 |
| Hypacrosaurus stebingeri Horner & Currie, 1994
in Bailleul, Zheng, Horner, et al., 2020.
|
Abstract
A histological ground-section from a duck-billed dinosaur nestling (Hypacrosaurus stebingeri) revealed microstructures morphologically consistent with nuclei and chromosomes in cells within calcified cartilage. We hypothesized that this exceptional cellular preservation extended to the molecular level and had molecular features in common with extant avian cartilage. Histochemical and immunological evidence supports in situ preservation of extracellular matrix components found in extant cartilage, including glycosaminoglycans and collagen type II. Furthermore, isolated Hypacrosaurus chondrocytes react positively with two DNA intercalating stains. Specific DNA staining is only observed inside the isolated cells, suggesting endogenous nuclear material survived fossilization. Our data support the hypothesis that calcified cartilage is preserved at the molecular level in this Mesozoic material, and suggest that remnants of once-living chondrocytes, including their DNA, may preserve for millions of years.
Keywords: cartilage, dinosaur, nuclei, chromosomes, collagen II, DNA markers
 |
| Ground section of Hypacrosaurus (MOR 548) supraoccipital shows exceptional histological preservation of calcified cartilage. (A) An isolated supraoccipital (So) of Hypacrosaurus in dorsal view. (B–D) Ground section of another So showing calcified cartilage with hypertrophic chondrocyte lacunae. (C) Some cell doublets appear empty (green arrow), but others (pink arrow) present darker, condensed material consistent in shape and location with a nucleus (white arrows). (D) Dark, condensed, and elongated material with morphological characteristics of metaphase chromosomes. The limit of the cell lacuna is visible (black arrow). (E) Caudal view of a juvenile emu skull (∼8–10 months old) showing the So and exoccipitals (Exo) in articulation. (F, G) Ground section (stained with Toluidine blue) of calcified cartilage from this emu skull showing cell doublets (pink arrows) with remnants of nuclei (white arrows) and others without intracellular content (green arrow). |
 |
| Reconstruction of the nesting ground of Hypacrosaurus stebingeri from the Two Medicine formation of Montana. In the center can be seen a deceased Hypacrosaurus nestling with the back of its skull embedded in shallow waters. A mourning adult is portrayed on the right.
Art by Michael Rothman.
|
CONCLUSIONS:
The identification of chemical markers of DNA in Hypacrosaurus suggest it may preserve much longer than originally proposed [30,31]. Even though it is clear that contamination does exist in fossil material and complicates identifications of original organic molecules, it can be accounted for with proper controls. Contamination is not a plausible explanation in this case, and to this date, the possible preservation of original proteins and DNA in deep time has not been convincingly eliminated with data. Although extensive research and sequencing is required to further understand DNA preservation in Mesozoic material, along with its chemical and molecular alterations, our data suggest the preserved nuclear material in Hypacrosaurus was in a condensed state at the time of the death of the organism, which may have contributed to its stability. We propose that DNA condensation may be a favorable process to its fossilization. Additionally, as was suggested for protein fossilization [20,45,46], crosslinking may be another mechanism involved in the preservation of DNA in deep time.
Alida M. Bailleul, Wenxia Zheng, John R. Horner, Brian K. Hall, Casey M. Holliday and Mary H. Schweitzer. 2020. Evidence of Proteins, Chromosomes and Chemical Markers of DNA in Exceptionally Preserved Dinosaur Cartilage. National Science Review. nwz206. DOI: 10.1093/nsr/nwz206
