Plant fossils can preserve external shape, internal anatomy, organic residues or evidence of biological activity. Identify the preservation process separately from the plant group: one specimen can record more than one process.
1. Mold and Cast Fossils
What it is: A mold is an empty hollow impression left in the rock after an organism rots away, and a cast is the 3D stone replica formed when that empty space gets filled back up with minerals or sediment.
Plant Example: The hollow, scaly trunk of an ancient clubmoss tree (Lepidodendron) falls into mud and rots away, leaving an empty hollow bark impression (mold). Sediment later fills that hollow and hardens into rock, creating a heavy, solid three-dimensional stone column (cast).
2. Permineralized Fossils (Petrified Fossils)
What it is: Mineral-bearing water enters pores in buried plant material. Dissolved substances precipitate within those spaces, preserving three-dimensional anatomy. Water evaporation is not a required explanation, and preservation quality varies.
Plant example: Silicified wood may preserve cellular anatomy. Permineralization and replacement often occur together; not every specimen preserves all growth rings or cell walls.
3. Replacement Fossils
What it is: Minerals substitute for original material as it is lost or dissolved. Replacement can retain shape and anatomical detail, but neither complete replacement nor perfect fidelity should be assumed.
Plant example: Silica can replace organic material in fossil wood. Distinguish replacement of tissue from mineral infilling of spaces within tissue.
4. Recrystallization Fossils
What it is: Recrystallization changes crystal structure or crystal size in mineral material. It is familiar in mineralised animal shells; ordinary organic plant cell walls are not originally calcite crystals.
Plant comparison: Coal balls are generally carbonate concretions containing permineralized peat. Mineral precipitation can preserve internal plant anatomy. They should be taught primarily as examples of permineralization, rather than assumed recrystallization of plant cell walls.
5. Carbonization Fossils
What it is: Burial and chemical alteration can leave a flattened, carbon-rich organic residue called a compression. An impression retains the outline or surface relief without that residue. Carbonization is not simply squeezing out gases to leave chemically pure carbon.
Plant example: Annularia is a form genus for foliage associated with extinct horsetail relatives, not fern foliage. Whorled leaves can occur as compressions or impressions in sedimentary rocks.
6. Preserved Remains, Including Amber Inclusions
What it is: Drying, freezing or entrapment in resin may preserve plant remains differently from mineral replacement. “Preserved remains” does not guarantee unchanged chemistry or the complete absence of decay.
Plant example: Flowers, needles or other plant material can be trapped in resin that becomes amber. An inclusion may retain structures or become a cavity or mold after decay; inspect the actual specimen before calling it unaltered.
7. Trace Fossils (Ichnofossils)
What it is: Trace fossils record biological activity, such as root penetration or insect boring, rather than the organism’s preserved body. A preserved root tissue specimen is a body fossil; a root channel can record growth through sediment.
Plant-related example: Ancient root channels or insect borings in fossil wood. The broad term rhizolith covers several root-related preservation forms, so distinguish the trace from any preserved root tissue.
How to classify an unfamiliar specimen
First ask what remains: a surface outline, an organic film, a three-dimensional shape or cellular anatomy? Then identify the plant organ and preservation process. A dark fossil need not be pure carbon, and a stone-like fossil does not prove that all original tissue was replaced.
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