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A three-dimensional scientific diagram of the microscopic structure of a leaf. The diagram shows the main tissue layers (listed from top: upper epithelium with cuticle, palisade mesophyll, spongy mesophyll, and lower epithelium with cuticle) as well as the guard cells and the main plant cell organelles.
The microscopic structure of a leaf, showing the position of the cuticle in relation to the epidermis and other key features.

Cuticle analysis, also known as fossil cuticle analysis and cuticular analysis, is an archaeobotanical method that uses plant cuticles to reconstruct the vegetation of past grassy environments. Cuticles comprise the protective layer of the skin, or epidermis, of leaves and blades of grass. They are made of cutin, a resilient substance that can preserve the shapes of underlying cells, a quality that aids in the identification of plants that are otherwise no longer visible in the archaeological record.[1] This can inform archaeobotanists on the floral makeup of a past environment, even when surviving remains from the plants are limited. Plant cuticles have also been incorporated into other areas of archaeobotanical research based on their susceptibility to environmental factors such as pCO2 levels and stresses such as water deficit and sodium chloride exposure.[2][3][4] Such research can help to reconstruct past environments and identify ecological events.

Method

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There is no one universal method to cuticle analysis. Rather, it is the shared principle on which the applications are based which underpins the methodology—namely, that a well-preserved plant cuticle can, through the use of microscopy, yield information regarding the nature of the plant from which it originated, including its species and the environmental stresses acting upon it. Depending on the desired outcome, both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) can be used, the main difference being that while SEM can provide information regarding the outer characteristics of an organism, TEM can be used to show details of the inner structure.[5][6][7] In SEM approaches, latex or silicone casts may be used to recreate epidermal and cuticular features in imperfectly preserved samples.[6] Atomic force microscopy (AFM) can also be used as a complementary method to provide high-resolution topographic imaging at submicron scale.[8] If the desired outcome is identification of the plant, the image created by one or a combination of these microscopy methods can then be compared to existing data, regarding both the impressions left in the cuticle of the underlying cell structure and the properties of the cuticle itself.

Cuticle extraction methods

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Depending on preservation conditions, a method of extraction for the fossil cuticle is required before the analysis can take place. One such method for separating cuticles from a rock matrix is acid maceration, which involves soaking the sample in agents such as dilute hydrogen peroxide or hydrochloric and hydrofluoric acid (known as the HCI/HF protocol) to break down the matrix.[9][10] However, this process is destructive and, if possible, avoided.[11] Cuticle transfer techniques are generally preferred, as these methods preserve the physiognomy and morphology of the sample, regardless of how fragmented the specimen may be.[11][12] Among the most practical and effective of these techniques is the polyester overlay transposition method, developed by Kouwenberg et al., which is both straightforward and non-destructive: the cuticle, either already accessible on the adaxial surface or exposed using cellophane tape from the abaxial surface, is pressed against the adhesive side of the polyester overlay and removed.[11]

Applications

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The most straightforward application of fossil cuticle analysis is in the identification of the plants which comprised a past environment. This is because the cutin layer preserves some of the defining characteristics of the plant's underlying cell structure, allowing it to be identified by experts at a microscopic level.[1] Taxonomical differences in the epidermises of the two species Pinus sylvestris L. and Pinus uncinata Ramond ex DC., for example, can often be observed from cuticle analysis, meaning the plants can still be reliably identified and distinguished from one another in cases where other methods such as pollen analysis are not possible.[13] In broader studies of past flora, this method can be expanded to not only inform researchers of the plant species present but also of patterns and trends underlying this distribution. In 2003, for example, cuticle analysis was used in a multi-proxy study to reconstruct changes in vegetation during the Late Pleistocene and Holocene in Kenya, with particular regard to the proportion of plants following the C4 photosynthetic pathway, and more specifically the NADP-ME C4 sub-pathway.[14] There are, however, a number of other ways that such data can be used, including the following:

In the reconstruction of past atmospheres

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The relationship between past CO2 levels and fossil cuticles has, particularly in the last few decades, become an important source of information on historic atmospheric change.[15] It has long been observed that the stomatal index of a leaf bears a direct, inverse correlation to the quantity of atmospheric CO2 at the time of growth.[16] Because of the properties of the cuticle, this means that fossil cuticles are often the best-preserved source of information on the stomatal qualities of a plant, including its stomatal index, stomatal ratio, and stomatal density.[17] The thickness of the cuticle can also indicate the properties of the atmosphere in which the plant grew.[17]

