Molds (concave surfaces) and casts (convex surfaces) are fossilization phenomena in which the actual plant, embedded in the surrounding background rock, was dissolved, leaving a hollow space (mold) that subsequently filled with other material. A cast was thus formed that preserved the plant’s external features.
Most petrifactions are fossils in which silica, carbonate, or other material permeated or replaced the internal structures of the plant and preserved them so well that in most specimens the finest cellular details can be observed. Compressions, another kind of petrifaction, are the pressed carbonized remains of the plant itself.
PENNSYLVANIAN FLORA
Aphthoroblattina
Teneopteron
The far-reaching Pennsylvanian swamplands had abundant species of trees and other plants that long since have become extinct. Today’s common deciduous trees were not present; flowering plants had not yet evolved. Instead, the tangled forests were dominated by giant ancestors of presently existing club-mosses, horsetails, ferns, conifers, and cycads. The undergrowth also was well developed, consisting mainly of ferns, fernlike plants, Sphenophyllum, and small club-mosses. The plant fossils give no indication of seasonal variations. The forests, evidently always green, grew rapidly and abundantly, with foliage of unprecedented size and luxuriance. Land animals were just beginning to develop and included sluggish, salamander-like amphibians, large primitive insects, and a few small reptiles. The insects flourished as never before or since in the damp forests and attained remarkable size. Insects more than four inches long were common and some are known to have been more than a foot long with a wingspread proportionately broad. Ancestors of the modern spiders, scorpions, centipedes (one fossil found in Illinois was twelve inches long), cockroaches, and dragonflies are represented by several hundred species.
The fossilized plants of Pennsylvanian time belonged to only a few main categories: scale and seal trees, ancient scouring rushes (horsetails), herbaceous Sphenophyllum, ferns, seed ferns, and cordaitean trees.