Fossilized cell nuclei are not that rare: Review of the histological evidence in the Phanerozoic

The preservation of cell nuclei in deep time is an area of research that is largely unexplored, likely because of the assumption that fine intracellular organelles are too fragile to enter the fossil record. However, the literature is full of histological reports of Phanerozoic fossils presenting exquisite subcellular details, such as nuclei, nucleoli and even chromosomes seen frozen in multiple stages of cell division and cell death. Starting in the Present and going back in time all the way to the Paleozoic, all histological examinations that recognize cell nuclei in crown multicellular eukaryotes are reviewed here. In the Quaternary, cell nuclei were reported in many mammal mummies found in arctic permafrosts; in the Neogene and Paleogene most reports come from plants and insects preserved in Baltic amber; in the Mesozoic, reports mostly come from dinosaur and plant material. In the Paleozoic, nuclei are reported only in a few Carboniferous plants. The oldest non-controversial nuclei (the 609 million year-old phosphatized Weng'an embryoids) predate the Paleozoic but will also be introduced here. Potential modes of nuclear preservation are also discussed, and it can be concluded that the most important factor is the instantaneous inhibition of autolysis after death. The importance of studying fossil nuclei should not be underestimated, as their morphology hold genetic information and can give insights on the evolution of genome sizes, stases, and karyotypes. Nuclei can also inform on the evolution of cell populations, cell death within the vertebrate tree, and on the preservation of ancient DNA in deep time.