The continuous record of atmospheric CO2 levels produced by Gregory Retallack used fossil plant cuticles in the above manner to produce an account of CO2 levels for the past 300 million years.[15] Though met with some methodological critique, and limited in overall accuracy by the preservation bias, the study demonstrates the merits of using stomatal indices via fossil cuticles as a paleobarometer for past CO2 levels in the pursuit of the reconstruction of past atmospheres.[18][19]

In the study of environmental stresses

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Environmental stresses such as drought, volcanic gas exposure, and climatic factors can sometimes be observed through the cuticle of a leaf or grass blade.[20] The presence of charred grass cuticles in African sediments, for example, demonstrates a pattern of grass fires still common in modern African savannahs.[21][22] The cuticles thus preserved can in turn be used in research regarding the nature of plants in a region as well as the biomes to which they belong in relation to the broader environmental factors by which they are influenced.[23]

In the case of volcanic sulphur dioxide (SO2) emissions, fossil cuticle analysis has of late been considered as a potential proxy of unique merits. In a 2018 study, a strong correlation was found between significant SO2 exposure during the Triassic–Jurassic transition and damage to the cuticles of nearby plants.[24] Given that there is prior research indicating that such exposure can result in distinct morphological changes to leaves and their cuticles, the main conclusion drawn was that volcanic activity and ensuing SO2 exposure played a significant part in the broader ecosystem response to the environmental challenges of the epoch.[25]

In the study of paleoaltimetry

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In 2004, Jennifer McElwain proposed a method for calculating paleoaltimetry using fossil cuticles and their record of stomatal density as a proxy for CO2 partial pressure (pCO2).[26] This technique utilises the inverse correlation between stomatal density and pCO2 to calculate paleo-elevation to within an average error range of ±300m, compared to ±400m in the comparable basaltic lava method.[27] The technique is not universally applicable, however, as there are many types of plants that are not CO2 sensitive and are thus incompatible with the pCO2 paleoaltimeter.[28]

In conjunction with geochemical fingerprinting

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Geochemical fingerprinting is a technique that can incorporate data from both fossil and extant cuticles to better understand paleoenvironmental conditions and changes in relation to individual plant species.[29] Defined by Jochen Hoefs as "a chemical signal that provides information about the origin, the formation and/or the environment of a geological sample", a geochemical fingerprint is a type of identification marker most remarkable because it will not typically change with time or age.[30] In the context of cuticle analysis, this means that a plant's original chemical signature can be observed from a fossil cuticle sample.[29] This can help to, among other things, establish correlative relationships between changes in historic plant geochemistry and events such as the end-Triassic extinction.[29]

Issues

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One of the drawbacks of fossil cuticle analysis is the range of chemical changes inherent to the natural preservation of plant cuticles. A comparative study of both modern and ancient Ginkgo cuticles, for example, revealed a number of distinct, consistent diagenetic changes that altered the features of the samples over time.[31] Ultimately, this means that, while incredibly informative, fossil cuticles cannot alone be used to reliably reconstruct features of past plants without consulting other sources of data, such as other contemporaneous plant matter or the modern descendants of either the species or its genus.

