THE BOTANIC GARDEN OF SMITH C OLLE G E
Much of what we know about the form and structure of extinct plant species comes from fossils. Four major types of plant fossils are:
Fossil Pecopteris fronds PHOTO BY PAMELA DODS; SPECIMEN FROM THE BENESKI MUSEUM, AMHERST COLLEGE
Compression and impression fossils originate in wet areas where plant parts become quickly buried by sediment, the weight of which squeezes out and flattens the plant material, leaving impressions in the fine-grained sediment. In some cases plant carbon or cuticle (waxy, water-repellent coating) also remain Fossil leaves of Ginkgo Fossil leaves and branches of Sphenophyllum miravallis within the impression. PHOTO BY GHEDOGHEDO PHOTO BY WOUDLOPER
Permineralized fossils result from infiltration of the plant tissue by water containing dissolved minerals. Over time, if the minerals precipitate, a rock forms that preserves the plant in three dimensions with detail down to the cellular level.
Above: Fossil wood from Madagascar PHOTO BY JAMES ST. JOHN
Petrified tree in Petrified Forest National Park in Arizona
Man standing next to fossilized Sigillaria trunk in a coal Petrified Calamites trunk Polished petrified wood from Arizona mine in France PHOTOS BY PAMELA DODS; SPECIMENS FROM THE BENESKI MUSEUM, AMHERST COLLEGE ALEXANDER WINCHELL, SKETCHES OF CREATION, NEW YORK, NY: HARPER & BROTHERS, 1870
Unaltered plant remains can occur Right: Flower of Strychnos electri in Dominican Republic where microbial activity, which would amber PHOTO BY GEORGE POINAR normally destroy plant remains, is inhibited by freezing, salinity, or acidity. Below: Hymenaea protera leaf in Dominican Republic amber Examples can be found in deep lake PHOTO BY GEORGE POINAR sediments, in amber (fossilized plant resin), and in pack rat middens, where animal nests made of plants become encased in crystallized urine.
Pack rat midden found in Utah PHOTO BY LARRY L. COATS
Casts of plants can form when sediment infiltrates into cavities left by the decay of buried plant parts. Casts preserve the three- dimensional shape of the plant parts but are without internal structure. Casts of ancient trees are among the most impressive plant fossils.
Calamites canniformis pith cast of Lepidodendron fossil branch in rusty siltstone, found in Ohio PHOTO BY JAMES ST. JOHN Above and below, Lepidodendron trunk surface casts hollow stem PHOTO BY PAMELA DODS; PHOTOS BY GHEDOGHEDO SPECIMEN FROM THE BENESKI MUSEUM, AMHERST COLLEGE