When a paleobotanist splits a rock that contains fossil plant fragments along a bedding plane, it is sometimes possible to see the carbonaceous film of a compression along one face, and a negative imprint of the plant part, with little or no carbon adhering, on the other face (FIG. 1.32 ) ; these two faces are called part and counterpart in paleobotany. The fossil with little or no carbonaceous material is called an impression

FIGURE 1.34 Impression of Sigillaria leaf bases showing parichnos scars and position of leaf trace (Pennsylvanian). Bar = 1 cm. (Courtesy BSPG.)

(FIGS. 1.33, 1.34). The imprint will show all the surface details of the compression, such as leaf shape and venation, but there is no actual plant material, that is no carbon, preserved. If you have ever seen a leaf imprint in a concrete sidewalk, you have seen an impression. The process involved in the formation of an impression is also analogous to these modern "fossils." Such imprints are formed when leaves fall and settle into the wet concrete just after it is poured. As the concrete hardens, it conforms to the contours of the lower side of the leaf that rests on it. Eventually, the leaf disintegrates and the pieces are blown away, but a negative replica of the leaf remains on the hardened concrete. If you have ever put your initials in wet concrete, you have formed an impression. The impression of dinosaur footprints represents an excellent example of this type of fossilization process. When several footprints are of the same type or a series of trackways are discovered in close proximity, it may be possible to extrapolate the stride of the organism and, from this, infer something of the biomechanics of the animal.

No cellular details can normally be seen on an impression because there is no adhering organic material, but, in some instances, especially where the matrix is exceedingly fine grained, a replica of the impression can be made with latex or similar material. The replica faithfully reproduces whatever surface details were on the original organism when it was impressed into the mud. Examination of part of the replica with the SEM may reveal details with great clarity, such as the pattern of the epidermal cells, hairs, glands, or other surface features. Some impression fossils are covered with mineral encrustations of different composition, for example iron (Spicer, 1977). These deposits may be the result of the

FIGURE 1.35 Cast of large seed fern seed (Pennsylvanian). Bar = 2 cm.

FIGURE 1.35 Cast of large seed fern seed (Pennsylvanian). Bar = 2 cm.

activities of microorganisms during the decay process. Regardless of their origin, however, the mineral crust may provide an excellent replica of the surface of the plant part, and this can be studied using a variety of imaging modes.

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