A cell nucleus with chromatin was found in the fossilized cartilage of a 125 million-year-old dinosaur
Semyon Morozov, N+1
Chinese paleontologists have announced the discovery of a cell nucleus containing chromatin in the fossilized cartilage of the Early Cretaceous dinosaur Caudipteryx. This is the second case of the discovery of fossil chromatin in vertebrate fossils — chromatin was previously detected in the cartilage of a duck-billed dinosaur that lived about 70 million years ago. The study was published in the journal Communications Biology (Zheng et al., Nuclear preservation in the cartilage of the Jehol dinosaur Caudipteryx).
Chondrocytes from the fossil cartilage of the dinosaur Caudipteryx. On the left is a cell with a nucleus. Drawings from the article by Zheng et al.
Recently, there have been more and more reports in the scientific literature about the finds of cell nuclei in the fossils of organisms, starting with mammoths that lived tens of thousands of years ago, and ending with plants of the Mesozoic era or even Proterozoic embryo-like fossils approximately 609 million years old (which, apparently, belong to protists). In some cases, according to the authors of the works, the nuclei were preserved not only at the histological level, but also at the molecular level: in some fossil plants (in particular, from the Jurassic period), using histochemical methods, it was possible to identify chromatin — a DNA complex with proteins in the nuclei.
Last year, researchers used histochemistry to identify chromatin in the nuclei of fossil vertebrates for the first time. This work was led by Alida M. Bailleul from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences. Her team discovered cell nuclei with DNA fragments in the fossil cartilage of the Late Cretaceous duck-billed dinosaur Hypacrosaurus stebingeri, which lived about 70 million years ago.
Many scientists are skeptical about the work to identify such ancient DNA fragments, since this molecule decays quite quickly by geological standards. The age of the oldest sequenced DNA belonging to the woolly mammoth is only 1.65 million years. Critics suggest that DNA from older fossils may actually belong to abstract bacteria (1, 2). Proponents of endogenous DNA respond that the nature of DNA distribution in fossil cells cannot be explained by bacterial origin.
Recently, Alida Bayel with colleagues from China has published another work on the cell nuclei of dinosaurs. This time paleontologists have studied the fossil cartilage of Oviraptorosaurus Caudipteryx sp. (the authors do not specify the species), a representative of the Zhehe biota (Dzhehol; Jehol Biota). This dinosaur lived in the early Cretaceous period — about 125 million years ago.
From the distal femur of caudipteryx, scientists took a sample of articular cartilage and divided it into three parts. From the first two, the researchers made slits (thin slices), which were studied using light and electron microscopy. As a result, paleontologists found cartilage cells in the fossil — chondrocytes.
Scientists decalcified the third fragment in ethylenediaminetetraacetic acid for 21 days, and then prepared histological sections from it, some of which were stained with hematoxylin (stains basophilic structures, including nucleic acids, blue or purple) and eosin (stains eosinophilic structures pink), and some were left unpainted. The obtained sections were studied by paleontologists using light (including polarization) microscopy and compared with similar sections of cartilage of a modern dinosaur — chicken. To avoid contamination, sections of caudipteryx and chicken cartilage were made in different rooms with different sets of tools.
After studying the stained sections of caudipteryx cartilage, the scientists found that most of the cells remained transparent, but in one cell hematoxylin stained a rounded structure purple, which the researchers identified as the nucleus. At the same time, it contained several dark purple threads, which, according to the authors, were chromatin. Scientists saw a similar picture on colored sections of chicken cartilage.
Hematoxylin and eosin—stained chondrocytes of caudipteryx (a, c, d, g, h) and chicken (b, e, f); h - in polarized light; nu — nucleus, ct — chromatin.
The authors note that this is the second known example of chromatin preservation in fossil vertebrates, and this chromatin is older than the previous one — from the cartilage of a duck—billed dinosaur - by 55 million years. Apparently, such ancient cellular structures were able to survive due to the special conditions of burial. The fossil remains of caudipteryx, as well as other organisms of the Zhehe biota, were found in lagerstettes — localities characterized by exceptional preservation of fossils. In the case of the Zhehe biota, the remains of organisms were well preserved due to the rapid burial by volcanic ash.
According to the authors, the type of tissue also contributed to the good preservation of cellular structures: there are no vessels and nerves in the cartilage, which reduces the likelihood of bacterial colonization. In addition, chondrocytes have mainly anaerobic metabolism, so they are resistant to hypoxia and can remain viable up to two weeks after the death of the organism — that is, burial does not necessarily have to occur immediately after death.
The burial of caudipteryx in the artist's view.
The discovered chemical differences between the nucleus and cytoplasm in the cartilage of caudipteryx suggest that DNA could have been partially preserved in the fossils, the authors conclude. However, they stipulate that such DNA is most likely impossible to isolate and sequence.
Many researchers doubt the possibility of preserving not only DNA in fossils, but also proteins, the finds of which in the fossils are explained by skeptics by bacterial origin or contamination in the laboratory. Last year , a preprint of the work of paleontologists was published, who analyzed the composition of the fossil shell of titanosaurs and came to the conclusion that only individual amino acids, but not proteins, can be preserved for tens of millions of years.
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