Pollen Grains Extracted from Oise Amber

Abrigded version from: Dario De Franceschi, Jean Dejax and Gaël de Ploëg, Extraction du pollen inclus dans l'ambre [Sparnacien du Quesnoy (Oise), bassin de Paris]: vers une nouvelle spécialité de la paléo-palynologie, C. R. Acad. Sci. Paris, Earth and Planetary Sciences 330, 2000, 227-233. Some of the figures are omitted, but the original figure numbers were left; the image texts are slightly modified.

  A new amber deposit has been discovered within a lignite clay from the Lower Eocene (Sparnacian) of the Paris Basin. This amber results from the fossilization of the resin secreted by an Angiosperm tree. Very few reports of pollen in amber have been recorded, and so far no detailed studies have been undertaken.
  The amber contains numerous inclusions: these are principally insects, but also various plant fragments. Among the latter, some flowers and abundant pollen, whose preservation seems excellent, were observed.
  The rough amber pieces, as they are taken away during the sifting of the sediment, present an irregular shape and an often frosted surface, which are not very favourable to the observation of these inclusions. The creation of plane and polished surfaces enables the spatial exploration of this transparent medium. Thus, among various and much more voluminous inclusions, pollen is present and is displayed in following different ’scattering states’:

Successive stages of extraction and observation of the pollen grains that are included in amber: 1, partial dissolution of the amber; 2, dissection of the softened amber; 3, deposit of pollen on a cover glass; 4, positioning and spotting of the grains; 5, gold coating; 6, observation with scanning electron microscope (REM); 7, recovery of the pollen glass; 8, mounting between a glass slide and this cover glass in Canada balsam; 9, observation with light microscope (LM).

  So, the extraction of the pollen included in the amber (figure 1), thanks to organic solvents, has been undertaken so that the same grains could be examined successively under scanning electron and light microscopes. The fossil pollen grains isolated in that way present the aspect and the ’freshness’ of modern grains, because their inclusion in the amber respected their volume and maintained their three-dimensional shape; moreover, these grains are not affected by the unknown factors, which could have modified the morphology of the grains extracted from the sediment itself – as it happens, these uncertainities result from the action of unknown parameters, linked to the diagenesis and to the organic matter extraction method from this sediment (standard preparation technique: physico-chemical treatment). Lastly, as in non-acetolysed modern grains, the cell contents of fossilized grains in the amber are still present, though it is contracted and probably partialy dehydrated (figures 5, 6). This permits to consider studying genetics (DNA extraction and sequencing) applied to different morphographic taxa, in order to try to validate or invalidate the supposed botanic affinities.

Figure 3.
Pollen grain partially cleared of the worked amber piece which coated it (SEM, n° PA 2448 2/2 L 1).
Scale bar = 10 µm in all pictures.

Figure 5.
Equatorial optical cut of one of a big mass of pollen grains (taxon Intratriporopollenites); note the presence of the cell content, retracted (LM with differential interference-contrast, n° PA 2438 2/2 P, coord. N 36).

Figure 6.
Grain belonging to the taxon Tricolporopollenites, whose cell content is strongly retracted
(LM with differential interference-contrast, n° PA 622 2/2, coord. F 36).

  Furthermore, the clayey sediment itself, within which this amber was gathered, provided – after a standard preparation technique – a palynological association, characteristic of the Sparnacian. The similarity between the palynofloras, which are fossilized in amber and in the clayey sediment itself, respectively, is checked.

Figure 10.
Grain belonging to the taxon Intratriporopollenites, species whose tectum is partial, reticulate (SEM, n° PA/P1/1).

  The comparison between the pollen observation under light microscopy and scanning electron microscopy (figures 10, 12-13) enables to better refine the family identifications of included flowers. The palynological variability study within a taxon itself was undertaken from pollen scattered in mass, notably of the Normapolles type and of the Intratriporopollenites Pflug & Thomson (1953, in Th. & Pf.) form genus.

Figure 12.
Grain belonging to the taxon Intratriporopollenites, species whose tectum is partial, micro-perforate (SEM, n° PA 2438 2/2 L 12).

Figure 13.
Detail of figure 12: note the micro-perforations of the tectum (SEM).
Scale bar = 10 µm in all pictures.

  Thus, it appears that the study of the fossilized pollen in the amber is a new palaeo-palynological speciality, which supplements the palynological analysis of the sediment containing this amber; particularly, it allows some deductions concerning the palaeo-environment.

V. Arnold thanks the authors for their permittance to copy their publication partially. Contact adress: dariodf@mnhn.fr


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