References

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  1. ^ a b Renfrew C, Bahn P (1996). Archaeology : theories, methods, and practice (2nd ed.). London: Thames and Hudson. pp. 224–5. ISBN 0-500-27867-9. OCLC 34521234.
  2. ^ Jardine PE, Kent M, Fraser WT, Lomax BH (2019). "Ginkgo leaf cuticle chemistry across changing pCO2 regimes". PalZ. 93 (3): 549–558. doi:10.1007/s12542-019-00486-7. ISSN 0031-0220. S2CID 202083207 – via Springer.
  3. ^ Kosma DK, Bourdenx B, Bernard A, Parsons EP, Lü S, Joubès J, Jenks MA (December 2009). "The impact of water deficiency on leaf cuticle lipids of Arabidopsis". Plant Physiology. 151 (4): 1918–1929. doi:10.1104/pp.109.141911. PMC 2785987. PMID 19819982.
  4. ^ Kolattukudy PE (1996). "Biosynthetic pathways of cutin and waxes, and their sensitivity to environmental stress.". In Kerstiens G (ed.). Plant cuticles: an integrated functional approach. Oxford: BIOS Scientific publishers Ltd. pp. 83–108. ISBN 978-1-85996-130-8. OCLC 247156946.
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  6. ^ a b Moisan P (2012-07-14). "The study of cuticular and epidermal features in fossil plant impressions using silicone replicas for scanning electron microscopy". Palaeontologia Electronica. 15 (2): 1–9. doi:10.26879/309. ISSN 1094-8074.
  7. ^ "Electron Microscopy | TEM vs SEM - US". www.thermofisher.com. Retrieved 2022-04-26.
  8. ^ Benítez JJ, Guzman-Puyol S, Domínguez E, Heredia A, Heredia-Guerrero JA (2019). "Applications and potentialities of Atomic Force Microscopy in fossil and extant plant cuticle characterization". Review of Palaeobotany and Palynology. 268: 125–132. Bibcode:2019RPaPa.268..125B. doi:10.1016/j.revpalbo.2019.06.015. S2CID 199095338.
  9. ^ Jones TP, Rowe NP (1999). Fossil Plants and Spores: Modern Techniques. Geological Society of London. ISBN 978-1-86239-035-5.
  10. ^ Mösle B, Finch P, Collinson ME, Scott AC (1997-05-01). "Comparison of modern and fossil plant cuticles by selective chemical extraction monitored by flash pyrolysis-gas chromatography-mass spectrometry and electron microscopy". Journal of Analytical and Applied Pyrolysis. PYROLYSIS '96. 40–41: 585–597. doi:10.1016/S0165-2370(97)00039-9. ISSN 0165-2370.
  11. ^ a b c Kouwenberg LL, Hines RR, McElwain JC (July 2007). "A new transfer technique to extract and process thin and fragmented fossil cuticle using polyester overlays". Review of Palaeobotany and Palynology. 145 (3): 243–248. Bibcode:2007RPaPa.145..243K. doi:10.1016/j.revpalbo.2006.11.002. ISSN 0034-6667.
  12. ^ Dilcher DL (January 1974). "Approaches to the identification of angiosperm leaf remains". The Botanical Review. 40 (1): 1–157. doi:10.1007/BF02860067. ISSN 1874-9372. S2CID 33281696.
  13. ^ Alvarez SG, Juaristi CM, Gutierrez JS, Garcia-Amorena I (2009-05-01). "Taxonomic differences between Pinus sylvestris and P. uncinata revealed in the stomata and cuticle characters for use in the study of fossil material". Review of Palaeobotany and Palynology. 155 (1): 61–68. Bibcode:2009RPaPa.155...61G. doi:10.1016/j.revpalbo.2009.01.002. ISSN 0034-6667.
  14. ^ Wooller MJ, Swain DL, Ficken KJ, Agnew AD, Street-Perrott FA, Eglinton G (2003). "Late Quaternary vegetation changes around Lake Rutundu, Mount Kenya, East Africa: evidence from grass cuticles, pollen and stable carbon isotopes". Journal of Quaternary Science. 18 (1): 3–15. Bibcode:2003JQS....18....3W. doi:10.1002/jqs.725. ISSN 0267-8179. S2CID 128739470.
  15. ^ a b Retallack GJ (May 2001). "A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles". Nature. 411 (6835): 287–290. Bibcode:2001Natur.411..287R. doi:10.1038/35077041. PMID 11357126. S2CID 4430851.
  16. ^ Beerling DJ, Royer DL (March 2002). "Reading a CO2 signal from fossil stomata". The New Phytologist. 153 (3): 387–397. doi:10.1046/j.0028-646X.2001.00335.x. PMID 33863224.
  17. ^ a b McElwain JC, Chaloner WG (1996). "The Fossil Cuticle as a Skeletal Record of Environmental Change". PALAIOS. 11 (4): 376–388. Bibcode:1996Palai..11..376M. doi:10.2307/3515247. ISSN 0883-1351. JSTOR 3515247.
  18. ^ Kerp H (January 2002). "Atmospheric CO2 from fossil plant cuticles". Nature. 415 (6867): 38. doi:10.1038/415038a. PMID 11780108. S2CID 31901296.
  19. ^ Retallack G (January 2002). "Atmospheric CO2 from fossil plant cuticles". Nature. 415 (6867): 38. doi:10.1038/415038b. ISSN 1476-4687. PMID 11780108.
