A Mineral Map for Fossil Hunters

Improving the localization of rare soft-tissue fossils

Think of a fossil. What do you see? Perhaps dinosaur bones, teeth, or mollusk shells? When we think of fossils, we traditionally picture hard tissue parts. However, more rare soft tissue fossils are also extremely valuable in piecing together the story of early life on Earth.

Not every part of every organism makes it into the fossil record. The fossil record is biased towards hard tissues like shells, teeth, and bones; soft tissues are usually left out of the fossil record because they tend to decay rather than fossilize. But many types of early organisms, like worms and shrimps, were made of soft tissues. Soft tissue fossils provide valuable information on early life—that is, when fossil-hunting researchers are able to find them.

A team of researchers at Yale, led by Professor Derek Briggs of the Geology and Geophysics Department, investigated where soft tissue fossils are likely to be found. In a recent study published in Geology, the researchers characterized the mineral signatures of rocks that tend to preserve soft tissue parts, in order to help scientists pinpoint places to find soft tissue fossils. This research accelerates the search for rare soft tissue fossils not only on Earth, but possibly even on other planets.

Treasures at the Burgess Shale

Most early organisms did not have shells or bones; it took until the Cambrian Explosion, around 500 million years ago, for organisms to develop hard body parts that were preserved in the fossil record. Studying soft tissue fossils thus provides insights into some of the earliest life forms on our planet, such as what they looked like and how they might have survived.

While soft tissue fossils are generally rarely found in the fossil record, there is an exception: the Burgess Shale, a geologic rock formation in the Canadian Rocky Mountains, containing fossilized creatures from around the Cambrian Explosion. The Burgess Shale preserves a remarkable diversity of organisms, including an unusually high number of soft tissue fossils. For this reason, a major category of soft tissue fossils is referred to as Burgess Shale-type fossils.

Briggs wondered what conditions are conducive to the formation of soft tissue fossils like those in the Burgess Shale. “I was interested in the preservation of fossils: the processes that account for the transfer of an organism into the fossil record, and the biases that that results in,” Briggs said. Pursuing this line of thinking, Briggs and his team began to study some of the mineralogical differences between rocks that do and do not contain soft tissue fossils. 

Clues from minerals

Ross Anderson, currently a fellow at All Souls College, Oxford and previously a graduate student in Briggs’ group at Yale, pioneered this study on the mineralogical conditions that lead to soft tissue preservation. While hunting for fossils around the world, he saw that different regions would preserve more soft tissue or hard tissue fossils and wondered why this was the case.

“If you go far back enough, before organisms made shells or bones, the fossils become much harder to find,” said Anderson. “I investigated the conditions conducive to those early fossils being preserved, so we can more easily find them.”

Anderson and Briggs collected about 200 rock samples from sites around the world, including the Burgess Shale. About half of these preserved fossils with soft tissues, like parts of primordial worms and shrimps. The others included more common hard tissues fossils, like shells and bones and trilobites.

The researchers ground up the rocks surrounding the fossils and analyzed them using a technique called X-ray diffraction, which provides a mineral signature for each rock. They compared the mineral signatures of those rocks containing soft tissue fossils with those that had only hard tissue fossils. Anderson noticed that the rocks with soft tissue fossils had high levels of a distinct mineral called berthierine, which tends to form in tropical and iron-rich areas such as the Burgess Shale.

In a previous study on the impact of bacteria on fossil formation, Briggs, Anderson, and colleagues had postulated that certain clay minerals are toxic to bacteria, promoting soft tissue fossil formation by protecting the fossils from bacterial decay. One such toxic clay mineral was berthierine. Thus, this mineral may prove to be the explanation for the preservation of soft tissues in fossils.

From the Burgess Shale to Mars

The findings of Briggs and Anderson will help other fossil researchers in their hunt for soft tissue fossils. Now, this knowledge can allow them to target specific areas and types of rocks in their search, saving time and resources. This information could also aid in the discovery of more Burgess Shale-type soft tissue fossils, leading to a larger number of clues about the different types of organisms existing on early Earth.

Additionally, microfossils of small organisms like bacteria are valuable to understanding early microbial life, but extremely hard to find. Typically, the process for identifying rocks likely to contain microfossils is arduous, requiring countless hours observing sections of rocks and looking for bacterial fossils.

“Imagine you go out and break up the rock with a hammer and find fossils—if you do this for the pre-Cambrian, you can’t see the fossils. So you collect large amounts of sedimentary rock and you process them in various ways,” Briggs explained. Now, Briggs hopes that their research will lead to expedited identification of rocks that are likely to contain these microfossils.

Another impact of their research lies in the search for life on other planets, such as Mars. “The rover on Mars—the Curiosity rover—has the ability to make mineralogical measurements,” Anderson said. “It has an X-ray diffractometer on it, and it has the ability to see if those rocks it looks at on a daily basis are conducive to finding soft tissue fossils or not.”

Briggs and Anderson’s research was partly funded by NASA, and they are hopeful that their mineral maps can help with the search for fossils, especially microfossils, at locations from the Burgess Shale to Mars. “The long term application would be to look for fossils on other planets,” Briggs said.

Their research helps scientists not only look for possible records of life on other planets, but also understand the factors that led to such intricate life forms on our own planet. “Understanding how complex life first evolved on Earth is one of the fundamental questions in the natural sciences,” Anderson said. “How did we go from a world with just microbes, to one where when you look out the window there are all sorts of plants and animals? We can make simple observations in rocks and learn a lot about how life evolved on Earth.”