Molds And Casts

In addition to two-dimensional plant parts, such as leaves, three-dimensional structures, such as stems, seeds, or fruits, can also be carried into sites where sediment is accumulating and buried. During flood events, massive trunks and tree branches can be moved some distance before they are eventually deposited. If these plant parts became crushed over time, they would be preserved as compression or impression fossils. If, however, the sediment surrounding the three-dimensional plant parts hardens before the plant fragment is crushed, the sediment will form a three-dimensional negative, or mold, of the plant fragment. As the plant material disintegrates, a hollow remains in the sediment, which can subsequently be filled in with sediment, thus forming a cast inside the mold. The surface of the cast and the mold can often faithfully reproduce the surface features of a particular plant part, such as characteristic leaf bases on the surface of a stem or the ornamentation of seeds (FIG. 1.35) and fruits. The sediment that fills in the cavity of the mold and solidifies becomes a three-dimensional cast of the original plant part

FIGURE 1.36 Cast of arborescent lycopod (Protostigmaria eggertina) (arrows) (Mississippian). Hammer = 15 cm.

(FIG. 1.36 ) . In almost all molds and casts no actual plant material remains, but the surface contours are the same as those of the original plant part. The formation of fossil molds and casts parallels the method by which a sculptor creates a bronze statue. The sculptor does not carve directly on a block of bronze, but creates a sculpture with some other medium— wood or wax perhaps. A mold is then constructed around the original sculpture and, when the mold is complete, the original is removed in some fashion (disassembling the mold temporarily or melting the wax). When the mold is reassembled, molten bronze is poured into it, and an exact replica of the original sculpture (but one that involves none of the original material in that sculpture) is formed. Rates of sedimentation in certain areas where molds and casts were formed must have been spectacular. As an example, the sea cliffs at Joggins, Nova Scotia, reveal exposed casts of Pennsylvanian tree trunks 3-8 m tall. The trees must have been buried quite

Figure 1.37 Compressed trunk (Devonian). (Courtesy W. E. Stein.)

cast of Eospermopteris

Figure 1.37 Compressed trunk (Devonian). (Courtesy W. E. Stein.)

cast of Eospermopteris

Figure 1.38 Large pith cast of Calamites gigas (Permian). Bar = 20 cm.

rapidly in place. Sediment hardened and the trees subsequently died, leaving hollows (molds) in the hardened rock that were subsequently filled with other sediments (casts). Compressions, and casts (FIG. 1.37) are important in showing the external form of plant parts in a three-dimensional fashion. Root casts of trees can provide important morphological information useful in determining the type of soil formation and soil drainage conditions when the plants were growing (Retallack, 1990). They also may reveal specialized taphonomic processes and how degradation of organic tissues may have proceeded (Driese et al., 1997).

A special form of cast is the calamite pith cast or steinkern, which is a common form of preservation of larger calamite stems and branches. Pith casts are casts (FIG. 1.38) of the hollow pith or medullary region in calamites and

FiguRE 1.39 Cast of several tracheids showing circular bordered pits (Miocene). Bar = 55 |im.

preserve an impression of the outside of the pith cavity, which represents the inside of the vascular tissue and cortex (see Chapter 10 for further details).

An unusual example of a mold and cast is represented by fossil wood that was exposed to colloidal silica during the diagenesis; the silica permeated the cell cavities, but somehow did not impregnate the cell walls. After precipitation of the silica within the cell cavities, the cell walls (=molds) disintegrated and all that is left are casts of the cavities of the wood cells (FIG. 1.39). These cell casts have the negative contours of the insides of the cell walls and show counterparts of specialized wall structures, such as bordered pits (Chapter 7).

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