Reply to Head's comment on “The Volyn biota (Ukraine) – indications of 1.5 Gyr old eukaryotes in 3D preservation, a spotlight on the `boring billion' ” by Franz et al. (2023)

Abstract

Head et al. (2024) emphasize the importance of the Volyn biota for evolution, especially in the so-called “boring billion”, in a detailed outline about the biological and geological context. However, they question the idea that the Volyn biota represent Precambrian fossils and instead argue that they contain young contaminants of “museum dust”. In addition, they question their biotic origin. We present here a detailed discussion of their points of concern based on presented data, including some additional information. Their points of concern were as follows: One object, shown by Franz et al. (2023a), is similar to a pollen grain, and another object is similar to trichomes; we show indications of fossilization and summarize our arguments against “museum dust”. They question the fossil character of the biota and argue for a biomineralization; we show that the biomineralization in trichomes is distinct from the mineralization of the biota. They missed information about the internal structure; we repeat the presented information about the internal structure in more detail, which is also indicative of fossil material and inconsistent with trichomes. They argue that we did not compare the biota with recent fungi via infrared spectroscopy; since the biota experienced temperatures near 300 °C, we think that a comparison with thermally degraded chitosan is more appropriate. They question the use of strongly negative δ13C as an argument for biotic origin, but we show that, in combination with positive δ15N values and the geological situation, a biotic origin is more likely than abiotic synthesis. In addition, Popov (2023) questioned the age of the Volyn biota, which we postulated as between approximately 1.5 and 1.7 Ga. He argues that the fossils could be Phanerozoic. We will also outline our arguments for the minimum age of 1.5 Ga.