National Fossil Day celebrates the curiosities beneath our feet

Dinosaurs aren’t the only fossil game in town. In fact, anything from an acorn to a meteorite can be a fossil. It’s not the shape or size that defines one, but the process — deep underground, through thousands of millennia, its matter goes through a fantastic transformation.

To fossilize, an object — say, an insect — must be buried quickly. Once beneath the earth, groundwater seeps into the insect’s body, carrying minerals with it. Over millions of years, those minerals replace the body’s original matter, leaving behind a stone cast.

These fossils are frozen moments of time that can give us all kinds of information about past animals, plants, climate and more. They can answer questions about how human ancestors lived and even what kinds of asteroids inhabited our solar system millions of years ago.

On Oct. 14, National Fossil Day, we’re celebrating these intriguing snapshots of the past. The National Park Service established this day “to promote public awareness and stewardship of fossils, as well as to foster a greater appreciation of their scientific and educational value.”

In that spirit, we’ve gathered insights on some of the world’s more unusual fossils from the Arizona State University scientists who study them.

Kathleen Pigg found that the stinging hairs of modern nettle leaves (right) were also present on nettle leaves from 50 million years ago (left). Photo courtesy of Kathleen Pigg.

The family tree of trees

Professor Kathleen Pigg from the School of Life Sciences studies fossilized plants and plant material (the fancy word for that is paleobotany). She also manages the entire fossil plant collection within the ASU Natural History Collections.

Pigg explains that there are two types of preservation when it comes to fossilized plants.

“Compressions are plants which are basically flattened. These come from lake deposits; the leaves fall in the water and get compressed by sediment. What you see is pretty much what you get,” she said.

“The other things we work on are specimens that have internal structures, three-dimensional things like acorns and seeds that fall to the forest floor. It’s kind of like fossilized compost.”

For the latter group, Pigg can use a CT scanner to look at slices of these specimens, which allows her to examine their internal structures.

“A lot of plants show their history of life in their structure,” she said.

For both fossil types, if the preservation is of excellent quality, high magnification can reveal a plant’s cell patterns. This is due to a unique characteristic of plant cells.

“Plants have cell walls, which animal cells do not have,” Pigg said. “These cell walls form a partition around the internal contents. Even though the cell contents are gone, sometimes that cell wall pattern is preserved in the fossil record.”

Kathleen Pigg introduces the fossil plant collection from the ASU Natural History Collections. Video credit: Knowledge Enterprise.

When two different plants mate to form a hybrid, a botanist can look at the hybrid and learn information about its parents. For example, a plant with a spiky-edged leaf mated with a plant with a smooth-edged leaf might create a hybrid with wavy edges. Pigg looks at ancient plant structures in this way to understand how they are related — tracking down literal family trees.

Her research also has important implications for the plants we have today. Ancient plants reveal information about climate over time, since different types of plants prefer different climates. She can also see how plants adapted to different climates or how they moved across the landscape. All of it, she says, gives scientists information about how plants work and allows them to better understand modern climates and ecosystems.

Pigg’s work focuses mainly on the northwestern region of the U.S. She often collaborates with the Stonerose Interpretive Center and Eocene Fossil Site in Washington state, which allows members of the public to search for fossils in the surrounding area. Typically, people can keep the fossils they find, but the center asks citizen scientists to donate rare specimens for study. Pigg admires this model that brings researchers and the public together and gives everyone the chance to contribute to science.

In the past, Pigg has brought small exhibits from the fossil plant collection, housed in ASU’s Alameda building, to events such as ASU Open Door. She’s currently working on an effort to make the collections available online through a digital archive and looks forward to being able to share specimens in person again.

The eroding landscape often leaves fossils like this ancient horse tooth exposed to the surface. Photo courtesy of Kaye Reed.

A safari’s worth of fossils

Fossils not only tell us about individual plants and animals, but also about their ecosystems. President’s Professor Kaye Reed uses fossils to get a big picture of all the animals that lived together in the same area and time period, including ancient human ancestors called hominids.

“My main goal is to look at all the fossils that we find, identify them and then reconstruct the community of mammals in which ancient hominids lived,” said Reed, faculty in the School of Human Evolution and Social Change and a research associate in the Institute of Human Origins.

Just as ecosystems can be different, so are the communities of animals that inhabit them. By knowing which animals were living together, Reed can use modern animals to infer what ancient animals ate, how they moved around and the role that each species played in its environment.

“Sometimes these communities can be 6,000 miles apart, not connected in a biogeographical sense, but have exactly the same community of what we call functional traits,” she said.

Reed mainly does fieldwork in the Afar region of Ethiopia, which is a particularly good place to look for fossils because it’s in the middle of the African rift system. As the ground breaks apart, it exposes more dirt, which erodes and reveals the fossils underneath.

Her research sites there are between 2.4 million and 3 million years old. Scientists can tell how old a fossil is by studying the radioactive elements left in the rock. Unstable elements change into stable ones at a constant rate; measuring the amount of an unstable element left in a fossil allows the scientist to count backwards and figure out its age.