  20. ^ Bartiromo A (2011). The cuticle micromorphology of extant and fossil plants as indicator of environmental conditions. A pioneer study on the influence of volcanic gases on the cuticle structure in extant plants (Ph.D. thesis). Université Claude Bernard-Lyon I. doi:10.6092/UNINA/FEDOA/8868. S2CID 129474005.
  21. ^ Kim SY, Scourse J, Marret F, Lim DI (2010-11-10). "A 26,000-year integrated record of marine and terrestrial environmental change off Gabon, west equatorial Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 297 (2): 428–438. Bibcode:2010PPP...297..428K. doi:10.1016/j.palaeo.2010.08.026. ISSN 0031-0182.
  22. ^ Wooller MJ, Street-Perrott FA, Agnew AD (2000-12-01). "Late Quaternary fires and grassland palaeoecology of Mount Kenya, East Africa: evidence from charred grass cuticles in lake sediments". Palaeogeography, Palaeoclimatology, Palaeoecology. Fire and the Palaeoenvironment. 164 (1): 207–230. Bibcode:2000PPP...164..207W. doi:10.1016/S0031-0182(00)00187-5. ISSN 0031-0182.
  23. ^ Kim SY, Scourse J, Marret F, Lim DI (2010-11-10). "A 26,000-year integrated record of marine and terrestrial environmental change off Gabon, west equatorial Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 297 (2): 428–438. Bibcode:2010PPP...297..428K. doi:10.1016/j.palaeo.2010.08.026. ISSN 0031-0182.
  24. ^ Steinthorsdottir M, Elliott-Kingston C, Bacon KL (2018-03-01). "Cuticle surfaces of fossil plants as a potential proxy for volcanic SO
    2     emissions: observations from the Triassic–Jurassic transition of East Greenland"    . Palaeobiodiversity and Palaeoenvironments. 98 (1): 49–69. doi:10.1007/s12549-017-0297-9. ISSN 1867-1608. S2CID 134565665.
  25. ^ Bacon KL, Belcher CM, Haworth M, McElwain JC (2013-04-10). "Increased atmospheric SO₂ detected from changes in leaf physiognomy across the Triassic-Jurassic boundary interval of East Greenland". PLOS ONE. 8 (4): e60614. Bibcode:2013PLoSO...860614B. doi:10.1371/journal.pone.0060614. PMC 3622679. PMID 23593262.
  26. ^ McElwain JC (2004-12-01). "Climate-independent paleoaltimetry using stomatal density in fossil leaves as a proxy for CO2 partial pressure". Geology. 32 (12): 1017–1020. Bibcode:2004Geo....32.1017M. doi:10.1130/G20915.1. ISSN 0091-7613.
  27. ^ Sahagian D, Proussevitch A, Carlson W (September 2002). "Timing of Colorado Plateau uplift: Initial constraints from vesicular basalt-derived paleoelevations". Geology. 30 (9): 807–810. Bibcode:2002Geo....30..807S. doi:10.1130/0091-7613(2002)030<0807:TOCPUI>2.0.CO;2. ISSN 0091-7613.
  28. ^ McElwain JC (January 2005). "Climate-independent paleoaltimetry using stomatal density in fossil leaves as a proxy for CO2 partial pressure: Comment and Reply: REPLY". Geology. 33 (1): e83. Bibcode:2005Geo....33E..83M. doi:10.1130/0091-7613-33.1.e83. ISSN 0091-7613.
  29. ^ a b c Vajda V, Pucetaite M, Steinthorsdottir M (September 2021). "Geochemical Fingerprints of Ginkgoales Across the Triassic-Jurassic Boundary of Greenland". International Journal of Plant Sciences. 182 (7): 649–662. doi:10.1086/715506. ISSN 1058-5893. S2CID 236422070.
  30. ^ Hoefs J (March 2010). "Geochemical fingerprints: a critical appraisal". European Journal of Mineralogy. 22 (1): 3–15. Bibcode:2010EJMin..22....3H. doi:10.1127/0935-1221/2010/0022-1997.
  31. ^ Collinson ME, Mösle B, Finch P, Scott AC, Wilson R (1998). "The preservation of plant cuticle in the fossil record: a chemical and microscopical investigation". Ancient Biomolecules. 2: 251–65 – via ResearchGate.

Further reading

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  • Allison PA, Bottjer DJ (2010). Taphonomy: Process and Bias Through Time (2nd ed.). Dordrecht: Springer. ISBN 978-90-481-8643-3.
  • Rowe NP, Jones TP (1999). Fossil Plants and Spores: Modern Techniques. London: Geological Society. ISBN 978-1-86239-035-5.
  • Warinner C, d'Alpoim J, Marston JM (2014). Method and Theory in Paleoethnobotany. Boulder: University Press of Colorado. ISBN 978-1-60732-316-7.
  • Willis K, McElwain J (2014). The Evolution of Plants (Second ed.). Oxford, United Kingdom. ISBN 978-0-19-929223-3.{{cite book}}: CS1 maint: location missing publisher (link)
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