The Araripe basin, at the boundary of the states of Ceará, Piauí and Pernambuco, is one of those rare places on the planet that holds a vast array of prehistoric animal fossils whose soft tissue is well preserved. Generally, these structures—eyes, conjunctive tissue and fragments of heart fiber—are the first to decompose so they rarely become fossilized. In those rare instances where they are preserved, they provide a basis for studies on the biology and evolution of species that have been extinct for millions of years. For some time, scientists have known that these structures are preserved as a result of specific geochemical processes that occur, such as the replacement of organic material by pyrite, a mineral composed essentially of iron and sulfur, or by kerogen, the insoluble part of organic matter that is retained in sedimentary rocks. In a May 2017 study published in the journal Scientific Reports, researchers analyzed these processes microscopically and suggested that they might be affected by the respiration of decomposing bacteria.
In the study, the team led by paleontologist Mírian Pacheco of the Department of Biology at the Federal University of São Carlos (UFSCar), Sorocaba campus, and geologist Setembrino Petri of the Institute of Geosciences at the University of São Paulo (IGc-USP), examined fossil samples from a species of primitive fish called Dastilbe crandalli, which lived in the area about 113 million years ago. The fossils, abundant in Araripe, were located in two types of limestone, sedimentary rock that is rich in calcium carbonate: one was gray and had more organic matter, and the other was beige and had fewer of these chemical compounds.
The first samples were wrapped in a grayish fiber material. The beige-colored limestone samples in turn were an orange, honeycomb-like color, with a fuzzy covering of microscopic crystals.
The researchers confirmed that the soft structures on the beige-colored fossils had been preserved through a pyritization process. “This means that the elements that these fossils were composed of were replaced by pyrite,” explains Pacheco. Pacheco says that this is the first time a case of pyritization has been observed in a fossilized vertebrate. The few known registered cases are of insects and various invertebrates. At the same time, the soft tissue of the gray-colored limestone samples was fossilized through the formation of kerogen. In this process, organic carbon takes on a more stable form, which can persist for millions of years. While the pyritization process helped to preserve tendons, cellular nuclei and membranes and eye tissue, the kerogenization process preserved conjunctive tissue, tegument and muscle fiber.
In both cases, however, these geochemical processes were affected by the action of decomposing bacteria. Through the process of anaerobic respiration—there is no oxygen involved—the microorganisms assisted in replacing organic matter with pyrite or kerogen, depending on the type of limestone in which the fossils were preserved. As these processes got underway, the elements making up the organic structures of these animals were slowly destroyed and replaced by pyrite or kerogen. At the same time, they left marks on the rocks that encased them.
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This hypothesis is essentially based on electronic microscopy analysis. In examining pyritized fossils, researchers identified the residue of microorganism activity. “We found smooth and flexible structures, similar to a spider’s web that are the result of the bacteria’s metabolization of pyrite,” says Pacheco. This would explain why each geochemical process would preserve the soft tissue in a particular way. Although both preserve these structures in their own way, the fossil fragments of soft tissue found in the beige-colored sediment, on a microscopic level, are even better preserved than those deposited in gray-colored limestone,” explains geologist Gabriel Osés, the study’s first author and master’s thesis advisor to Setembrino Petri during some of his research on the project.
“The study is important because it broadens the area where these processes occurred to include other geological deposits, and it clarifies the geochemical conditions that led to the preservation of soft tissue in fossils in the Araripe basin,” says paleontologist Marcello Guimarães Simões, at the Institute of Biosciences at Sao Paulo State University (Unesp), Botucatu campus, who did not participate in the study. Simões says that prior to this point, people thought that these processes occurred in the soft tissue of fossils only in specific regions, like the Gaojiashan Formation in China, and during geological periods prior to the Cretaceous period, which stretched from 145 to 66 million years ago. “Now we know that these geochemical processes can also be observed in fossils from more recent geological eras and possibly, in other geological deposits around the world.”
Applications of Raman Spectroscopy in paleobiology and astrobiology (No. 12/18936-0); Grant Mechanism Regular Research Grant; Principal Investigator Setembrino Petri (USP); Investment R$584,668.54.
OSÉS, G. L. et al. Deciphering pyritization-kerogenization gradient for fish soft-tissue preservation. Scientific Reports. Vol. 7. No. 1468, pp. 1-15. May 2017.