinct 200 million years ago. But their fossil remains are a touchstone for geologists.

Conodont teeth are the earliest preserved in the fossil record. They're microscopic – and they look like no other fossil. Some are conical – others have bizarre, serrated shapes. These distinctive teeth are ideal as “index fossils.”

Conodonts evolved rapidly. Successive layers of rock preserve that evolutionary journey, in conodont fossils of distinct species.

The Guadalupe Mountains, as well as the Glass and Apache mountains, are the exposed remains of a reef created in Permian time. Two hundred and seventy million years ago, the planet's land masses were fused as the supercontinent of Pangaea. Much of West Texas – located then near the equator – was covered by a warm inland sea, bounded by the reef.

Geologists began to trace the history of Permian time in these reef rocks as early as the 1850s. Indeed, the middle stretch of the Permian – a span of 12 million years – is known as the Guadalupian Epoch. The Guadalupian is divided into three ages – each age is associated with a location in the mountains.

They're part of a system of “Global Stratotype Sections and Points,” or GSSPs – which serve as reference points for geologists. Each of the Guadalupe GSSPs has a unique conodont species. If a geologist elsewhere finds that index fossil, they can place the rock they're working with in Guadalupian time. Fortunately, conodonts were both abundant and ubiquitous.

“The nice thing about these little guys – they were cosmopolitan in the Permian ocean,” Hearst said, “so you find them all over in marine sediments. And, there can be thousands of them in a single hand-sized sample of rock.”

In their recent visit, the international team collected rocks from the GSSPs, at Nipple Hill, Stratotype Canyon and Getaway Hill in Guadalupe Pass. In the years to come, micro-paleontologists will examine fossils from those rocks – of conodonts and other marine creatures. It's painstaking work – but it will add to our understanding of the history of the planet, and its life.

“This requires the work of hundreds and thousands of geologists,” Hearst said. “This has been going on for 100, almost 200 years, and we're still in the process of refining our knowledge. Our knowledge now of how the rock record of the Earth evolved is becoming more and more complete as better and better correlations are made. We can start pinpointing events in geologic history with greater and greater accuracy and precision. And that way we can build a more complete story of the evolution of the Earth.”

The Guadalupes are one of the great landmarks of the Southwest. They're also a lodestar for geologists around the world.