Ultrarare, soft-tissue fossils are more likely to survive in rocks containing bacteria-inhibiting minerals, according to new research that identified common mineralogical signatures for fossil-bearing rocks. Scientists hope to use these results to dig into the complexity of life surrounding the Cambrian explosion, an intense period of evolution and diversification that occurred approximately 540 million years ago, and to increase their efficiency at finding soft-tissue fossils.
“It’s really important for interpreting the [fossilized] organisms that we have a good understanding of the types of preservations and how those preservations happen,” said Ross Anderson, a postdoctoral research fellow at All Souls College in the University of Oxford in the United Kingdom and lead scientist on the project. With a better understanding of when, where, and why soft-tissue fossilization happens, “we might be able to more easily find fossils,” he added.
The team compared the mineralogies of hundreds of rocks that hosted soft-tissue fossils with those that supported mineralized skeletons. They discovered that soft-tissue, or Burgess Shale–type (BST), fossils were more likely to exist in materials that slow down the rate of tissue decay and also promote the fossilization of organic matter. These results offer the first statistical evidence of a connection between fossil occurrence and clay mineralogy, according to the researchers.
Seeing the Guts
Nowadays, much of the life that walks, swims, crawls, or slithers has a hard skeleton, and most fossils from the past 400 million years trace those skeletons. But when alive, those creatures are made primarily of soft tissues—brains, muscles, organs, etc.—that often do not get preserved because they decay too quickly. Before the Cambrian explosion, most life-forms did not have a skeleton at all—they were all soft tissue.
Fossils of soft tissues are incredibly rare but can provide a wealth of information about the ecology and biology of the creature when it was alive, Anderson explained. For a bone to fossilize, its rigid organic molecular structure gets slowly replaced by more time-resistant minerals, a process called mineralization. The type of fossilization seen in the Burgess Shale, however, preserves those delicate soft tissues without chemical alteration by compressing and sealing them within sediments, keeping them carbon-rich and nonmineralized.
“The Burgess Shale and other similar deposits…preserve some of the original organic matter in shales,” Anderson said. “You do get the soft parts preserved. You see the guts. You see wholly organic organisms like worms.”
But, until now, it has been difficult to understand why soft tissues fossilized in some locations but not others. Lab experiments that Anderson’s colleagues conducted in the mid-1990s suggested that the mineralogy of the surrounding material might play a key role in the preservation of soft tissues, and he sought to test that with actual fossils.
Finding the Right Minerals
To do this, the researchers examined the mineralogy of more than 200 Cambrian fossils, some newly collected and some from museums and archives, originating from 19 sedimentary layers spanning four different continents. Using X-ray diffraction, they measured the chemical compositions of the sediments surrounding soft-tissue fossils and mineralized skeletons.
The team found that if a rock’s mineralogy was more than 40% illite, BST fossilization was nearly impossible. Conversely, samples made of more than 20% berthierine had a greater than 90% chance of supporting a soft-tissue fossil. Berthierine, a mineral with antibacterial properties that forms in tropical, iron-rich sediments, likely inhibits enzymes that break down soft tissues and promotes fossilization, Anderson explained. These results agree with past lab experiments that explored possible fossilization pathways.
“You need to slow that process of decay down such that they can become stable on geological timescales,” he said. “And that, as it turns out, is quite rare and quite difficult to do. You need to have some unique environmental conditions.”
With the two mineralogical signatures, Anderson’s team could predict which clays contained BST fossils with approximately 80% accuracy. They noted, however, that the presence of berthierine does not guarantee that soft-tissue fossils will exist at a site. Climate, water chemistry, runoff, and, of course, available fauna also are important factors. High berthierine concentrations likely help tissue preservation along, Anderson said. The team published these results in Geology on 15 February.
The analysis is “fascinating,” said Emma Hammarlund, a geobiology researcher at Lund University in Sweden, because it describes a “symptom of Burgess Shale–type preservation rather than the mechanism that led to the preservation of nonmineralized tissue.” Hammarlund, who was not involved with the research, called the study “the first ‘mineralogical guide’ of its kind, predicting a mineralogy of where else to dig deeper for nonmineralized tissue in the Cambrian and earlier.”
Expanding the Search
These Cambrian fossils have mainly been found in the Burgess Shale in British Columbia in Canada, and finding new sites that have similar soft-tissue fossils has been a hit-or-miss process, according to Anderson.
The researchers plan to use the results of this study to more efficiently identify locations for additional deposits of Burgess Shale–type fossils around the world and to discover fossil deposits from before the Cambrian explosion, when most life did not have hard skeletons.
More BST fossils could “greatly deepen our understanding of the diversification of animal life on Earth in the Cambrian,” said Hammarlund, “and, possibly, even of the preceding dawn of animal life.”
—Kimberly M. S. Cartier (@AstroKimCartier), News Writing and Production Intern
Cartier, K. M. S. (2018), Rocks with soft-tissue fossils share a mineral fingerprint, Eos, 99, https://doi.org/10.1029/2018EO095237. Published on 20 March 2018.
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