Abstract
Animal fossils preserved in various geological materials, such as limestone, claystone, or amber, provide detailed information on extinct species that is indispensable for retracing the evolution of terrestrial life. Here, we present the first record of an animal fossil preserved in opal formed by weathering with such high-resolution details that even individual cuticle hairs are observed. The fossil consists of the exoskeleton of a nymphal insect belonging to the order Hemiptera and either the family Tettigarctidae or the Cicadidae. This identification is based on anatomical details such as the tibial and femoral morphology of the forelegs. The exoskeleton of the insect was primarily zeolitized during the alteration of the host rocks and later sealed in opal deposited by silica-rich fluids derived from the continental weathering of the volcanic host rocks. Organic matter is preserved in the form of amorphous carbon. This finding makes opal formed by rocks weathering a new, complementary source of animal fossils, offering new prospects for the search for ancient life in the early history of Earth and possibly other terrestrial planets such as Mars, where weathering-formed opal occurs.
Highlights
Animal fossils preserved in various geological materials, such as limestone, claystone, or amber, provide detailed information on extinct species that is indispensable for retracing the evolution of terrestrial life
The most sought-after type is precious opal, which differs from common opal only by the presence of colored patches that appear to move as the stone is viewed from different angles, an optical phenomenon attributed to the diffraction of visible light by a regular arrangement of silica spheres[9,10]
The degradation of organic matter may favor the precipitation of silica by providing functional groups, making amorphous silica an important preservation agent of fossils[11,12]
Summary
Animal fossils preserved in various geological materials, such as limestone, claystone, or amber, provide detailed information on extinct species that is indispensable for retracing the evolution of terrestrial life. The most sought-after type is precious opal, which differs from common opal only by the presence of colored patches (called play-of-color) that appear to move as the stone is viewed from different angles, an optical phenomenon attributed to the diffraction of visible light by a regular arrangement of silica spheres[9,10] Both precious and common opal form mainly in two geological settings: (1) hydrothermal alteration occurring when hot water circulates underground and precipitates opal when the temperature drops, e.g., as siliceous sinter (a hard incrustation around hot springs); and (2) continental weathering that occurs when meteoric waters percolate, hydrolyzing silicate while transforming the rock into a soil. Permineralization is especially documented for wood and bone when a silica-rich fluid fills the cavities between organ tissues[12,13,14,15,16,17]
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