Reed’s team excavated this fossil skull of an antelope related to modern wildebeests. Photo courtesy of Kaye Reed.

Most of her finds are antelope, but she does end up with quite a menagerie: giraffes, rhinos, elephants, hippos, monkeys and sabertooth cats have all made an appearance at her sites. Occasionally, she digs up a mystery that baffles her whole team.

“I have a fossil right now, and none of us can figure out what it is,” she said. “It’s this thin hand or foot bone with a little hook on it. Maybe someday we’ll figure it out.”

Before working in Ethiopia, Reed had been doing fieldwork at a cave site in South Africa. The site is about 2.8 million years old, around the same age as her Ethiopian sites. The species of animals are different between the two regions, she says, but the structure of the animal communities are strikingly similar.

By studying the animals of ancient environments, Reed can gain insights into the lives of hominids. For example, scientists have debated about whether Australopithecus afarensis — species of the famed Lucy fossil — climbed trees. Analysis of shoulder bones suggested that these hominids could climb like apes. But you need more than the right shoulders to climb trees — you also need trees. Reed set out to find whether trees were present in A. afarensis’s environment.

Lots of tree-dwelling, fruit-eating animals at that time would point to a forest, while lots of land-walking, grass-eating animals would indicate an open grassland.

“We reconstructed the habitat, and in fact it did have quite a few trees, so it would have been a possibility,” she said. “And I think if there are giant hyenas and sabertooth cats, it would be really nice to climb a tree.”

Additionally, by identifying the types of environments found at different periods, Reed can use climate data from similar modern environments to learn about how Africa’s rainy and dry seasons changed over time. Through this method, she found that as the seasons became longer, some hominid species were able to adapt while others were not. This can provide clues to the origins of different characteristics found in the human lineage.

Aside from the gains to scientific knowledge, Reed finds fieldwork fulfilling just for the plain fun of it.

“The best part is to go into the field, walk around, look down and pick up something that lived 3 million years ago.”

Fossil meteorites reveal what types of asteroids were impacting the Earth millions of years ago.

Space and time

Meteorites are some of the oldest objects in the solar system, though that doesn’t automatically make them fossils. What’s more, they’re rocks, not living material. So how is it we have fossil meteorites?

These special meteorites, often found in quarries around the world, are ones that fell hundreds of millions of years ago, were buried and underwent a transformation underground. A unique mineral called chromite, which is resistant to alteration, is all the material that remains of the meteorite as it was when it fell.

“It's not like a fossilized tree where organic matter is replaced with terrestrial minerals. Still, almost none of the minerals of the original meteorite remain. It’s been nearly completely replaced by terrestrial processes. By that definition, it really is a fossil,” said Devin Schrader, an associate research professor in the School of Earth and Space Exploration and the interim director of the Center for Meteorite Studies. Fun fact: asteroid (117581) Devinschrader is named after him.

He specializes in (nonfossilized) chondrites, which are meteorites that haven’t changed much since they initially formed 4.5 billion years ago. These tell him about the composition of our solar system when it first began. (Read more about Schrader’s latest research.)

Even so, he knows a thing or two about what we can learn from fossil meteorites.

A fossil meteorite from a quarry looks like a rock lodged within stone and has a blurry halo of lighter stone surrounding it. Within the central rock-like area are the surviving grains of chromite, which hold clues to the history of our solar system.

Scientists can identify what type of meteorite the fossil is by analyzing a unique chemical footprint — the isotopes of oxygen — in the chromite and comparing it to the oxygen isotopes of chromite in known meteorites.

“It gives a history of what’s been hitting the Earth over a longer time scale than what we can record now as humans. The types of meteorites falling to Earth today are falling in different abundances than what we think was falling in the past,” Schrader said.

Meteorites tell scientists about the asteroids they come from. For example, most meteorites hitting Earth today — about 86% — are known as ordinary chondrites and come from at least three different asteroids. However, Schrader believes they weren’t so ordinary in the past.

The meteorites we know of, he explains, are a biased sample. They only show what is currently in our solar system and has happened to come our way since we started recording them. Fossil meteorites are valuable because they offer scientists a glimpse at the asteroids of our solar system that were impacting Earth millions of years ago.

Although the vast Center for Meteorite Studies collection doesn’t include any fossil meteorites, two of its specimens stand out in Schrader’s mind for having been on Earth a long, long time.

Muonionalusta, an iron meteorite recovered from Sweden, fell in fragments on a glacier during an ice age sometime between 800,000 and 1 million years ago, where it was covered by glacial debris. The meteorite did not go through the fossilization process. Instead, the exterior of the meteorite fragments slowly rusted away as they were exposed to water, with their remains becoming smaller over time.

Another meteorite, Lake Murray, fell between 90 and 110 million years ago in what is now Oklahoma, where it was buried in sandstone that preserved it.

“Only about 30% of the original meteorite has survived to this day. The rest has weathered to iron oxide,” Schrader said.

Virtual tours of the meteorite collection are available on the center’s website.