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, 5 (10), e13334

Influence of Microbial Biofilms on the Preservation of Primary Soft Tissue in Fossil and Extant Archosaurs

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Influence of Microbial Biofilms on the Preservation of Primary Soft Tissue in Fossil and Extant Archosaurs

Joseph E Peterson et al. PLoS One.

Abstract

Background: Mineralized and permineralized bone is the most common form of fossilization in the vertebrate record. Preservation of gross soft tissues is extremely rare, but recent studies have suggested that primary soft tissues and biomolecules are more commonly preserved within preserved bones than had been presumed. Some of these claims have been challenged, with presentation of evidence suggesting that some of the structures are microbial artifacts, not primary soft tissues. The identification of biomolecules in fossil vertebrate extracts from a specimen of Brachylophosaurus canadensis has shown the interpretation of preserved organic remains as microbial biofilm to be highly unlikely. These discussions also propose a variety of potential mechanisms that would permit the preservation of soft-tissues in vertebrate fossils over geologic time.

Methodology/principal findings: This study experimentally examines the role of microbial biofilms in soft-tissue preservation in vertebrate fossils by quantitatively establishing the growth and morphology of biofilms on extant archosaur bone. These results are microscopically and morphologically compared with soft-tissue extracts from vertebrate fossils from the Hell Creek Formation of southeastern Montana (Latest Maastrichtian) in order to investigate the potential role of microbial biofilms on the preservation of fossil bone and bound organic matter in a variety of taphonomic settings. Based on these analyses, we highlight a mechanism whereby this bound organic matter may be preserved.

Conclusions/significance: Results of the study indicate that the crystallization of microbial biofilms on decomposing organic matter within vertebrate bone in early taphonomic stages may contribute to the preservation of primary soft tissues deeper in the bone structure.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Location of Xcolak Cenote where samples were placed for biofilm growth.
Figure 2
Figure 2. Interpretive sketch showing geological setting and variation in salinity with depth.
Illustration courtesy of Gene Perry and Mark Howland.
Figure 3
Figure 3. Examples of biofilm infiltration classes for extant archosaur bone.
A) Class 0: a pore on a fractured bone sample from A. mississippiensis showing no biofilm infilling; B) Class 1: a pore on a fractured bone sample from A. mississippiensis showing a biofilm coating the inner pore wall; C) Class 2: pores from a whole bone sample of A. mississippiensis showing a significant biofilm coating on the inner pore walls; D) Class 4: a pore from a fractured bone sample of G. gallus showing a complete infiltration or covering of the pore. All images shown at 300× magnification.
Figure 4
Figure 4. Results of PC-ORD statistical analysis.
Testing for significant differences in the amount of PLFA assemblages in each sample, these results show a significant difference between samples of whole bone from aerobic conditions (2W) and fractured bone from aerobic conditions (2H), suggesting a difference in microbial communities due to bone condition prior to placement in the cenote. Axis 1 is based on variation in cenote environmental conditions, while Axis 2 is based on whole or fractured bones.
Figure 5
Figure 5. SEM and EDS results of G. gallus control sample.
A) SEM image of primary soft tissue in G. gallus control sample with primary blood cells observable. B) EDS results of primary soft tissue in G gallus control sample showing a high abundance of carbon.
Figure 6
Figure 6. Biofilm growth on an extant G. gallus bone.
A) Biofilm surrounding surface pore. B) Biofilm partially covering a surface pore and lining the inner pore surface. C) Encrusting biofilm on the surface of a bone. D) Biofilm on the surface of a bone and on the inner pore surface.
Figure 7
Figure 7. Higher magnification SEM images of biofilm growth on extant G. gallus bone.
A) Biofilm on inner pore surface. B) Higher magnification of Figure 7A, showing individual bacterial cells and EPS. Debris and diatoms are adhered to the biofilm. C) Biofilm surrounding surface pore and clean bone surface. D) Higher magnification of Figure 7C, showing biofilm on inner pore surface.
Figure 8
Figure 8. Results of thescelosaur specimen BMR P2006.4.309.
A) SEM images of thescelosaur samples prior to demineralization. Scale bar equals 10 cm. B) Thescelosaur samples at 330× magnificaiton, showing bacterial cells filling the surface pores. C) Thescelosaur samples at 2200× magnification, showing the mineralized bacterial cells in fossil specimens. D) EDS results of the mineralized bacterial cells in fossilized thescelosaur samples, showing a Ca- and P-rich signature.
Figure 9
Figure 9. Thescelosaur bone sample extracts after complete demineralization in EDTA.
A) Light microscopy image of branching, fragile, semi-transparent vessels remaining after demineralization. Scale bar equals 1 mm. B) SEM image of thescelosaur extract, at 55× magnification. C) EDS analysis of frangible thescelosaur extracts, rich in calcium an d phosphate.
Figure 10
Figure 10. Results of theropod specimen BMR P2002.4.1.
A) Theropod bone sample prior to EDTA demineralization. Scale bar equals 5 cm. B) Partial demineralization of theropod sample, showing “etched” vessels released during demineralization. Scale bar equals 1 mm. C) Release of resistant vessels from fossil bone. Scale bar equals 1 mm.
Figure 11
Figure 11. Complete demineralization of theropod bone.
(A) SEM image of hollow, vascular theropod extract at 75× magnification. B) EDS signature of 11(A) vessel, rich in Ca, O, Mn, and P. C) SEM image of theropod vessel at 120× magnification. D) EDS signature of 11(C) vessel, rich in calcium and phosphate.
Figure 12
Figure 12. Ceratopsian specimen BMR P2006.4.1 bone fragments.
A) Bone fragments prior to demineralization. Scale bar equals 5 cm. B) Light microscopy image of ceratopsian sample after demineralization, revealing a network of pliable, how vessels and tubes. Scale bar equals 1 mm. C) SEM image of pliable ceratopsian extracts at 4,500× magnification. D) EDS signature of SEM image 12(C), high in carbon, iron, and silica.
Figure 13
Figure 13. SEM and EDS results of ceratopsian specimen BMR P2006.4.1.
A) SEM image at 650× magnification of pliable ceratopsian vessels. B) SEM image at 4,500× magnification of framboids identified in pliable ceratopsian vessels. C) EDS signature of framboids, rich in iron.
Figure 14
Figure 14. Histograms of (A) pore size distributions and (B) pore infiltration distribution among extant archosaur samples.

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